Changed: DB Params

templ/.dockerignore

@@ -0,0 +1,3 @@
+.git
+Dockerfile
+.dockerignore

templ/.envrc

@@ -0,0 +1 @@
+use flake

templ/.github/FUNDING.yml

@@ -0,0 +1 @@
+github: [a-h, joerdav]

templ/.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Before you begin**
+Please make sure you're using the latest version of the templ CLI (`go install github.com/a-h/templ/cmd/templ@latest`), and have upgraded your project to use the latest version of the templ runtime (`go get -u github.com/a-h/templ@latest`)
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+A small, self-container, complete reproduction, uploaded to a GitHub repo, containing the minimum amount of files required to reproduce the behaviour, along with a list of commands that need to be run. Keep it simple.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots or screen captures to help explain your problem.
+
+**Logs**
+If the issue is related to IDE support, run through the LSP troubleshooting section at https://templ.guide/commands-and-tools/ide-support/#troubleshooting-1 and include logs from templ
+
+**`templ info` output**
+Run `templ info` and include the output.
+
+**Desktop (please complete the following information):**
+ - OS: [e.g. MacOS, Linux, Windows, WSL]
+ - templ CLI version (`templ version`)
+- Go version (`go version`)
+- `gopls` version (`gopls version`)
+
+**Additional context**
+Add any other context about the problem here.

templ/.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@v16
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+
+      - name: Test
+        run: nix develop --command xc test-cover
+
+      - name: Copy coverage.out to temp
+        run: cp coverage.out $RUNNER_TEMP
+      
+      - name: Update coverage report
+        uses: ncruces/go-coverage-report@57ac6f0f19874f7afbab596105154f08004f482e
+        with:
+          coverage-file: ${{ runner.temp }}/coverage.out
+          report: 'true'
+          chart: 'true'
+          reuse-go: 'true'
+        if: |
+          github.event_name == 'push'  
+
+      - name: Build
+        run: nix build
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@v16
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+
+      - name: Lint
+        run: nix develop --command xc lint
+
+  ensure-generated:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@v16
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+
+      - name: Generate
+        run: nix develop --command xc ensure-generated
+
+  ensure-fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@v16
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+
+      - name: Fmt
+        run: nix develop --command xc fmt
+
+      - name: Ensure clean
+        run: git diff --exit-code

templ/.github/workflows/docs.yaml

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+name: Deploy Docs
+
+on:
+  release:
+    types: [published]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  build-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+          fetch-depth: 0
+      - name: Setup Pages
+        id: pages
+        uses: actions/configure-pages@v5
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: npm
+          cache-dependency-path: "./docs/package-lock.json"
+      - name: Install Node.js dependencies
+        run: |
+          cd docs
+          npm ci
+      - name: Build
+        run: |
+          cd docs
+          npm run build
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: ./docs/build
+
+  deploy-docs:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build-docs
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

templ/.github/workflows/release.yml

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+name: Release
+
+on:
+  push:
+    tags:
+      - 'v*'
+  workflow_dispatch:
+
+permissions:
+   contents: write
+   packages: write
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: actions/setup-go@v5
+        with:
+          go-version: 1.22
+          cache: true
+      - uses: ko-build/setup-ko@v0.7
+      - uses: sigstore/cosign-installer@v3.7.0
+        with:
+          cosign-release: v2.2.3
+      - uses: goreleaser/goreleaser-action@v5
+        with:
+          version: v1.24.0
+          args: release --clean
+        env:
+          GITHUB_TOKEN: '${{ secrets.GITHUB_TOKEN }}'
+          COSIGN_PASSWORD: '${{ secrets.COSIGN_PASSWORD }}'
+          COSIGN_PRIVATE_KEY: '${{ secrets.COSIGN_PRIVATE_KEY }}'
+          COSIGN_PUBLIC_KEY: '${{ secrets.COSIGN_PUBLIC_KEY }}'

templ/.gitignore

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+# Output.
+cmd/templ/templ
+
+# Logs.
+cmd/templ/lspcmd/*log.txt
+
+# Go code coverage.
+coverage.out
+coverage
+
+# Mac filesystem jank.
+.DS_Store
+
+# Docusaurus.
+docs/build/
+docs/resources/_gen/
+node_modules/
+dist/
+
+# Nix artifacts.
+result
+
+# Editors
+## nvim
+.null-ls*
+
+# Go workspace.
+go.work
+
+# direnv
+.direnv
+
+# templ txt files.
+*_templ.txt

templ/.goreleaser.yaml

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+builds:
+  - env:
+      - CGO_ENABLED=0
+    dir: cmd/templ
+    mod_timestamp: '{{ .CommitTimestamp }}'
+    flags:
+      - -trimpath
+    ldflags:
+      - -s -w
+    goos:
+      - linux
+      - windows
+      - darwin
+
+checksum:
+  name_template: 'checksums.txt'
+
+signs:
+  - id: checksums
+    cmd: cosign
+    stdin: '{{ .Env.COSIGN_PASSWORD }}'
+    output: true
+    artifacts: checksum
+    args:
+      - sign-blob
+      - --yes
+      - --key
+      - env://COSIGN_PRIVATE_KEY
+      - '--output-certificate=${certificate}'
+      - '--output-signature=${signature}'
+      - '${artifact}'
+
+archives:
+  - format: tar.gz
+    name_template: >-
+      {{ .ProjectName }}_
+      {{- title .Os }}_
+      {{- if eq .Arch "amd64" }}x86_64
+      {{- else if eq .Arch "386" }}i386
+      {{- else }}{{ .Arch }}{{ end }}
+      {{- if .Arm }}v{{ .Arm }}{{ end }}
+
+kos:
+  - repository: ghcr.io/a-h/templ
+    platforms:
+    - linux/amd64
+    - linux/arm64
+    tags:
+    - latest
+    - '{{.Tag}}'
+    bare: true
+
+docker_signs:
+  - cmd: cosign
+    artifacts: all
+    output: true
+    args:
+      - sign
+      - --yes
+      - --key
+      - env://COSIGN_PRIVATE_KEY
+      - '${artifact}'
+
+snapshot:
+  name_template: "{{ incpatch .Version }}-next"
+
+changelog:
+  sort: asc
+  filters:
+    exclude:
+      - '^docs:'
+      - '^test:'

templ/.ignore

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+*_templ.go
+examples/integration-ct/static/index.js
+examples/counter/assets/css/bulma.*
+examples/counter/assets/js/htmx.min.js
+examples/counter-basic/assets/css/bulma.*
+examples/typescript/assets/index.js
+package-lock.json
+go.sum
+docs/static/llms.md

templ/.version

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+0.3.833

templ/.vscode/settings.json

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+{
+    "cSpell.words": [
+        "blockquote",
+        "fieldset",
+        "figcaption",
+        "formatstring",
+        "goexpression",
+        "keygen",
+        "strs",
+        "templ"
+    ]
+}

templ/CODE_OF_CONDUCT.md

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+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to make participation in our
+community a harassment-free experience for everyone, regardless of age, body
+size, visible or invisible disability, ethnicity, sex characteristics, gender
+identity and expression, level of experience, education, socio-economic status,
+nationality, personal appearance, race, religion, or sexual identity
+and orientation.
+
+We pledge to act and interact in ways that contribute to an open, welcoming,
+diverse, inclusive, and healthy community.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment for our
+community include:
+
+* Demonstrating empathy and kindness toward other people
+* Being respectful of differing opinions, viewpoints, and experiences
+* Giving and gracefully accepting constructive feedback
+* Accepting responsibility and apologizing to those affected by our mistakes,
+  and learning from the experience
+* Focusing on what is best not just for us as individuals, but for the
+  overall community
+
+Examples of unacceptable behavior include:
+
+* The use of sexualized language or imagery, and sexual attention or
+  advances of any kind
+* Trolling, insulting or derogatory comments, and personal or political attacks
+* Public or private harassment
+* Publishing others' private information, such as a physical or email
+  address, without their explicit permission
+* Other conduct which could reasonably be considered inappropriate in a
+  professional setting
+
+## Enforcement Responsibilities
+
+Community leaders are responsible for clarifying and enforcing our standards of
+acceptable behavior and will take appropriate and fair corrective action in
+response to any behavior that they deem inappropriate, threatening, offensive,
+or harmful.
+
+Community leaders have the right and responsibility to remove, edit, or reject
+comments, commits, code, wiki edits, issues, and other contributions that are
+not aligned to this Code of Conduct, and will communicate reasons for moderation
+decisions when appropriate.
+
+## Scope
+
+This Code of Conduct applies within all community spaces, and also applies when
+an individual is officially representing the community in public spaces.
+Examples of representing our community include using an official e-mail address,
+posting via an official social media account, or acting as an appointed
+representative at an online or offline event.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be
+reported to the community leaders responsible for enforcement at
+adrianhesketh@hushail.com.
+All complaints will be reviewed and investigated promptly and fairly.
+
+All community leaders are obligated to respect the privacy and security of the
+reporter of any incident.
+
+## Enforcement Guidelines
+
+Community leaders will follow these Community Impact Guidelines in determining
+the consequences for any action they deem in violation of this Code of Conduct:
+
+### 1. Correction
+
+**Community Impact**: Use of inappropriate language or other behavior deemed
+unprofessional or unwelcome in the community.
+
+**Consequence**: A private, written warning from community leaders, providing
+clarity around the nature of the violation and an explanation of why the
+behavior was inappropriate. A public apology may be requested.
+
+### 2. Warning
+
+**Community Impact**: A violation through a single incident or series
+of actions.
+
+**Consequence**: A warning with consequences for continued behavior. No
+interaction with the people involved, including unsolicited interaction with
+those enforcing the Code of Conduct, for a specified period of time. This
+includes avoiding interactions in community spaces as well as external channels
+like social media. Violating these terms may lead to a temporary or
+permanent ban.
+
+### 3. Temporary Ban
+
+**Community Impact**: A serious violation of community standards, including
+sustained inappropriate behavior.
+
+**Consequence**: A temporary ban from any sort of interaction or public
+communication with the community for a specified period of time. No public or
+private interaction with the people involved, including unsolicited interaction
+with those enforcing the Code of Conduct, is allowed during this period.
+Violating these terms may lead to a permanent ban.
+
+### 4. Permanent Ban
+
+**Community Impact**: Demonstrating a pattern of violation of community
+standards, including sustained inappropriate behavior,  harassment of an
+individual, or aggression toward or disparagement of classes of individuals.
+
+**Consequence**: A permanent ban from any sort of public interaction within
+the community.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage],
+version 2.0, available at
+https://www.contributor-covenant.org/version/2/0/code_of_conduct.html.
+
+Community Impact Guidelines were inspired by [Mozilla's code of conduct
+enforcement ladder](https://github.com/mozilla/diversity).
+
+[homepage]: https://www.contributor-covenant.org
+
+For answers to common questions about this code of conduct, see the FAQ at
+https://www.contributor-covenant.org/faq. Translations are available at
+https://www.contributor-covenant.org/translations.

templ/CONTRIBUTING.md

@@ -0,0 +1,244 @@
+# Contributing to templ
+
+## Vision
+
+Enable Go developers to build strongly typed, component-based HTML user interfaces with first-class developer tooling, and a short learning curve.
+
+## Come up with a design and share it
+
+Before starting work on any major pull requests or code changes, start a discussion at https://github.com/a-h/templ/discussions or raise an issue.
+
+We don't want you to spend time on a PR or feature that ultimately doesn't get merged because it doesn't fit with the project goals, or the design doesn't work for some reason.
+
+For issues, it really helps if you provide a reproduction repo, or can create a failing unit test to describe the behaviour.
+
+In designs, we need to consider:
+
+* Backwards compatibility - Not changing the public API between releases, introducing gradual deprecation - don't break people's code.
+* Correctness over time - How can we reduce the risk of defects both now, and in future releases?
+* Threat model - How could each change be used to inject vulnerabilities into web pages?
+* Go version - We target the oldest supported version of Go as per https://go.dev/doc/devel/release
+* Automatic migration - If we need to force through a change.
+* Compile time vs runtime errors - Prefer compile time.
+* Documentation - New features are only useful if people can understand the new feature, what would the documentation look like?
+* Examples - How will we demonstrate the feature?
+
+## Project structure
+
+templ is structured into a few areas:
+
+### Parser `./parser`
+
+The parser directory currently contains both v1 and v2 parsers.
+
+The v1 parser is not maintained, it's only used to migrate v1 code over to the v2 syntax.
+
+The parser is responsible for parsing templ files into an object model. The types that make up the object model are in `types.go`. Automatic formatting of the types is tested in `types_test.go`.
+
+A templ file is parsed into the `TemplateFile` struct object model.
+
+```go
+type TemplateFile struct {
+	// Header contains comments or whitespace at the top of the file.
+	Header []GoExpression
+	// Package expression.
+	Package Package
+	// Nodes in the file.
+	Nodes []TemplateFileNode
+}
+```
+
+Parsers are individually tested using two types of unit test.
+
+One test covers the successful parsing of text into an object. For example, the `HTMLCommentParser` test checks for successful patterns.
+
+```go
+func TestHTMLCommentParser(t *testing.T) {
+	var tests = []struct {
+		name     string
+		input    string
+		expected HTMLComment
+	}{
+		{
+			name:  "comment - single line",
+			input: `<!-- single line comment -->`,
+			expected: HTMLComment{
+				Contents: " single line comment ",
+			},
+		},
+		{
+			name:  "comment - no whitespace",
+			input: `<!--no whitespace between sequence open and close-->`,
+			expected: HTMLComment{
+				Contents: "no whitespace between sequence open and close",
+			},
+		},
+		{
+			name: "comment - multiline",
+			input: `<!-- multiline
+								comment
+					-->`,
+			expected: HTMLComment{
+				Contents: ` multiline
+								comment
+					`,
+			},
+		},
+		{
+			name:  "comment - with tag",
+			input: `<!-- <p class="test">tag</p> -->`,
+			expected: HTMLComment{
+				Contents: ` <p class="test">tag</p> `,
+			},
+		},
+		{
+			name:  "comments can contain tags",
+			input: `<!-- <div> hello world </div> -->`,
+			expected: HTMLComment{
+				Contents: ` <div> hello world </div> `,
+			},
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			input := parse.NewInput(tt.input)
+			result, ok, err := htmlComment.Parse(input)
+			if err != nil {
+				t.Fatalf("parser error: %v", err)
+			}
+			if !ok {
+				t.Fatalf("failed to parse at %d", input.Index())
+			}
+			if diff := cmp.Diff(tt.expected, result); diff != "" {
+				t.Errorf(diff)
+			}
+		})
+	}
+}
+```
+
+Alongside each success test, is a similar test to check that invalid syntax is detected.
+
+```go
+func TestHTMLCommentParserErrors(t *testing.T) {
+	var tests = []struct {
+		name     string
+		input    string
+		expected error
+	}{
+		{
+			name:  "unclosed HTML comment",
+			input: `<!-- unclosed HTML comment`,
+			expected: parse.Error("expected end comment literal '-->' not found",
+				parse.Position{
+					Index: 26,
+					Line:  0,
+					Col:   26,
+				}),
+		},
+		{
+			name:  "comment in comment",
+			input: `<!-- <-- other --> -->`,
+			expected: parse.Error("comment contains invalid sequence '--'", parse.Position{
+				Index: 8,
+				Line:  0,
+				Col:   8,
+			}),
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			input := parse.NewInput(tt.input)
+			_, _, err := htmlComment.Parse(input)
+			if diff := cmp.Diff(tt.expected, err); diff != "" {
+				t.Error(diff)
+			}
+		})
+	}
+}
+```
+
+### Generator
+
+The generator takes the object model and writes out Go code that produces the expected output. Any changes to Go code output by templ are made in this area.
+
+Testing of the generator is carried out by creating a templ file, and a matching expected output file.
+
+For example, `./generator/test-a-href` contains a templ file of:
+
+```templ
+package testahref
+
+templ render() {
+	<a href="javascript:alert(&#39;unaffected&#39;);">Ignored</a>
+	<a href={ templ.URL("javascript:alert('should be sanitized')") }>Sanitized</a>
+	<a href={ templ.SafeURL("javascript:alert('should not be sanitized')") }>Unsanitized</a>
+}
+```
+
+It also contains an expected output file.
+
+```html
+<a href="javascript:alert(&#39;unaffected&#39;);">Ignored</a>
+<a href="about:invalid#TemplFailedSanitizationURL">Sanitized</a>
+<a href="javascript:alert(&#39;should not be sanitized&#39;)">Unsanitized</a>
+```
+
+These tests contribute towards the code coverage metrics by building an instrumented test CLI program. See the `test-cover` task in the `README.md` file.
+
+### CLI
+
+The command line interface for templ is used to generate Go code from templ files, format templ files, and run the LSP.
+
+The code for this is at `./cmd/templ`.
+
+Testing of the templ command line is done with unit tests to check the argument parsing.
+
+The `templ generate` command is tested by generating templ files in the project, and testing that the expected output HTML is present.
+
+### Runtime
+
+The runtime is used by generated code, and by template authors, to serve template content over HTTP, and to carry out various operations.
+
+It is in the root directory of the project at `./runtime.go`. The runtime is unit tested, as well as being tested as part of the `generate` tests.
+
+### LSP
+
+The LSP is structured within the command line interface, and proxies commands through to the `gopls` LSP.
+
+### Docs
+
+The docs are a Docusaurus project at `./docs`.
+
+## Coding
+
+### Build tasks
+
+templ uses the `xc` task runner - https://github.com/joerdav/xc
+
+If you run `xc` you can get see a list of the development tasks that can be run, or you can read the `README.md` file and see the `Tasks` section.
+
+The most useful tasks for local development are:
+
+* `install-snapshot` - this builds the templ CLI and installs it into `~/bin`. Ensure that this is in your path.
+* `test` - this regenerates all templates, and runs the unit tests.
+* `fmt` - run the `gofmt` tool to format all Go code.
+* `lint` - run the same linting as run in the CI process.
+* `docs-run` - run the Docusaurus documentation site.
+
+### Commit messages
+
+The project using https://www.conventionalcommits.org/en/v1.0.0/
+
+Examples:
+
+* `feat: support Go comments in templates, fixes #234"`
+
+### Coding style
+
+* Reduce nesting - i.e. prefer early returns over an `else` block, as per https://danp.net/posts/reducing-go-nesting/ or https://go.dev/doc/effective_go#if
+* Use line breaks to separate "paragraphs" of code - don't use line breaks in between lines, or at the start/end of functions etc.
+* Use the `fmt` and `lint` build tasks to format and lint your code before submitting a PR.
+

templ/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2021 Adrian Hesketh
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

templ/README.md

@@ -0,0 +1,184 @@
+![templ](https://github.com/a-h/templ/raw/main/templ.png)
+
+## An HTML templating language for Go that has great developer tooling.
+
+![templ](ide-demo.gif)
+
+
+## Documentation
+
+See user documentation at https://templ.guide
+
+<p align="center">
+<a href="https://pkg.go.dev/github.com/a-h/templ"><img src="https://pkg.go.dev/badge/github.com/a-h/templ.svg" alt="Go Reference" /></a>
+<a href="https://xcfile.dev"><img src="https://xcfile.dev/badge.svg" alt="xc compatible" /></a>
+<a href="https://raw.githack.com/wiki/a-h/templ/coverage.html"><img src="https://github.com/a-h/templ/wiki/coverage.svg" alt="Go Coverage" /></a>
+<a href="https://goreportcard.com/report/github.com/a-h/templ"><img src="https://goreportcard.com/badge/github.com/a-h/templ" alt="Go Report Card" /></a>
+</p>
+
+## Tasks
+
+### build
+
+Build a local version.
+
+```sh
+go run ./get-version > .version
+cd cmd/templ
+go build
+```
+
+### install-snapshot
+
+Build and install current version.
+
+```sh
+# Remove templ from the non-standard ~/bin/templ path
+# that this command previously used.
+rm -f ~/bin/templ
+# Clear LSP logs.
+rm -f cmd/templ/lspcmd/*.txt
+# Update version.
+go run ./get-version > .version
+# Install to $GOPATH/bin or $HOME/go/bin
+cd cmd/templ && go install
+```
+
+### build-snapshot
+
+Use goreleaser to build the command line binary using goreleaser.
+
+```sh
+goreleaser build --snapshot --clean
+```
+
+### generate
+
+Run templ generate using local version.
+
+```sh
+go run ./cmd/templ generate -include-version=false
+```
+
+### test
+
+Run Go tests.
+
+```sh
+go run ./get-version > .version
+go run ./cmd/templ generate -include-version=false
+go test ./...
+```
+
+### test-short
+
+Run Go tests.
+
+```sh
+go run ./get-version > .version
+go run ./cmd/templ generate -include-version=false
+go test ./... -short
+```
+
+### test-cover
+
+Run Go tests.
+
+```sh
+# Create test profile directories.
+mkdir -p coverage/fmt
+mkdir -p coverage/generate
+mkdir -p coverage/version
+mkdir -p coverage/unit
+# Build the test binary.
+go build -cover -o ./coverage/templ-cover ./cmd/templ
+# Run the covered generate command.
+GOCOVERDIR=coverage/fmt ./coverage/templ-cover fmt .
+GOCOVERDIR=coverage/generate ./coverage/templ-cover generate -include-version=false
+GOCOVERDIR=coverage/version ./coverage/templ-cover version
+# Run the unit tests.
+go test -cover ./... -coverpkg ./... -args -test.gocoverdir="$PWD/coverage/unit"
+# Display the combined percentage.
+go tool covdata percent -i=./coverage/fmt,./coverage/generate,./coverage/version,./coverage/unit
+# Generate a text coverage profile for tooling to use.
+go tool covdata textfmt -i=./coverage/fmt,./coverage/generate,./coverage/version,./coverage/unit -o coverage.out
+# Print total
+go tool cover -func coverage.out | grep total
+```
+
+### test-cover-watch
+
+```sh
+gotestsum --watch -- -coverprofile=coverage.out
+```
+
+### test-fuzz
+
+```sh
+./parser/v2/fuzz.sh
+./parser/v2/goexpression/fuzz.sh
+```
+
+### benchmark
+
+Run benchmarks.
+
+```sh
+go run ./cmd/templ generate -include-version=false && go test ./... -bench=. -benchmem
+```
+
+### fmt
+
+Format all Go and templ code.
+
+```sh
+gofmt -s -w .
+go run ./cmd/templ fmt .
+```
+
+### lint
+
+Run the lint operations that are run as part of the CI.
+
+```sh
+golangci-lint run --verbose
+```
+
+### ensure-generated
+
+Ensure that templ files have been generated with the local version of templ, and that those files have been added to git.
+
+Requires: generate
+
+```sh
+git diff --exit-code
+```
+
+### push-release-tag
+
+Push a semantic version number to GitHub to trigger the release process.
+
+```sh
+./push-tag.sh
+```
+
+### docs-run
+
+Run the development server.
+
+Directory: docs
+
+```sh
+npm run start
+```
+
+### docs-build
+
+Build production docs site.
+
+Directory: docs
+
+```sh
+npm run build
+```
+

templ/SECURITY.md

@@ -0,0 +1,9 @@
+# Security Policy
+
+## Supported Versions
+
+The latest version of templ is supported.
+
+## Reporting a Vulnerability
+
+Use the "Security" tab in GitHub and fill out the "Report a vulnerability" form.

templ/benchmarks/react/.gitignore

@@ -0,0 +1,3 @@
+index.js
+node_modules
+

templ/benchmarks/react/README.md

@@ -0,0 +1,23 @@
+# React benchmark
+
+## Tasks
+
+### install
+
+```
+npm i
+```
+
+### build
+
+```sh
+npm run build
+```
+
+### run
+
+requires: build
+
+```sh
+npm start
+```

templ/benchmarks/react/package-lock.json

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+        "esbuild": "0.19.2",
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+        "react": "^18.2.0",
+        "react-dom": "^18.2.0"
+      }
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+}

templ/benchmarks/react/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "react-benchmark",
+  "version": "1.0.0",
+  "description": "",
+  "main": "./src/index.jsx",
+  "scripts": {
+    "build": "esbuild ./src/index.jsx --bundle --outfile=index.js",
+    "start": "node index.js"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "benchmark": "^2.1.4",
+    "esbuild": "0.19.2",
+    "microtime": "^3.0.0",
+    "react": "^18.2.0",
+    "react-dom": "^18.2.0"
+  }
+}

templ/benchmarks/react/src/index.jsx

@@ -0,0 +1,34 @@
+import * as React from 'react'
+import * as Server from 'react-dom/server'
+import Benchmark from 'benchmark';
+
+const component = (p) =>
+        <div>
+                <h1>{p.Name}</h1>
+                <div style={{ fontFamily: "sans-serif" }} id="test" data-contents="something with &#34;quotes&#34; and a &lt;tag&gt;">
+                        <div>email:<a href="mailto: luiz@example.com">luiz@example.com</a></div>
+                </div>
+                <hr noshade /><hr optionA optionB optionC="other" /><hr noshade />
+        </div>;
+
+const p = {
+        Name: "Luiz Bonfa",
+        Email: "luiz@example.com",
+};
+
+// Benchmark.
+// Outputs...
+// Render test x 114,131 ops/sec ±0.27% (97 runs sampled)
+// There are 1,000,000,000 nanoseconds in a second.
+// 1,000,000,000ns / 114,131 ops = 8,757.5 ns per operation.
+// The templ equivalent is 340 ns per operation.
+const suite = new Benchmark.Suite;
+
+const test = suite.add('Render test',
+        () => Server.renderToString(component(p)))
+
+test.on('cycle', (event) => {
+        console.log(String(event.target));
+});
+
+test.run();

templ/benchmarks/templ/README.md

@@ -0,0 +1,27 @@
+# templ benchmark
+
+Used to test code generation strategies for improvements to render time.
+
+## Tasks
+
+### run
+
+```
+go test -bench .
+```
+
+## Results as of 2023-08-17
+
+```
+go test -bench .
+goos: darwin
+goarch: arm64
+pkg: github.com/a-h/templ/benchmarks/templ
+BenchmarkTempl-10                3291883               369.1 ns/op           536 B/op          6 allocs/op
+BenchmarkGoTemplate-10            481052              2475 ns/op            1400 B/op         38 allocs/op
+BenchmarkIOWriteString-10       20353198                56.64 ns/op          320 B/op          1 allocs/op
+PASS
+ok      github.com/a-h/templ/benchmarks/templ   4.650s
+```
+
+React comes in at 1,000,000,000ns / 114,131 ops/s = 8,757.5 ns per operation.

templ/benchmarks/templ/data.go

@@ -0,0 +1,6 @@
+package testhtml
+
+type Person struct {
+	Name  string
+	Email string
+}

templ/benchmarks/templ/render_test.go

@@ -0,0 +1,87 @@
+package testhtml
+
+import (
+	"context"
+	"html/template"
+	"io"
+	"strings"
+	"testing"
+
+	_ "embed"
+
+	"github.com/a-h/templ/parser/v2"
+)
+
+func BenchmarkTemplRender(b *testing.B) {
+	b.ReportAllocs()
+	t := Render(Person{
+		Name:  "Luiz Bonfa",
+		Email: "luiz@example.com",
+	})
+
+	w := new(strings.Builder)
+	for i := 0; i < b.N; i++ {
+		err := t.Render(context.Background(), w)
+		if err != nil {
+			b.Errorf("failed to render: %v", err)
+		}
+		w.Reset()
+	}
+}
+
+//go:embed template.templ
+var parserBenchmarkTemplate string
+
+func BenchmarkTemplParser(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		tf, err := parser.ParseString(parserBenchmarkTemplate)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if tf.Package.Expression.Value == "" {
+			b.Fatal("unexpected nil template")
+		}
+	}
+}
+
+var goTemplate = template.Must(template.New("example").Parse(`<div>
+	<h1>{{.Name}}</h1>
+	<div style="font-family: &#39;sans-serif&#39;" id="test" data-contents="something with &#34;quotes&#34; and a &lt;tag&gt;">
+		<div>
+			email:<a href="mailto: {{.Email}}">{{.Email}}</a></div>
+		</div>
+	</div>
+	<hr noshade>
+	<hr optionA optionB optionC="other">
+	<hr noshade>
+`))
+
+func BenchmarkGoTemplateRender(b *testing.B) {
+	w := new(strings.Builder)
+	person := Person{
+		Name:  "Luiz Bonfa",
+		Email: "luiz@exapmle.com",
+	}
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		err := goTemplate.Execute(w, person)
+		if err != nil {
+			b.Errorf("failed to render: %v", err)
+		}
+		w.Reset()
+	}
+}
+
+const html = `<div><h1>Luiz Bonfa</h1><div style="font-family: &#39;sans-serif&#39;" id="test" data-contents="something with &#34;quotes&#34; and a &lt;tag&gt;"><div>email:<a href="mailto: luiz@example.com">luiz@example.com</a></div></div></div><hr noshade><hr optionA optionB optionC="other"><hr noshade>`
+
+func BenchmarkIOWriteString(b *testing.B) {
+	b.ReportAllocs()
+	w := new(strings.Builder)
+	for i := 0; i < b.N; i++ {
+		_, err := io.WriteString(w, html)
+		if err != nil {
+			b.Errorf("failed to render: %v", err)
+		}
+		w.Reset()
+	}
+}

templ/benchmarks/templ/template.templ

@@ -0,0 +1,13 @@
+package testhtml
+
+templ Render(p Person) {
+	<div>
+		<h1>{ p.Name }</h1>
+		<div style="font-family: 'sans-serif'" id="test" data-contents={ `something with "quotes" and a <tag>` }>
+			<div>email:<a href={ templ.URL("mailto: " + p.Email) }>{ p.Email }</a></div>
+		</div>
+	</div>
+	<hr noshade?={ true }/>
+	<hr optionA optionB?={ true } optionC="other" optionD?={ false }/>
+	<hr noshade/>
+}

templ/benchmarks/templ/template_templ.go

@@ -0,0 +1,118 @@
+// Code generated by templ - DO NOT EDIT.
+
+// templ: version: v0.3.833
+package testhtml
+
+//lint:file-ignore SA4006 This context is only used if a nested component is present.
+
+import "github.com/a-h/templ"
+import templruntime "github.com/a-h/templ/runtime"
+
+func Render(p Person) templ.Component {
+	return templruntime.GeneratedTemplate(func(templ_7745c5c3_Input templruntime.GeneratedComponentInput) (templ_7745c5c3_Err error) {
+		templ_7745c5c3_W, ctx := templ_7745c5c3_Input.Writer, templ_7745c5c3_Input.Context
+		if templ_7745c5c3_CtxErr := ctx.Err(); templ_7745c5c3_CtxErr != nil {
+			return templ_7745c5c3_CtxErr
+		}
+		templ_7745c5c3_Buffer, templ_7745c5c3_IsBuffer := templruntime.GetBuffer(templ_7745c5c3_W)
+		if !templ_7745c5c3_IsBuffer {
+			defer func() {
+				templ_7745c5c3_BufErr := templruntime.ReleaseBuffer(templ_7745c5c3_Buffer)
+				if templ_7745c5c3_Err == nil {
+					templ_7745c5c3_Err = templ_7745c5c3_BufErr
+				}
+			}()
+		}
+		ctx = templ.InitializeContext(ctx)
+		templ_7745c5c3_Var1 := templ.GetChildren(ctx)
+		if templ_7745c5c3_Var1 == nil {
+			templ_7745c5c3_Var1 = templ.NopComponent
+		}
+		ctx = templ.ClearChildren(ctx)
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 1, "<div><h1>")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		var templ_7745c5c3_Var2 string
+		templ_7745c5c3_Var2, templ_7745c5c3_Err = templ.JoinStringErrs(p.Name)
+		if templ_7745c5c3_Err != nil {
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templ/benchmarks/templ/template.templ`, Line: 5, Col: 14}
+		}
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var2))
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 2, "</h1><div style=\"font-family: &#39;sans-serif&#39;\" id=\"test\" data-contents=\"")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		var templ_7745c5c3_Var3 string
+		templ_7745c5c3_Var3, templ_7745c5c3_Err = templ.JoinStringErrs(`something with "quotes" and a <tag>`)
+		if templ_7745c5c3_Err != nil {
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templ/benchmarks/templ/template.templ`, Line: 6, Col: 104}
+		}
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var3))
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 3, "\"><div>email:<a href=\"")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		var templ_7745c5c3_Var4 templ.SafeURL = templ.URL("mailto: " + p.Email)
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(string(templ_7745c5c3_Var4)))
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 4, "\">")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		var templ_7745c5c3_Var5 string
+		templ_7745c5c3_Var5, templ_7745c5c3_Err = templ.JoinStringErrs(p.Email)
+		if templ_7745c5c3_Err != nil {
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templ/benchmarks/templ/template.templ`, Line: 7, Col: 67}
+		}
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var5))
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 5, "</a></div></div></div><hr")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		if true {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 6, " noshade")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 7, "><hr optionA")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		if true {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 8, " optionB")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 9, " optionC=\"other\"")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		if false {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 10, " optionD")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 11, "><hr noshade>")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		return nil
+	})
+}
+
+var _ = templruntime.GeneratedTemplate

templ/cfg/cfg.go

@@ -0,0 +1,20 @@
+// This package is inspired by the GOEXPERIMENT approach of allowing feature flags for experimenting with breaking changes.
+package cfg
+
+import (
+	"os"
+	"strings"
+)
+
+type Flags struct{}
+
+var Experiment = parse()
+
+func parse() *Flags {
+	m := map[string]bool{}
+	for _, f := range strings.Split(os.Getenv("TEMPL_EXPERIMENT"), ",") {
+		m[strings.ToLower(f)] = true
+	}
+
+	return &Flags{}
+}

templ/cmd/templ/fmtcmd/main.go

@@ -0,0 +1,166 @@
+package fmtcmd
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"runtime"
+	"sync"
+	"time"
+
+	"github.com/a-h/templ/cmd/templ/imports"
+	"github.com/a-h/templ/cmd/templ/processor"
+	parser "github.com/a-h/templ/parser/v2"
+	"github.com/natefinch/atomic"
+)
+
+type Arguments struct {
+	FailIfChanged bool
+	ToStdout      bool
+	StdinFilepath string
+	Files         []string
+	WorkerCount   int
+}
+
+func Run(log *slog.Logger, stdin io.Reader, stdout io.Writer, args Arguments) (err error) {
+	// If no files are provided, read from stdin and write to stdout.
+	if len(args.Files) == 0 {
+		out, _ := format(writeToWriter(stdout), readFromReader(stdin, args.StdinFilepath), true)
+		return out
+	}
+	process := func(fileName string) (error, bool) {
+		read := readFromFile(fileName)
+		write := writeToFile
+		if args.ToStdout {
+			write = writeToWriter(stdout)
+		}
+		writeIfUnchanged := args.ToStdout
+		return format(write, read, writeIfUnchanged)
+	}
+	dir := args.Files[0]
+	return NewFormatter(log, dir, process, args.WorkerCount, args.FailIfChanged).Run()
+}
+
+type Formatter struct {
+	Log          *slog.Logger
+	Dir          string
+	Process      func(fileName string) (error, bool)
+	WorkerCount  int
+	FailIfChange bool
+}
+
+func NewFormatter(log *slog.Logger, dir string, process func(fileName string) (error, bool), workerCount int, failIfChange bool) *Formatter {
+	f := &Formatter{
+		Log:          log,
+		Dir:          dir,
+		Process:      process,
+		WorkerCount:  workerCount,
+		FailIfChange: failIfChange,
+	}
+	if f.WorkerCount == 0 {
+		f.WorkerCount = runtime.NumCPU()
+	}
+	return f
+}
+
+func (f *Formatter) Run() (err error) {
+	changesMade := 0
+	start := time.Now()
+	results := make(chan processor.Result)
+	f.Log.Debug("Walking directory", slog.String("path", f.Dir))
+	go processor.Process(f.Dir, f.Process, f.WorkerCount, results)
+	var successCount, errorCount int
+	for r := range results {
+		if r.ChangesMade {
+			changesMade += 1
+		}
+		if r.Error != nil {
+			f.Log.Error(r.FileName, slog.Any("error", r.Error))
+			errorCount++
+			continue
+		}
+		f.Log.Debug(r.FileName, slog.Duration("duration", r.Duration))
+		successCount++
+	}
+
+	if f.FailIfChange && changesMade > 0 {
+		f.Log.Error("Templates were valid but not properly formatted", slog.Int("count", successCount+errorCount), slog.Int("changed", changesMade), slog.Int("errors", errorCount), slog.Duration("duration", time.Since(start)))
+		return fmt.Errorf("templates were not formatted properly")
+	}
+
+	f.Log.Info("Format Complete", slog.Int("count", successCount+errorCount), slog.Int("errors", errorCount), slog.Int("changed", changesMade), slog.Duration("duration", time.Since(start)))
+
+	if errorCount > 0 {
+		return fmt.Errorf("formatting failed")
+	}
+
+	return
+}
+
+type reader func() (fileName, src string, err error)
+
+func readFromReader(r io.Reader, stdinFilepath string) func() (fileName, src string, err error) {
+	return func() (fileName, src string, err error) {
+		b, err := io.ReadAll(r)
+		if err != nil {
+			return "", "", fmt.Errorf("failed to read stdin: %w", err)
+		}
+		return stdinFilepath, string(b), nil
+	}
+}
+
+func readFromFile(name string) reader {
+	return func() (fileName, src string, err error) {
+		b, err := os.ReadFile(name)
+		if err != nil {
+			return "", "", fmt.Errorf("failed to read file %q: %w", fileName, err)
+		}
+		return name, string(b), nil
+	}
+}
+
+type writer func(fileName, tgt string) error
+
+var mu sync.Mutex
+
+func writeToWriter(w io.Writer) func(fileName, tgt string) error {
+	return func(fileName, tgt string) error {
+		mu.Lock()
+		defer mu.Unlock()
+		_, err := w.Write([]byte(tgt))
+		return err
+	}
+}
+
+func writeToFile(fileName, tgt string) error {
+	return atomic.WriteFile(fileName, bytes.NewBufferString(tgt))
+}
+
+func format(write writer, read reader, writeIfUnchanged bool) (err error, fileChanged bool) {
+	fileName, src, err := read()
+	if err != nil {
+		return err, false
+	}
+	t, err := parser.ParseString(src)
+	if err != nil {
+		return err, false
+	}
+	t.Filepath = fileName
+	t, err = imports.Process(t)
+	if err != nil {
+		return err, false
+	}
+	w := new(bytes.Buffer)
+	if err = t.Write(w); err != nil {
+		return fmt.Errorf("formatting error: %w", err), false
+	}
+
+	fileChanged = (src != w.String())
+
+	if !writeIfUnchanged && !fileChanged {
+		return nil, fileChanged
+	}
+	return write(fileName, w.String()), fileChanged
+}

templ/cmd/templ/fmtcmd/main_test.go

@@ -0,0 +1,163 @@
+package fmtcmd
+
+import (
+	_ "embed"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"golang.org/x/tools/txtar"
+)
+
+//go:embed testdata.txtar
+var testDataTxTar []byte
+
+type testProject struct {
+	dir       string
+	cleanup   func()
+	testFiles map[string]testFile
+}
+
+type testFile struct {
+	name            string
+	input, expected string
+}
+
+func setupProjectDir() (tp testProject, err error) {
+	tp.dir, err = os.MkdirTemp("", "fmtcmd_test_*")
+	if err != nil {
+		return tp, fmt.Errorf("failed to make test dir: %w", err)
+	}
+	tp.testFiles = make(map[string]testFile)
+	testData := txtar.Parse(testDataTxTar)
+	for i := 0; i < len(testData.Files); i += 2 {
+		file := testData.Files[i]
+		err = os.WriteFile(filepath.Join(tp.dir, file.Name), file.Data, 0660)
+		if err != nil {
+			return tp, fmt.Errorf("failed to write file: %w", err)
+		}
+		tp.testFiles[file.Name] = testFile{
+			name:     filepath.Join(tp.dir, file.Name),
+			input:    string(file.Data),
+			expected: string(testData.Files[i+1].Data),
+		}
+	}
+	tp.cleanup = func() {
+		os.RemoveAll(tp.dir)
+	}
+	return tp, nil
+}
+
+func TestFormat(t *testing.T) {
+	log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+	t.Run("can format a single file from stdin to stdout", func(t *testing.T) {
+		tp, err := setupProjectDir()
+		if err != nil {
+			t.Fatalf("failed to setup project dir: %v", err)
+		}
+		defer tp.cleanup()
+		stdin := strings.NewReader(tp.testFiles["a.templ"].input)
+		stdout := new(strings.Builder)
+		if err = Run(log, stdin, stdout, Arguments{
+			ToStdout: true,
+		}); err != nil {
+			t.Fatalf("failed to run format command: %v", err)
+		}
+		if diff := cmp.Diff(tp.testFiles["a.templ"].expected, stdout.String()); diff != "" {
+			t.Error(diff)
+		}
+	})
+	t.Run("can process a single file to stdout", func(t *testing.T) {
+		tp, err := setupProjectDir()
+		if err != nil {
+			t.Fatalf("failed to setup project dir: %v", err)
+		}
+		defer tp.cleanup()
+		stdout := new(strings.Builder)
+		if err = Run(log, nil, stdout, Arguments{
+			ToStdout: true,
+			Files: []string{
+				tp.testFiles["a.templ"].name,
+			},
+			FailIfChanged: false,
+		}); err != nil {
+			t.Fatalf("failed to run format command: %v", err)
+		}
+		if diff := cmp.Diff(tp.testFiles["a.templ"].expected, stdout.String()); diff != "" {
+			t.Error(diff)
+		}
+	})
+	t.Run("can process a single file in place", func(t *testing.T) {
+		tp, err := setupProjectDir()
+		if err != nil {
+			t.Fatalf("failed to setup project dir: %v", err)
+		}
+		defer tp.cleanup()
+		if err = Run(log, nil, nil, Arguments{
+			Files: []string{
+				tp.testFiles["a.templ"].name,
+			},
+			FailIfChanged: false,
+		}); err != nil {
+			t.Fatalf("failed to run format command: %v", err)
+		}
+		data, err := os.ReadFile(tp.testFiles["a.templ"].name)
+		if err != nil {
+			t.Fatalf("failed to read file: %v", err)
+		}
+		if diff := cmp.Diff(tp.testFiles["a.templ"].expected, string(data)); diff != "" {
+			t.Error(diff)
+		}
+	})
+
+	t.Run("fails when fail flag used and change occurs", func(t *testing.T) {
+		tp, err := setupProjectDir()
+		if err != nil {
+			t.Fatalf("failed to setup project dir: %v", err)
+		}
+		defer tp.cleanup()
+		if err = Run(log, nil, nil, Arguments{
+			Files: []string{
+				tp.testFiles["a.templ"].name,
+			},
+			FailIfChanged: true,
+		}); err == nil {
+			t.Fatal("command should have exited with an error and did not")
+		}
+		data, err := os.ReadFile(tp.testFiles["a.templ"].name)
+		if err != nil {
+			t.Fatalf("failed to read file: %v", err)
+		}
+		if diff := cmp.Diff(tp.testFiles["a.templ"].expected, string(data)); diff != "" {
+			t.Error(diff)
+		}
+	})
+
+	t.Run("passes when fail flag used and no change occurs", func(t *testing.T) {
+		tp, err := setupProjectDir()
+		if err != nil {
+			t.Fatalf("failed to setup project dir: %v", err)
+		}
+		defer tp.cleanup()
+		if err = Run(log, nil, nil, Arguments{
+			Files: []string{
+				tp.testFiles["c.templ"].name,
+			},
+			FailIfChanged: true,
+		}); err != nil {
+			t.Fatalf("failed to run format command: %v", err)
+		}
+		data, err := os.ReadFile(tp.testFiles["c.templ"].name)
+		if err != nil {
+			t.Fatalf("failed to read file: %v", err)
+		}
+		if diff := cmp.Diff(tp.testFiles["c.templ"].expected, string(data)); diff != "" {
+			t.Error(diff)
+		}
+	})
+}

templ/cmd/templ/fmtcmd/testdata.txtar

@@ -0,0 +1,54 @@
+-- a.templ --
+package test
+
+templ a() {
+	<div><p class={templ.Class("mapped")}>A
+</p></div>
+}
+-- a.templ --
+package test
+
+templ a() {
+	<div>
+		<p class={ templ.Class("mapped") }>
+			A
+		</p>
+	</div>
+}
+-- b.templ --
+package test
+
+templ b() {
+	<div><p>B
+</p></div>
+}
+-- b.templ --
+package test
+
+templ b() {
+	<div>
+		<p>
+			B
+		</p>
+	</div>
+}
+-- c.templ --
+package test
+
+templ c() {
+	<div>
+		<p>
+			C
+		</p>
+	</div>
+}
+-- c.templ --
+package test
+
+templ c() {
+	<div>
+		<p>
+			C
+		</p>
+	</div>
+}

templ/cmd/templ/generatecmd/cmd.go

@@ -0,0 +1,403 @@
+package generatecmd
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/url"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/a-h/templ"
+	"github.com/a-h/templ/cmd/templ/generatecmd/modcheck"
+	"github.com/a-h/templ/cmd/templ/generatecmd/proxy"
+	"github.com/a-h/templ/cmd/templ/generatecmd/run"
+	"github.com/a-h/templ/cmd/templ/generatecmd/watcher"
+	"github.com/a-h/templ/generator"
+	"github.com/cenkalti/backoff/v4"
+	"github.com/cli/browser"
+	"github.com/fsnotify/fsnotify"
+)
+
+const defaultWatchPattern = `(.+\.go$)|(.+\.templ$)|(.+_templ\.txt$)`
+
+func NewGenerate(log *slog.Logger, args Arguments) (g *Generate, err error) {
+	g = &Generate{
+		Log:  log,
+		Args: &args,
+	}
+	if g.Args.WorkerCount == 0 {
+		g.Args.WorkerCount = runtime.NumCPU()
+	}
+	if g.Args.WatchPattern == "" {
+		g.Args.WatchPattern = defaultWatchPattern
+	}
+	g.WatchPattern, err = regexp.Compile(g.Args.WatchPattern)
+	if err != nil {
+		return nil, fmt.Errorf("failed to compile watch pattern %q: %w", g.Args.WatchPattern, err)
+	}
+	return g, nil
+}
+
+type Generate struct {
+	Log          *slog.Logger
+	Args         *Arguments
+	WatchPattern *regexp.Regexp
+}
+
+type GenerationEvent struct {
+	Event       fsnotify.Event
+	Updated     bool
+	GoUpdated   bool
+	TextUpdated bool
+}
+
+func (cmd Generate) Run(ctx context.Context) (err error) {
+	if cmd.Args.NotifyProxy {
+		return proxy.NotifyProxy(cmd.Args.ProxyBind, cmd.Args.ProxyPort)
+	}
+	if cmd.Args.Watch && cmd.Args.FileName != "" {
+		return fmt.Errorf("cannot watch a single file, remove the -f or -watch flag")
+	}
+	writingToWriter := cmd.Args.FileWriter != nil
+	if cmd.Args.FileName == "" && writingToWriter {
+		return fmt.Errorf("only a single file can be output to stdout, add the -f flag to specify the file to generate code for")
+	}
+	// Default to writing to files.
+	if cmd.Args.FileWriter == nil {
+		cmd.Args.FileWriter = FileWriter
+	}
+	if cmd.Args.PPROFPort > 0 {
+		go func() {
+			_ = http.ListenAndServe(fmt.Sprintf("localhost:%d", cmd.Args.PPROFPort), nil)
+		}()
+	}
+
+	// Use absolute path.
+	if !path.IsAbs(cmd.Args.Path) {
+		cmd.Args.Path, err = filepath.Abs(cmd.Args.Path)
+		if err != nil {
+			return fmt.Errorf("failed to get absolute path: %w", err)
+		}
+	}
+
+	// Configure generator.
+	var opts []generator.GenerateOpt
+	if cmd.Args.IncludeVersion {
+		opts = append(opts, generator.WithVersion(templ.Version()))
+	}
+	if cmd.Args.IncludeTimestamp {
+		opts = append(opts, generator.WithTimestamp(time.Now()))
+	}
+
+	// Check the version of the templ module.
+	if err := modcheck.Check(cmd.Args.Path); err != nil {
+		cmd.Log.Warn("templ version check: " + err.Error())
+	}
+
+	fseh := NewFSEventHandler(
+		cmd.Log,
+		cmd.Args.Path,
+		cmd.Args.Watch,
+		opts,
+		cmd.Args.GenerateSourceMapVisualisations,
+		cmd.Args.KeepOrphanedFiles,
+		cmd.Args.FileWriter,
+		cmd.Args.Lazy,
+	)
+
+	// If we're processing a single file, don't bother setting up the channels/multithreaing.
+	if cmd.Args.FileName != "" {
+		_, err = fseh.HandleEvent(ctx, fsnotify.Event{
+			Name: cmd.Args.FileName,
+			Op:   fsnotify.Create,
+		})
+		return err
+	}
+
+	// Start timer.
+	start := time.Now()
+
+	// Create channels:
+	// For the initial filesystem walk and subsequent (optional) fsnotify events.
+	events := make(chan fsnotify.Event)
+	// Count of events currently being processed by the event handler.
+	var eventsWG sync.WaitGroup
+	// Used to check that the event handler has completed.
+	var eventHandlerWG sync.WaitGroup
+	// For errs from the watcher.
+	errs := make(chan error)
+	// Tracks whether errors occurred during the generation process.
+	var errorCount atomic.Int64
+	// For triggering actions after generation has completed.
+	postGeneration := make(chan *GenerationEvent, 256)
+	// Used to check that the post-generation handler has completed.
+	var postGenerationWG sync.WaitGroup
+	var postGenerationEventsWG sync.WaitGroup
+
+	// Waitgroup for the push process.
+	var pushHandlerWG sync.WaitGroup
+
+	// Start process to push events into the channel.
+	pushHandlerWG.Add(1)
+	go func() {
+		defer pushHandlerWG.Done()
+		defer close(events)
+		cmd.Log.Debug(
+			"Walking directory",
+			slog.String("path", cmd.Args.Path),
+			slog.Bool("devMode", cmd.Args.Watch),
+		)
+		if err := watcher.WalkFiles(ctx, cmd.Args.Path, cmd.WatchPattern, events); err != nil {
+			cmd.Log.Error("WalkFiles failed, exiting", slog.Any("error", err))
+			errs <- FatalError{Err: fmt.Errorf("failed to walk files: %w", err)}
+			return
+		}
+		if !cmd.Args.Watch {
+			cmd.Log.Debug("Dev mode not enabled, process can finish early")
+			return
+		}
+		cmd.Log.Info("Watching files")
+		rw, err := watcher.Recursive(ctx, cmd.Args.Path, cmd.WatchPattern, events, errs)
+		if err != nil {
+			cmd.Log.Error("Recursive watcher setup failed, exiting", slog.Any("error", err))
+			errs <- FatalError{Err: fmt.Errorf("failed to setup recursive watcher: %w", err)}
+			return
+		}
+		cmd.Log.Debug("Waiting for context to be cancelled to stop watching files")
+		<-ctx.Done()
+		cmd.Log.Debug("Context cancelled, closing watcher")
+		if err := rw.Close(); err != nil {
+			cmd.Log.Error("Failed to close watcher", slog.Any("error", err))
+		}
+		cmd.Log.Debug("Waiting for events to be processed")
+		eventsWG.Wait()
+		cmd.Log.Debug(
+			"All pending events processed, waiting for pending post-generation events to complete",
+		)
+		postGenerationEventsWG.Wait()
+		cmd.Log.Debug(
+			"All post-generation events processed, deleting watch mode text files",
+			slog.Int64("errorCount", errorCount.Load()),
+		)
+
+		fileEvents := make(chan fsnotify.Event)
+		go func() {
+			if err := watcher.WalkFiles(ctx, cmd.Args.Path, cmd.WatchPattern, fileEvents); err != nil {
+				cmd.Log.Error("Post dev mode WalkFiles failed", slog.Any("error", err))
+				errs <- FatalError{Err: fmt.Errorf("failed to walk files: %w", err)}
+				return
+			}
+			close(fileEvents)
+		}()
+		for event := range fileEvents {
+			if strings.HasSuffix(event.Name, "_templ.txt") {
+				if err = os.Remove(event.Name); err != nil {
+					cmd.Log.Warn("Failed to remove watch mode text file", slog.Any("error", err))
+				}
+			}
+		}
+	}()
+
+	// Start process to handle events.
+	eventHandlerWG.Add(1)
+	sem := make(chan struct{}, cmd.Args.WorkerCount)
+	go func() {
+		defer eventHandlerWG.Done()
+		defer close(postGeneration)
+		cmd.Log.Debug("Starting event handler")
+		for event := range events {
+			eventsWG.Add(1)
+			sem <- struct{}{}
+			go func(event fsnotify.Event) {
+				cmd.Log.Debug("Processing file", slog.String("file", event.Name))
+				defer eventsWG.Done()
+				defer func() { <-sem }()
+				r, err := fseh.HandleEvent(ctx, event)
+				if err != nil {
+					errs <- err
+				}
+				if !(r.GoUpdated || r.TextUpdated) {
+					cmd.Log.Debug("File not updated", slog.String("file", event.Name))
+					return
+				}
+				e := &GenerationEvent{
+					Event:       event,
+					Updated:     r.Updated,
+					GoUpdated:   r.GoUpdated,
+					TextUpdated: r.TextUpdated,
+				}
+				cmd.Log.Debug("File updated", slog.String("file", event.Name))
+				postGeneration <- e
+			}(event)
+		}
+		// Wait for all events to be processed before closing.
+		eventsWG.Wait()
+	}()
+
+	// Start process to handle post-generation events.
+	var updates int
+	postGenerationWG.Add(1)
+	var firstPostGenerationExecuted bool
+	go func() {
+		defer close(errs)
+		defer postGenerationWG.Done()
+		cmd.Log.Debug("Starting post-generation handler")
+		timeout := time.NewTimer(time.Hour * 24 * 365)
+		var goUpdated, textUpdated bool
+		var p *proxy.Handler
+		for {
+			select {
+			case ge := <-postGeneration:
+				if ge == nil {
+					cmd.Log.Debug("Post-generation event channel closed, exiting")
+					return
+				}
+				goUpdated = goUpdated || ge.GoUpdated
+				textUpdated = textUpdated || ge.TextUpdated
+				if goUpdated || textUpdated {
+					updates++
+				}
+				// Reset timer.
+				if !timeout.Stop() {
+					<-timeout.C
+				}
+				timeout.Reset(time.Millisecond * 100)
+			case <-timeout.C:
+				if !goUpdated && !textUpdated {
+					// Nothing to process, reset timer and wait again.
+					timeout.Reset(time.Hour * 24 * 365)
+					break
+				}
+				postGenerationEventsWG.Add(1)
+				if cmd.Args.Command != "" && goUpdated {
+					cmd.Log.Debug("Executing command", slog.String("command", cmd.Args.Command))
+					if cmd.Args.Watch {
+						os.Setenv("TEMPL_DEV_MODE", "true")
+					}
+					if _, err := run.Run(ctx, cmd.Args.Path, cmd.Args.Command); err != nil {
+						cmd.Log.Error("Error executing command", slog.Any("error", err))
+					}
+				}
+				if !firstPostGenerationExecuted {
+					cmd.Log.Debug("First post-generation event received, starting proxy")
+					firstPostGenerationExecuted = true
+					p, err = cmd.StartProxy(ctx)
+					if err != nil {
+						cmd.Log.Error("Failed to start proxy", slog.Any("error", err))
+					}
+				}
+				// Send server-sent event.
+				if p != nil && (textUpdated || goUpdated) {
+					cmd.Log.Debug("Sending reload event")
+					p.SendSSE("message", "reload")
+				}
+				postGenerationEventsWG.Done()
+				// Reset timer.
+				timeout.Reset(time.Millisecond * 100)
+				textUpdated = false
+				goUpdated = false
+			}
+		}
+	}()
+
+	// Read errors.
+	for err := range errs {
+		if err == nil {
+			continue
+		}
+		if errors.Is(err, FatalError{}) {
+			cmd.Log.Debug("Fatal error, exiting")
+			return err
+		}
+		cmd.Log.Error("Error", slog.Any("error", err))
+		errorCount.Add(1)
+	}
+
+	// Wait for everything to complete.
+	cmd.Log.Debug("Waiting for push handler to complete")
+	pushHandlerWG.Wait()
+	cmd.Log.Debug("Waiting for event handler to complete")
+	eventHandlerWG.Wait()
+	cmd.Log.Debug("Waiting for post-generation handler to complete")
+	postGenerationWG.Wait()
+	if cmd.Args.Command != "" {
+		cmd.Log.Debug("Killing command", slog.String("command", cmd.Args.Command))
+		if err := run.KillAll(); err != nil {
+			cmd.Log.Error("Error killing command", slog.Any("error", err))
+		}
+	}
+
+	// Check for errors after everything has completed.
+	if errorCount.Load() > 0 {
+		return fmt.Errorf("generation completed with %d errors", errorCount.Load())
+	}
+
+	cmd.Log.Info(
+		"Complete",
+		slog.Int("updates", updates),
+		slog.Duration("duration", time.Since(start)),
+	)
+	return nil
+}
+
+func (cmd *Generate) StartProxy(ctx context.Context) (p *proxy.Handler, err error) {
+	if cmd.Args.Proxy == "" {
+		cmd.Log.Debug("No proxy URL specified, not starting proxy")
+		return nil, nil
+	}
+	var target *url.URL
+	target, err = url.Parse(cmd.Args.Proxy)
+	if err != nil {
+		return nil, FatalError{Err: fmt.Errorf("failed to parse proxy URL: %w", err)}
+	}
+	if cmd.Args.ProxyPort == 0 {
+		cmd.Args.ProxyPort = 7331
+	}
+	if cmd.Args.ProxyBind == "" {
+		cmd.Args.ProxyBind = "127.0.0.1"
+	}
+	p = proxy.New(cmd.Log, cmd.Args.ProxyBind, cmd.Args.ProxyPort, target)
+	go func() {
+		cmd.Log.Info("Proxying", slog.String("from", p.URL), slog.String("to", p.Target.String()))
+		if err := http.ListenAndServe(fmt.Sprintf("%s:%d", cmd.Args.ProxyBind, cmd.Args.ProxyPort), p); err != nil {
+			cmd.Log.Error("Proxy failed", slog.Any("error", err))
+		}
+	}()
+	if !cmd.Args.OpenBrowser {
+		cmd.Log.Debug("Not opening browser")
+		return p, nil
+	}
+	go func() {
+		cmd.Log.Debug("Waiting for proxy to be ready", slog.String("url", p.URL))
+		backoff := backoff.NewExponentialBackOff()
+		backoff.InitialInterval = time.Second
+		var client http.Client
+		client.Timeout = 1 * time.Second
+		for {
+			if _, err := client.Get(p.URL); err == nil {
+				break
+			}
+			d := backoff.NextBackOff()
+			cmd.Log.Debug(
+				"Proxy not ready, retrying",
+				slog.String("url", p.URL),
+				slog.Any("backoff", d),
+			)
+			time.Sleep(d)
+		}
+		if err := browser.OpenURL(p.URL); err != nil {
+			cmd.Log.Error("Failed to open browser", slog.Any("error", err))
+		}
+	}()
+	return p, nil
+}

templ/cmd/templ/generatecmd/eventhandler.go

@@ -0,0 +1,366 @@
+package generatecmd
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"fmt"
+	"go/format"
+	"go/scanner"
+	"go/token"
+	"io"
+	"log/slog"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/a-h/templ/cmd/templ/visualize"
+	"github.com/a-h/templ/generator"
+	"github.com/a-h/templ/parser/v2"
+	"github.com/fsnotify/fsnotify"
+)
+
+type FileWriterFunc func(name string, contents []byte) error
+
+func FileWriter(fileName string, contents []byte) error {
+	return os.WriteFile(fileName, contents, 0o644)
+}
+
+func WriterFileWriter(w io.Writer) FileWriterFunc {
+	return func(_ string, contents []byte) error {
+		_, err := w.Write(contents)
+		return err
+	}
+}
+
+func NewFSEventHandler(
+	log *slog.Logger,
+	dir string,
+	devMode bool,
+	genOpts []generator.GenerateOpt,
+	genSourceMapVis bool,
+	keepOrphanedFiles bool,
+	fileWriter FileWriterFunc,
+	lazy bool,
+) *FSEventHandler {
+	if !path.IsAbs(dir) {
+		dir, _ = filepath.Abs(dir)
+	}
+	fseh := &FSEventHandler{
+		Log:                        log,
+		dir:                        dir,
+		fileNameToLastModTime:      make(map[string]time.Time),
+		fileNameToLastModTimeMutex: &sync.Mutex{},
+		fileNameToError:            make(map[string]struct{}),
+		fileNameToErrorMutex:       &sync.Mutex{},
+		fileNameToOutput:           make(map[string]generator.GeneratorOutput),
+		fileNameToOutputMutex:      &sync.Mutex{},
+		devMode:                    devMode,
+		hashes:                     make(map[string][sha256.Size]byte),
+		hashesMutex:                &sync.Mutex{},
+		genOpts:                    genOpts,
+		genSourceMapVis:            genSourceMapVis,
+		keepOrphanedFiles:          keepOrphanedFiles,
+		writer:                     fileWriter,
+		lazy:                       lazy,
+	}
+	return fseh
+}
+
+type FSEventHandler struct {
+	Log *slog.Logger
+	// dir is the root directory being processed.
+	dir                        string
+	fileNameToLastModTime      map[string]time.Time
+	fileNameToLastModTimeMutex *sync.Mutex
+	fileNameToError            map[string]struct{}
+	fileNameToErrorMutex       *sync.Mutex
+	fileNameToOutput           map[string]generator.GeneratorOutput
+	fileNameToOutputMutex      *sync.Mutex
+	devMode                    bool
+	hashes                     map[string][sha256.Size]byte
+	hashesMutex                *sync.Mutex
+	genOpts                    []generator.GenerateOpt
+	genSourceMapVis            bool
+	Errors                     []error
+	keepOrphanedFiles          bool
+	writer                     func(string, []byte) error
+	lazy                       bool
+}
+
+type GenerateResult struct {
+	// Updated indicates that the file was updated.
+	Updated bool
+	// GoUpdated indicates that Go expressions were updated.
+	GoUpdated bool
+	// TextUpdated indicates that text literals were updated.
+	TextUpdated bool
+}
+
+func (h *FSEventHandler) HandleEvent(ctx context.Context, event fsnotify.Event) (result GenerateResult, err error) {
+	// Handle _templ.go files.
+	if !event.Has(fsnotify.Remove) && strings.HasSuffix(event.Name, "_templ.go") {
+		_, err = os.Stat(strings.TrimSuffix(event.Name, "_templ.go") + ".templ")
+		if !os.IsNotExist(err) {
+			return GenerateResult{}, err
+		}
+		// File is orphaned.
+		if h.keepOrphanedFiles {
+			return GenerateResult{}, nil
+		}
+		h.Log.Debug("Deleting orphaned Go file", slog.String("file", event.Name))
+		if err = os.Remove(event.Name); err != nil {
+			h.Log.Warn("Failed to remove orphaned file", slog.Any("error", err))
+		}
+		return GenerateResult{Updated: true, GoUpdated: true, TextUpdated: false}, nil
+	}
+	// Handle _templ.txt files.
+	if !event.Has(fsnotify.Remove) && strings.HasSuffix(event.Name, "_templ.txt") {
+		if h.devMode {
+			// Don't delete the file in dev mode, ignore changes to it, since the .templ file
+			// must have been updated in order to trigger a change in the _templ.txt file.
+			return GenerateResult{Updated: false, GoUpdated: false, TextUpdated: false}, nil
+		}
+		h.Log.Debug("Deleting watch mode file", slog.String("file", event.Name))
+		if err = os.Remove(event.Name); err != nil {
+			h.Log.Warn("Failed to remove watch mode text file", slog.Any("error", err))
+			return GenerateResult{}, nil
+		}
+		return GenerateResult{}, nil
+	}
+
+	// If the file hasn't been updated since the last time we processed it, ignore it.
+	lastModTime, updatedModTime := h.UpsertLastModTime(event.Name)
+	if !updatedModTime {
+		h.Log.Debug("Skipping file because it wasn't updated", slog.String("file", event.Name))
+		return GenerateResult{}, nil
+	}
+
+	// Process anything that isn't a templ file.
+	if !strings.HasSuffix(event.Name, ".templ") {
+		// If it's a Go file, mark it as updated.
+		if strings.HasSuffix(event.Name, ".go") {
+			result.GoUpdated = true
+		}
+		result.Updated = true
+		return result, nil
+	}
+
+	// Handle templ files.
+
+	// If the go file is newer than the templ file, skip generation, because it's up-to-date.
+	if h.lazy && goFileIsUpToDate(event.Name, lastModTime) {
+		h.Log.Debug("Skipping file because the Go file is up-to-date", slog.String("file", event.Name))
+		return GenerateResult{}, nil
+	}
+
+	// Start a processor.
+	start := time.Now()
+	var diag []parser.Diagnostic
+	result, diag, err = h.generate(ctx, event.Name)
+	if err != nil {
+		h.SetError(event.Name, true)
+		return result, fmt.Errorf("failed to generate code for %q: %w", event.Name, err)
+	}
+	if len(diag) > 0 {
+		for _, d := range diag {
+			h.Log.Warn(d.Message,
+				slog.String("from", fmt.Sprintf("%d:%d", d.Range.From.Line, d.Range.From.Col)),
+				slog.String("to", fmt.Sprintf("%d:%d", d.Range.To.Line, d.Range.To.Col)),
+			)
+		}
+		return result, nil
+	}
+	if errorCleared, errorCount := h.SetError(event.Name, false); errorCleared {
+		h.Log.Info("Error cleared", slog.String("file", event.Name), slog.Int("errors", errorCount))
+	}
+	h.Log.Debug("Generated code", slog.String("file", event.Name), slog.Duration("in", time.Since(start)))
+
+	return result, nil
+}
+
+func goFileIsUpToDate(templFileName string, templFileLastMod time.Time) (upToDate bool) {
+	goFileName := strings.TrimSuffix(templFileName, ".templ") + "_templ.go"
+	goFileInfo, err := os.Stat(goFileName)
+	if err != nil {
+		return false
+	}
+	return goFileInfo.ModTime().After(templFileLastMod)
+}
+
+func (h *FSEventHandler) SetError(fileName string, hasError bool) (previouslyHadError bool, errorCount int) {
+	h.fileNameToErrorMutex.Lock()
+	defer h.fileNameToErrorMutex.Unlock()
+	_, previouslyHadError = h.fileNameToError[fileName]
+	delete(h.fileNameToError, fileName)
+	if hasError {
+		h.fileNameToError[fileName] = struct{}{}
+	}
+	return previouslyHadError, len(h.fileNameToError)
+}
+
+func (h *FSEventHandler) UpsertLastModTime(fileName string) (modTime time.Time, updated bool) {
+	fileInfo, err := os.Stat(fileName)
+	if err != nil {
+		return modTime, false
+	}
+	h.fileNameToLastModTimeMutex.Lock()
+	defer h.fileNameToLastModTimeMutex.Unlock()
+	previousModTime := h.fileNameToLastModTime[fileName]
+	currentModTime := fileInfo.ModTime()
+	if !currentModTime.After(previousModTime) {
+		return currentModTime, false
+	}
+	h.fileNameToLastModTime[fileName] = currentModTime
+	return currentModTime, true
+}
+
+func (h *FSEventHandler) UpsertHash(fileName string, hash [sha256.Size]byte) (updated bool) {
+	h.hashesMutex.Lock()
+	defer h.hashesMutex.Unlock()
+	lastHash := h.hashes[fileName]
+	if lastHash == hash {
+		return false
+	}
+	h.hashes[fileName] = hash
+	return true
+}
+
+// generate Go code for a single template.
+// If a basePath is provided, the filename included in error messages is relative to it.
+func (h *FSEventHandler) generate(ctx context.Context, fileName string) (result GenerateResult, diagnostics []parser.Diagnostic, err error) {
+	t, err := parser.Parse(fileName)
+	if err != nil {
+		return GenerateResult{}, nil, fmt.Errorf("%s parsing error: %w", fileName, err)
+	}
+	targetFileName := strings.TrimSuffix(fileName, ".templ") + "_templ.go"
+
+	// Only use relative filenames to the basepath for filenames in runtime error messages.
+	absFilePath, err := filepath.Abs(fileName)
+	if err != nil {
+		return GenerateResult{}, nil, fmt.Errorf("failed to get absolute path for %q: %w", fileName, err)
+	}
+	relFilePath, err := filepath.Rel(h.dir, absFilePath)
+	if err != nil {
+		return GenerateResult{}, nil, fmt.Errorf("failed to get relative path for %q: %w", fileName, err)
+	}
+	// Convert Windows file paths to Unix-style for consistency.
+	relFilePath = filepath.ToSlash(relFilePath)
+
+	var b bytes.Buffer
+	generatorOutput, err := generator.Generate(t, &b, append(h.genOpts, generator.WithFileName(relFilePath))...)
+	if err != nil {
+		return GenerateResult{}, nil, fmt.Errorf("%s generation error: %w", fileName, err)
+	}
+
+	formattedGoCode, err := format.Source(b.Bytes())
+	if err != nil {
+		err = remapErrorList(err, generatorOutput.SourceMap, fileName)
+		return GenerateResult{}, nil, fmt.Errorf("%s source formatting error %w", fileName, err)
+	}
+
+	// Hash output, and write out the file if the goCodeHash has changed.
+	goCodeHash := sha256.Sum256(formattedGoCode)
+	if h.UpsertHash(targetFileName, goCodeHash) {
+		result.Updated = true
+		if err = h.writer(targetFileName, formattedGoCode); err != nil {
+			return result, nil, fmt.Errorf("failed to write target file %q: %w", targetFileName, err)
+		}
+	}
+
+	// Add the txt file if it has changed.
+	if h.devMode {
+		txtFileName := strings.TrimSuffix(fileName, ".templ") + "_templ.txt"
+		joined := strings.Join(generatorOutput.Literals, "\n")
+		txtHash := sha256.Sum256([]byte(joined))
+		if h.UpsertHash(txtFileName, txtHash) {
+			result.TextUpdated = true
+			if err = os.WriteFile(txtFileName, []byte(joined), 0o644); err != nil {
+				return result, nil, fmt.Errorf("failed to write string literal file %q: %w", txtFileName, err)
+			}
+		}
+
+		// Check whether the change would require a recompilation to take effect.
+		h.fileNameToOutputMutex.Lock()
+		defer h.fileNameToOutputMutex.Unlock()
+		previous := h.fileNameToOutput[fileName]
+		if generator.HasChanged(previous, generatorOutput) {
+			result.GoUpdated = true
+		}
+		h.fileNameToOutput[fileName] = generatorOutput
+	}
+
+	parsedDiagnostics, err := parser.Diagnose(t)
+	if err != nil {
+		return result, nil, fmt.Errorf("%s diagnostics error: %w", fileName, err)
+	}
+
+	if h.genSourceMapVis {
+		err = generateSourceMapVisualisation(ctx, fileName, targetFileName, generatorOutput.SourceMap)
+	}
+
+	return result, parsedDiagnostics, err
+}
+
+// Takes an error from the formatter and attempts to convert the positions reported in the target file to their positions
+// in the source file.
+func remapErrorList(err error, sourceMap *parser.SourceMap, fileName string) error {
+	list, ok := err.(scanner.ErrorList)
+	if !ok || len(list) == 0 {
+		return err
+	}
+	for i, e := range list {
+		// The positions in the source map are off by one line because of the package definition.
+		srcPos, ok := sourceMap.SourcePositionFromTarget(uint32(e.Pos.Line-1), uint32(e.Pos.Column))
+		if !ok {
+			continue
+		}
+		list[i].Pos = token.Position{
+			Filename: fileName,
+			Offset:   int(srcPos.Index),
+			Line:     int(srcPos.Line) + 1,
+			Column:   int(srcPos.Col),
+		}
+	}
+	return list
+}
+
+func generateSourceMapVisualisation(ctx context.Context, templFileName, goFileName string, sourceMap *parser.SourceMap) error {
+	if err := ctx.Err(); err != nil {
+		return err
+	}
+	var templContents, goContents []byte
+	var templErr, goErr error
+	var wg sync.WaitGroup
+	wg.Add(2)
+	go func() {
+		defer wg.Done()
+		templContents, templErr = os.ReadFile(templFileName)
+	}()
+	go func() {
+		defer wg.Done()
+		goContents, goErr = os.ReadFile(goFileName)
+	}()
+	wg.Wait()
+	if templErr != nil {
+		return templErr
+	}
+	if goErr != nil {
+		return templErr
+	}
+
+	targetFileName := strings.TrimSuffix(templFileName, ".templ") + "_templ_sourcemap.html"
+	w, err := os.Create(targetFileName)
+	if err != nil {
+		return fmt.Errorf("%s sourcemap visualisation error: %w", templFileName, err)
+	}
+	defer w.Close()
+	b := bufio.NewWriter(w)
+	defer b.Flush()
+
+	return visualize.HTML(templFileName, string(templContents), string(goContents), sourceMap).Render(ctx, b)
+}

templ/cmd/templ/generatecmd/fatalerror.go

@@ -0,0 +1,23 @@
+package generatecmd
+
+type FatalError struct {
+	Err error
+}
+
+func (e FatalError) Error() string {
+	return e.Err.Error()
+}
+
+func (e FatalError) Unwrap() error {
+	return e.Err
+}
+
+func (e FatalError) Is(target error) bool {
+	_, ok := target.(FatalError)
+	return ok
+}
+
+func (e FatalError) As(target any) bool {
+	_, ok := target.(*FatalError)
+	return ok
+}

templ/cmd/templ/generatecmd/main.go

@@ -0,0 +1,39 @@
+package generatecmd
+
+import (
+	"context"
+	_ "embed"
+	"log/slog"
+
+	_ "net/http/pprof"
+)
+
+type Arguments struct {
+	FileName                        string
+	FileWriter                      FileWriterFunc
+	Path                            string
+	Watch                           bool
+	WatchPattern                    string
+	OpenBrowser                     bool
+	Command                         string
+	ProxyBind                       string
+	ProxyPort                       int
+	Proxy                           string
+	NotifyProxy                     bool
+	WorkerCount                     int
+	GenerateSourceMapVisualisations bool
+	IncludeVersion                  bool
+	IncludeTimestamp                bool
+	// PPROFPort is the port to run the pprof server on.
+	PPROFPort         int
+	KeepOrphanedFiles bool
+	Lazy              bool
+}
+
+func Run(ctx context.Context, log *slog.Logger, args Arguments) (err error) {
+	g, err := NewGenerate(log, args)
+	if err != nil {
+		return err
+	}
+	return g.Run(ctx)
+}

templ/cmd/templ/generatecmd/main_test.go

@@ -0,0 +1,170 @@
+package generatecmd
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"os"
+	"path"
+	"regexp"
+	"testing"
+	"time"
+
+	"github.com/a-h/templ/cmd/templ/testproject"
+	"golang.org/x/sync/errgroup"
+)
+
+func TestGenerate(t *testing.T) {
+	log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+	t.Run("can generate a file in place", func(t *testing.T) {
+		// templ generate -f templates.templ
+		dir, err := testproject.Create("github.com/a-h/templ/cmd/templ/testproject")
+		if err != nil {
+			t.Fatalf("failed to create test project: %v", err)
+		}
+		defer os.RemoveAll(dir)
+
+		// Delete the templates_templ.go file to ensure it is generated.
+		err = os.Remove(path.Join(dir, "templates_templ.go"))
+		if err != nil {
+			t.Fatalf("failed to remove templates_templ.go: %v", err)
+		}
+
+		// Run the generate command.
+		err = Run(context.Background(), log, Arguments{
+			FileName: path.Join(dir, "templates.templ"),
+		})
+		if err != nil {
+			t.Fatalf("failed to run generate command: %v", err)
+		}
+
+		// Check the templates_templ.go file was created.
+		_, err = os.Stat(path.Join(dir, "templates_templ.go"))
+		if err != nil {
+			t.Fatalf("templates_templ.go was not created: %v", err)
+		}
+	})
+	t.Run("can generate a file in watch mode", func(t *testing.T) {
+		// templ generate -f templates.templ
+		dir, err := testproject.Create("github.com/a-h/templ/cmd/templ/testproject")
+		if err != nil {
+			t.Fatalf("failed to create test project: %v", err)
+		}
+		defer os.RemoveAll(dir)
+
+		// Delete the templates_templ.go file to ensure it is generated.
+		err = os.Remove(path.Join(dir, "templates_templ.go"))
+		if err != nil {
+			t.Fatalf("failed to remove templates_templ.go: %v", err)
+		}
+		ctx, cancel := context.WithCancel(context.Background())
+
+		var eg errgroup.Group
+		eg.Go(func() error {
+			// Run the generate command.
+			return Run(ctx, log, Arguments{
+				Path:  dir,
+				Watch: true,
+			})
+		})
+
+		// Check the templates_templ.go file was created, with backoff.
+		for i := 0; i < 5; i++ {
+			time.Sleep(time.Second * time.Duration(i))
+			_, err = os.Stat(path.Join(dir, "templates_templ.go"))
+			if err != nil {
+				continue
+			}
+			_, err = os.Stat(path.Join(dir, "templates_templ.txt"))
+			if err != nil {
+				continue
+			}
+			break
+		}
+		if err != nil {
+			t.Fatalf("template files were not created: %v", err)
+		}
+
+		cancel()
+		if err := eg.Wait(); err != nil {
+			t.Fatalf("generate command failed: %v", err)
+		}
+
+		// Check the templates_templ.txt file was removed.
+		_, err = os.Stat(path.Join(dir, "templates_templ.txt"))
+		if err == nil {
+			t.Fatalf("templates_templ.txt was not removed")
+		}
+	})
+}
+
+func TestDefaultWatchPattern(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		matches bool
+	}{
+		{
+			name:    "empty file names do not match",
+			input:   "",
+			matches: false,
+		},
+		{
+			name:    "*_templ.txt matches, Windows",
+			input:   `C:\Users\adrian\github.com\a-h\templ\cmd\templ\testproject\strings_templ.txt`,
+			matches: true,
+		},
+		{
+			name:    "*_templ.txt matches, Unix",
+			input:   "/Users/adrian/github.com/a-h/templ/cmd/templ/testproject/strings_templ.txt",
+			matches: true,
+		},
+		{
+			name:    "*.templ files match, Windows",
+			input:   `C:\Users\adrian\github.com\a-h\templ\cmd\templ\testproject\templates.templ`,
+			matches: true,
+		},
+		{
+			name:    "*.templ files match, Unix",
+			input:   "/Users/adrian/github.com/a-h/templ/cmd/templ/testproject/templates.templ",
+			matches: true,
+		},
+		{
+			name:    "*_templ.go files match, Windows",
+			input:   `C:\Users\adrian\github.com\a-h\templ\cmd\templ\testproject\templates_templ.go`,
+			matches: true,
+		},
+		{
+			name:    "*_templ.go files match, Unix",
+			input:   "/Users/adrian/github.com/a-h/templ/cmd/templ/testproject/templates_templ.go",
+			matches: true,
+		},
+		{
+			name:    "*.go files match, Windows",
+			input:   `C:\Users\adrian\github.com\a-h\templ\cmd\templ\testproject\templates.go`,
+			matches: true,
+		},
+		{
+			name:    "*.go files match, Unix",
+			input:   "/Users/adrian/github.com/a-h/templ/cmd/templ/testproject/templates.go",
+			matches: true,
+		},
+		{
+			name:    "*.css files do not match",
+			input:   "/Users/adrian/github.com/a-h/templ/cmd/templ/testproject/templates.css",
+			matches: false,
+		},
+	}
+	wpRegexp, err := regexp.Compile(defaultWatchPattern)
+	if err != nil {
+		t.Fatalf("failed to compile default watch pattern: %v", err)
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			if wpRegexp.MatchString(test.input) != test.matches {
+				t.Fatalf("expected match of %q to be %v", test.input, test.matches)
+			}
+		})
+	}
+}

templ/cmd/templ/generatecmd/modcheck/modcheck.go

@@ -0,0 +1,82 @@
+package modcheck
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/a-h/templ"
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/semver"
+)
+
+// WalkUp the directory tree, starting at dir, until we find a directory containing
+// a go.mod file.
+func WalkUp(dir string) (string, error) {
+	dir, err := filepath.Abs(dir)
+	if err != nil {
+		return "", fmt.Errorf("failed to get absolute path: %w", err)
+	}
+
+	var modFile string
+	for {
+		modFile = filepath.Join(dir, "go.mod")
+		_, err := os.Stat(modFile)
+		if err != nil && !os.IsNotExist(err) {
+			return "", fmt.Errorf("failed to stat go.mod file: %w", err)
+		}
+		if os.IsNotExist(err) {
+			// Move up.
+			prev := dir
+			dir = filepath.Dir(dir)
+			if dir == prev {
+				break
+			}
+			continue
+		}
+		break
+	}
+
+	// No file found.
+	if modFile == "" {
+		return dir, fmt.Errorf("could not find go.mod file")
+	}
+	return dir, nil
+}
+
+func Check(dir string) error {
+	dir, err := WalkUp(dir)
+	if err != nil {
+		return err
+	}
+
+	// Found a go.mod file.
+	// Read it and find the templ version.
+	modFile := filepath.Join(dir, "go.mod")
+	m, err := os.ReadFile(modFile)
+	if err != nil {
+		return fmt.Errorf("failed to read go.mod file: %w", err)
+	}
+
+	mf, err := modfile.Parse(modFile, m, nil)
+	if err != nil {
+		return fmt.Errorf("failed to parse go.mod file: %w", err)
+	}
+	if mf.Module.Mod.Path == "github.com/a-h/templ" {
+		// The go.mod file is for templ itself.
+		return nil
+	}
+	for _, r := range mf.Require {
+		if r.Mod.Path == "github.com/a-h/templ" {
+			cmp := semver.Compare(r.Mod.Version, templ.Version())
+			if cmp < 0 {
+				return fmt.Errorf("generator %v is newer than templ version %v found in go.mod file, consider running `go get -u github.com/a-h/templ` to upgrade", templ.Version(), r.Mod.Version)
+			}
+			if cmp > 0 {
+				return fmt.Errorf("generator %v is older than templ version %v found in go.mod file, consider upgrading templ CLI", templ.Version(), r.Mod.Version)
+			}
+			return nil
+		}
+	}
+	return fmt.Errorf("templ not found in go.mod file, run `go get github.com/a-h/templ` to install it")
+}

templ/cmd/templ/generatecmd/modcheck/modcheck_test.go

@@ -0,0 +1,47 @@
+package modcheck
+
+import (
+	"testing"
+
+	"golang.org/x/mod/modfile"
+)
+
+func TestPatchGoVersion(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{
+			input:    "go 1.20",
+			expected: "1.20",
+		},
+		{
+			input:    "go 1.20.123",
+			expected: "1.20.123",
+		},
+		{
+			input:    "go 1.20.1",
+			expected: "1.20.1",
+		},
+		{
+			input:    "go 1.20rc1",
+			expected: "1.20rc1",
+		},
+		{
+			input:    "go 1.15",
+			expected: "1.15",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.input, func(t *testing.T) {
+			input := "module github.com/a-h/templ\n\n" + string(test.input) + "\n" + "toolchain go1.27.9\n"
+			mf, err := modfile.Parse("go.mod", []byte(input), nil)
+			if err != nil {
+				t.Fatalf("failed to parse go.mod: %v", err)
+			}
+			if test.expected != mf.Go.Version {
+				t.Errorf("expected %q, got %q", test.expected, mf.Go.Version)
+			}
+		})
+	}
+}

templ/cmd/templ/generatecmd/proxy/proxy.go

@@ -0,0 +1,284 @@
+package proxy
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"html"
+	"io"
+	stdlog "log"
+	"log/slog"
+	"math"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/PuerkitoBio/goquery"
+	"github.com/a-h/templ/cmd/templ/generatecmd/sse"
+	"github.com/andybalholm/brotli"
+
+	_ "embed"
+)
+
+//go:embed script.js
+var script string
+
+type Handler struct {
+	log    *slog.Logger
+	URL    string
+	Target *url.URL
+	p      *httputil.ReverseProxy
+	sse    *sse.Handler
+}
+
+func getScriptTag(nonce string) string {
+	if nonce != "" {
+		var sb strings.Builder
+		sb.WriteString(`<script src="/_templ/reload/script.js" nonce="`)
+		sb.WriteString(html.EscapeString(nonce))
+		sb.WriteString(`"></script>`)
+		return sb.String()
+	}
+	return `<script src="/_templ/reload/script.js"></script>`
+}
+
+func insertScriptTagIntoBody(nonce, body string) (updated string) {
+	doc, err := goquery.NewDocumentFromReader(strings.NewReader(body))
+	if err != nil {
+		return strings.Replace(body, "</body>", getScriptTag(nonce)+"</body>", -1)
+	}
+	doc.Find("body").AppendHtml(getScriptTag(nonce))
+	r, err := doc.Html()
+	if err != nil {
+		return strings.Replace(body, "</body>", getScriptTag(nonce)+"</body>", -1)
+	}
+	return r
+}
+
+type passthroughWriteCloser struct {
+	io.Writer
+}
+
+func (pwc passthroughWriteCloser) Close() error {
+	return nil
+}
+
+const unsupportedContentEncoding = "Unsupported content encoding, hot reload script not inserted."
+
+func (h *Handler) modifyResponse(r *http.Response) error {
+	log := h.log.With(slog.String("url", r.Request.URL.String()))
+	if r.Header.Get("templ-skip-modify") == "true" {
+		log.Debug("Skipping response modification because templ-skip-modify header is set")
+		return nil
+	}
+	if contentType := r.Header.Get("Content-Type"); !strings.HasPrefix(contentType, "text/html") {
+		log.Debug("Skipping response modification because content type is not text/html", slog.String("content-type", contentType))
+		return nil
+	}
+
+	// Set up readers and writers.
+	newReader := func(in io.Reader) (out io.Reader, err error) {
+		return in, nil
+	}
+	newWriter := func(out io.Writer) io.WriteCloser {
+		return passthroughWriteCloser{out}
+	}
+	switch r.Header.Get("Content-Encoding") {
+	case "gzip":
+		newReader = func(in io.Reader) (out io.Reader, err error) {
+			return gzip.NewReader(in)
+		}
+		newWriter = func(out io.Writer) io.WriteCloser {
+			return gzip.NewWriter(out)
+		}
+	case "br":
+		newReader = func(in io.Reader) (out io.Reader, err error) {
+			return brotli.NewReader(in), nil
+		}
+		newWriter = func(out io.Writer) io.WriteCloser {
+			return brotli.NewWriter(out)
+		}
+	case "":
+		log.Debug("No content encoding header found")
+	default:
+		h.log.Warn(unsupportedContentEncoding, slog.String("encoding", r.Header.Get("Content-Encoding")))
+	}
+
+	// Read the encoded body.
+	encr, err := newReader(r.Body)
+	if err != nil {
+		return err
+	}
+	defer r.Body.Close()
+	body, err := io.ReadAll(encr)
+	if err != nil {
+		return err
+	}
+
+	// Update it.
+	csp := r.Header.Get("Content-Security-Policy")
+	updated := insertScriptTagIntoBody(parseNonce(csp), string(body))
+	if log.Enabled(r.Request.Context(), slog.LevelDebug) {
+		if len(updated) == len(body) {
+			log.Debug("Reload script not inserted")
+		} else {
+			log.Debug("Reload script inserted")
+		}
+	}
+
+	// Encode the response.
+	var buf bytes.Buffer
+	encw := newWriter(&buf)
+	_, err = encw.Write([]byte(updated))
+	if err != nil {
+		return err
+	}
+	err = encw.Close()
+	if err != nil {
+		return err
+	}
+
+	// Update the response.
+	r.Body = io.NopCloser(&buf)
+	r.ContentLength = int64(buf.Len())
+	r.Header.Set("Content-Length", strconv.Itoa(buf.Len()))
+	return nil
+}
+
+func parseNonce(csp string) (nonce string) {
+outer:
+	for _, rawDirective := range strings.Split(csp, ";") {
+		parts := strings.Fields(rawDirective)
+		if len(parts) < 2 {
+			continue
+		}
+		if parts[0] != "script-src" {
+			continue
+		}
+		for _, source := range parts[1:] {
+			source = strings.TrimPrefix(source, "'")
+			source = strings.TrimSuffix(source, "'")
+			if strings.HasPrefix(source, "nonce-") {
+				nonce = source[6:]
+				break outer
+			}
+		}
+	}
+	return nonce
+}
+
+func New(log *slog.Logger, bind string, port int, target *url.URL) (h *Handler) {
+	p := httputil.NewSingleHostReverseProxy(target)
+	p.ErrorLog = stdlog.New(os.Stderr, "Proxy to target error: ", 0)
+	p.Transport = &roundTripper{
+		maxRetries:      20,
+		initialDelay:    100 * time.Millisecond,
+		backoffExponent: 1.5,
+	}
+	h = &Handler{
+		log:    log,
+		URL:    fmt.Sprintf("http://%s:%d", bind, port),
+		Target: target,
+		p:      p,
+		sse:    sse.New(),
+	}
+	p.ModifyResponse = h.modifyResponse
+	return h
+}
+
+func (p *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path == "/_templ/reload/script.js" {
+		// Provides a script that reloads the page.
+		w.Header().Add("Content-Type", "text/javascript")
+		_, err := io.WriteString(w, script)
+		if err != nil {
+			fmt.Printf("failed to write script: %v\n", err)
+		}
+		return
+	}
+	if r.URL.Path == "/_templ/reload/events" {
+		switch r.Method {
+		case http.MethodGet:
+			// Provides a list of messages including a reload message.
+			p.sse.ServeHTTP(w, r)
+			return
+		case http.MethodPost:
+			// Send a reload message to all connected clients.
+			p.sse.Send("message", "reload")
+			return
+		}
+		http.Error(w, "only GET or POST method allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	p.p.ServeHTTP(w, r)
+}
+
+func (p *Handler) SendSSE(eventType string, data string) {
+	p.sse.Send(eventType, data)
+}
+
+type roundTripper struct {
+	maxRetries      int
+	initialDelay    time.Duration
+	backoffExponent float64
+}
+
+func (rt *roundTripper) setShouldSkipResponseModificationHeader(r *http.Request, resp *http.Response) {
+	// Instruct the modifyResponse function to skip modifying the response if the
+	// HTTP request has come from HTMX.
+	if r.Header.Get("HX-Request") != "true" {
+		return
+	}
+	resp.Header.Set("templ-skip-modify", "true")
+}
+
+func (rt *roundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
+	// Read and buffer the body.
+	var bodyBytes []byte
+	if r.Body != nil && r.Body != http.NoBody {
+		var err error
+		bodyBytes, err = io.ReadAll(r.Body)
+		if err != nil {
+			return nil, err
+		}
+		r.Body.Close()
+	}
+
+	// Retry logic.
+	var resp *http.Response
+	var err error
+	for retries := 0; retries < rt.maxRetries; retries++ {
+		// Clone the request and set the body.
+		req := r.Clone(r.Context())
+		if bodyBytes != nil {
+			req.Body = io.NopCloser(bytes.NewReader(bodyBytes))
+		}
+
+		// Execute the request.
+		resp, err = http.DefaultTransport.RoundTrip(req)
+		if err != nil {
+			time.Sleep(rt.initialDelay * time.Duration(math.Pow(rt.backoffExponent, float64(retries))))
+			continue
+		}
+
+		rt.setShouldSkipResponseModificationHeader(r, resp)
+
+		return resp, nil
+	}
+
+	return nil, fmt.Errorf("max retries reached: %q", r.URL.String())
+}
+
+func NotifyProxy(host string, port int) error {
+	urlStr := fmt.Sprintf("http://%s:%d/_templ/reload/events", host, port)
+	req, err := http.NewRequest(http.MethodPost, urlStr, nil)
+	if err != nil {
+		return err
+	}
+	_, err = http.DefaultClient.Do(req)
+	return err
+}

templ/cmd/templ/generatecmd/proxy/proxy_test.go

@@ -0,0 +1,627 @@
+package proxy
+
+import (
+	"bufio"
+	"bytes"
+	"compress/gzip"
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/andybalholm/brotli"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestRoundTripper(t *testing.T) {
+	t.Run("if the HX-Request header is present, set the templ-skip-modify header on the response", func(t *testing.T) {
+		rt := &roundTripper{}
+		req, err := http.NewRequest("GET", "http://example.com", nil)
+		if err != nil {
+			t.Fatalf("unexpected error creating request: %v", err)
+		}
+		req.Header.Set("HX-Request", "true")
+		resp := &http.Response{Header: make(http.Header)}
+		rt.setShouldSkipResponseModificationHeader(req, resp)
+		if resp.Header.Get("templ-skip-modify") != "true" {
+			t.Errorf("expected templ-skip-modify header to be true, got %v", resp.Header.Get("templ-skip-modify"))
+		}
+	})
+	t.Run("if the HX-Request header is not present, do not set the templ-skip-modify header on the response", func(t *testing.T) {
+		rt := &roundTripper{}
+		req, err := http.NewRequest("GET", "http://example.com", nil)
+		if err != nil {
+			t.Fatalf("unexpected error creating request: %v", err)
+		}
+		resp := &http.Response{Header: make(http.Header)}
+		rt.setShouldSkipResponseModificationHeader(req, resp)
+		if resp.Header.Get("templ-skip-modify") != "" {
+			t.Errorf("expected templ-skip-modify header to be empty, got %v", resp.Header.Get("templ-skip-modify"))
+		}
+	})
+}
+
+func TestProxy(t *testing.T) {
+	t.Run("plain: non-html content is not modified", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`{"key": "value"}`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "application/json")
+		r.Header.Set("Content-Length", "16")
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != "16" {
+			t.Errorf("expected content length to be 16, got %v", r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(`{"key": "value"}`, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("plain: if the response contains templ-skip-modify header, it is not modified", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`Hello`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html")
+		r.Header.Set("Content-Length", "5")
+		r.Header.Set("templ-skip-modify", "true")
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != "5" {
+			t.Errorf("expected content length to be 5, got %v", r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(`Hello`, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("plain: body tags get the script inserted", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`<html><body></body></html>`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Length", "26")
+
+		expectedString := insertScriptTagIntoBody("", `<html><body></body></html>`)
+		if !strings.Contains(expectedString, getScriptTag("")) {
+			t.Fatalf("expected the script tag to be inserted, but it wasn't: %q", expectedString)
+		}
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != fmt.Sprintf("%d", len(expectedString)) {
+			t.Errorf("expected content length to be %d, got %v", len(expectedString), r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(expectedString, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("plain: body tags get the script inserted with nonce", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`<html><body></body></html>`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Length", "26")
+		const nonce = "this-is-the-nonce"
+		r.Header.Set("Content-Security-Policy", fmt.Sprintf("script-src 'nonce-%s'", nonce))
+
+		expectedString := insertScriptTagIntoBody(nonce, `<html><body></body></html>`)
+		if !strings.Contains(expectedString, getScriptTag(nonce)) {
+			t.Fatalf("expected the script tag to be inserted, but it wasn't: %q", expectedString)
+		}
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != fmt.Sprintf("%d", len(expectedString)) {
+			t.Errorf("expected content length to be %d, got %v", len(expectedString), r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(expectedString, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("plain: body tags get the script inserted ignoring js with body tags", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`<html><body><script>console.log("<body></body>")</script></body></html>`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Length", "26")
+
+		expectedString := insertScriptTagIntoBody("", `<html><body><script>console.log("<body></body>")</script></body></html>`)
+		if !strings.Contains(expectedString, getScriptTag("")) {
+			t.Fatalf("expected the script tag to be inserted, but it wasn't: %q", expectedString)
+		}
+		if !strings.Contains(expectedString, `console.log("<body></body>")`) {
+			t.Fatalf("expected the script tag to be inserted, but mangled the html: %q", expectedString)
+		}
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != fmt.Sprintf("%d", len(expectedString)) {
+			t.Errorf("expected content length to be %d, got %v", len(expectedString), r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(expectedString, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("gzip: non-html content is not modified", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(strings.NewReader(`{"key": "value"}`)),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "application/json")
+		// It's not actually gzipped here, but it doesn't matter, it shouldn't get that far.
+		r.Header.Set("Content-Encoding", "gzip")
+		// Similarly, this is not the actual length of the gzipped content.
+		r.Header.Set("Content-Length", "16")
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != "16" {
+			t.Errorf("expected content length to be 16, got %v", r.Header.Get("Content-Length"))
+		}
+		actualBody, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(`{"key": "value"}`, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("gzip: body tags get the script inserted", func(t *testing.T) {
+		// Arrange
+		body := `<html><body></body></html>`
+		var buf bytes.Buffer
+		gzw := gzip.NewWriter(&buf)
+		_, err := gzw.Write([]byte(body))
+		if err != nil {
+			t.Fatalf("unexpected error writing gzip: %v", err)
+		}
+		gzw.Close()
+
+		expectedString := insertScriptTagIntoBody("", body)
+
+		var expectedBytes bytes.Buffer
+		gzw = gzip.NewWriter(&expectedBytes)
+		_, err = gzw.Write([]byte(expectedString))
+		if err != nil {
+			t.Fatalf("unexpected error writing gzip: %v", err)
+		}
+		gzw.Close()
+		expectedLength := len(expectedBytes.Bytes())
+
+		r := &http.Response{
+			Body:   io.NopCloser(&buf),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Encoding", "gzip")
+		r.Header.Set("Content-Length", fmt.Sprintf("%d", expectedLength))
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err = h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != fmt.Sprintf("%d", expectedLength) {
+			t.Errorf("expected content length to be %d, got %v", expectedLength, r.Header.Get("Content-Length"))
+		}
+
+		gr, err := gzip.NewReader(r.Body)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		actualBody, err := io.ReadAll(gr)
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(expectedString, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("brotli: body tags get the script inserted", func(t *testing.T) {
+		// Arrange
+		body := `<html><body></body></html>`
+		var buf bytes.Buffer
+		brw := brotli.NewWriter(&buf)
+		_, err := brw.Write([]byte(body))
+		if err != nil {
+			t.Fatalf("unexpected error writing gzip: %v", err)
+		}
+		brw.Close()
+
+		expectedString := insertScriptTagIntoBody("", body)
+
+		var expectedBytes bytes.Buffer
+		brw = brotli.NewWriter(&expectedBytes)
+		_, err = brw.Write([]byte(expectedString))
+		if err != nil {
+			t.Fatalf("unexpected error writing gzip: %v", err)
+		}
+		brw.Close()
+		expectedLength := len(expectedBytes.Bytes())
+
+		r := &http.Response{
+			Body:   io.NopCloser(&buf),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Encoding", "br")
+		r.Header.Set("Content-Length", fmt.Sprintf("%d", expectedLength))
+
+		// Act
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err = h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if r.Header.Get("Content-Length") != fmt.Sprintf("%d", expectedLength) {
+			t.Errorf("expected content length to be %d, got %v", expectedLength, r.Header.Get("Content-Length"))
+		}
+
+		actualBody, err := io.ReadAll(brotli.NewReader(r.Body))
+		if err != nil {
+			t.Fatalf("unexpected error reading response: %v", err)
+		}
+		if diff := cmp.Diff(expectedString, string(actualBody)); diff != "" {
+			t.Errorf("unexpected response body (-got +want):\n%s", diff)
+		}
+	})
+	t.Run("notify-proxy: sending POST request to /_templ/reload/events should receive reload sse event", func(t *testing.T) {
+		// Arrange 1: create a test proxy server.
+		dummyHandler := func(w http.ResponseWriter, r *http.Request) {}
+		dummyServer := httptest.NewServer(http.HandlerFunc(dummyHandler))
+		defer dummyServer.Close()
+
+		u, err := url.Parse(dummyServer.URL)
+		if err != nil {
+			t.Fatalf("unexpected error parsing URL: %v", err)
+		}
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+		handler := New(log, "0.0.0.0", 0, u)
+		proxyServer := httptest.NewServer(handler)
+		defer proxyServer.Close()
+
+		u2, err := url.Parse(proxyServer.URL)
+		if err != nil {
+			t.Fatalf("unexpected error parsing URL: %v", err)
+		}
+		port, err := strconv.Atoi(u2.Port())
+		if err != nil {
+			t.Fatalf("unexpected error parsing port: %v", err)
+		}
+
+		// Arrange 2: start a goroutine to listen for sse events.
+		ctx := context.Background()
+		ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
+		defer cancel()
+
+		errChan := make(chan error)
+		sseRespCh := make(chan string)
+		sseListening := make(chan bool) // Coordination channel that ensures the SSE listener is started before notifying the proxy.
+		go func() {
+			req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("%s/_templ/reload/events", proxyServer.URL), nil)
+			if err != nil {
+				errChan <- err
+				return
+			}
+			resp, err := http.DefaultClient.Do(req)
+			if err != nil {
+				errChan <- err
+				return
+			}
+			defer resp.Body.Close()
+
+			sseListening <- true
+			lines := []string{}
+			scanner := bufio.NewScanner(resp.Body)
+			for scanner.Scan() {
+				lines = append(lines, scanner.Text())
+				if scanner.Text() == "data: reload" {
+					sseRespCh <- strings.Join(lines, "\n")
+					return
+				}
+			}
+			err = scanner.Err()
+			if err != nil {
+				errChan <- err
+				return
+			}
+		}()
+
+		// Act: notify the proxy.
+		select { // Either SSE is listening or an error occurred.
+		case <-sseListening:
+			err = NotifyProxy(u2.Hostname(), port)
+			if err != nil {
+				t.Fatalf("unexpected error notifying proxy: %v", err)
+			}
+		case err := <-errChan:
+			if err == nil {
+				t.Fatalf("unexpected sse response: %v", err)
+			}
+		}
+
+		// Assert.
+		select { // Either SSE has a expected response or an error or timeout occurred.
+		case resp := <-sseRespCh:
+			if !strings.Contains(resp, "event: message\ndata: reload") {
+				t.Errorf("expected sse reload event to be received, got: %q", resp)
+			}
+		case err := <-errChan:
+			if err == nil {
+				t.Fatalf("unexpected sse response: %v", err)
+			}
+		case <-ctx.Done():
+			t.Fatalf("timeout waiting for sse response")
+		}
+	})
+	t.Run("unsupported encodings result in a warning", func(t *testing.T) {
+		// Arrange
+		r := &http.Response{
+			Body:   io.NopCloser(bytes.NewReader([]byte("<p>Data</p>"))),
+			Header: make(http.Header),
+			Request: &http.Request{
+				URL: &url.URL{
+					Scheme: "http",
+					Host:   "example.com",
+				},
+			},
+		}
+		r.Header.Set("Content-Type", "text/html, charset=utf-8")
+		r.Header.Set("Content-Encoding", "weird-encoding")
+
+		// Act
+		lh := newTestLogHandler(slog.LevelInfo)
+		log := slog.New(lh)
+		h := New(log, "127.0.0.1", 7474, &url.URL{Scheme: "http", Host: "example.com"})
+		err := h.modifyResponse(r)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		// Assert
+		if len(lh.records) != 1 {
+			var sb strings.Builder
+			for _, record := range lh.records {
+				sb.WriteString(record.Message)
+				sb.WriteString("\n")
+			}
+			t.Fatalf("expected 1 log entry, but got %d: \n%s", len(lh.records), sb.String())
+		}
+		record := lh.records[0]
+		if record.Message != unsupportedContentEncoding {
+			t.Errorf("expected warning message %q, got %q", unsupportedContentEncoding, record.Message)
+		}
+		if record.Level != slog.LevelWarn {
+			t.Errorf("expected warning, got level %v", record.Level)
+		}
+	})
+}
+
+func newTestLogHandler(level slog.Level) *testLogHandler {
+	return &testLogHandler{
+		m:       new(sync.Mutex),
+		records: nil,
+		level:   level,
+	}
+}
+
+type testLogHandler struct {
+	m       *sync.Mutex
+	records []slog.Record
+	level   slog.Level
+}
+
+func (h *testLogHandler) Enabled(ctx context.Context, l slog.Level) bool {
+	return l >= h.level
+}
+
+func (h *testLogHandler) Handle(ctx context.Context, r slog.Record) error {
+	h.m.Lock()
+	defer h.m.Unlock()
+	if r.Level < h.level {
+		return nil
+	}
+	h.records = append(h.records, r)
+	return nil
+}
+
+func (h *testLogHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return h
+}
+
+func (h *testLogHandler) WithGroup(name string) slog.Handler {
+	return h
+}
+
+func TestParseNonce(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		csp      string
+		expected string
+	}{
+		{
+			name:     "empty csp",
+			csp:      "",
+			expected: "",
+		},
+		{
+			name:     "simple csp",
+			csp:      "script-src 'nonce-oLhVst3hTAcxI734qtB0J9Qc7W4qy09C'",
+			expected: "oLhVst3hTAcxI734qtB0J9Qc7W4qy09C",
+		},
+		{
+			name:     "simple csp without single quote",
+			csp:      "script-src nonce-oLhVst3hTAcxI734qtB0J9Qc7W4qy09C",
+			expected: "oLhVst3hTAcxI734qtB0J9Qc7W4qy09C",
+		},
+		{
+			name:     "complete csp",
+			csp:      "default-src 'self'; frame-ancestors 'self'; form-action 'self'; script-src 'strict-dynamic' 'nonce-4VOtk0Uo1l7pwtC';",
+			expected: "4VOtk0Uo1l7pwtC",
+		},
+		{
+			name:     "mdn example 1",
+			csp:      "default-src 'self'",
+			expected: "",
+		},
+		{
+			name:     "mdn example 2",
+			csp:      "default-src 'self' *.trusted.com",
+			expected: "",
+		},
+		{
+			name:     "mdn example 3",
+			csp:      "default-src 'self'; img-src *; media-src media1.com media2.com; script-src userscripts.example.com",
+			expected: "",
+		},
+		{
+			name:     "mdn example 3 multiple sources",
+			csp:      "default-src 'self'; img-src *; media-src media1.com media2.com; script-src userscripts.example.com foo.com 'strict-dynamic' 'nonce-4VOtk0Uo1l7pwtC'",
+			expected: "4VOtk0Uo1l7pwtC",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			nonce := parseNonce(tc.csp)
+			if nonce != tc.expected {
+				t.Errorf("expected nonce to be %s, but got %s", tc.expected, nonce)
+			}
+		})
+	}
+}

templ/cmd/templ/generatecmd/proxy/script.js

@@ -0,0 +1,10 @@
+(function() {
+  let templ_reloadSrc = window.templ_reloadSrc || new EventSource("/_templ/reload/events");
+  templ_reloadSrc.onmessage = (event) => {
+    if (event && event.data === "reload") {
+      window.location.reload();
+    }
+  };
+  window.templ_reloadSrc = templ_reloadSrc;
+  window.onbeforeunload = () => window.templ_reloadSrc.close();
+})();

templ/cmd/templ/generatecmd/run/run_test.go

@@ -0,0 +1,108 @@
+package run_test
+
+import (
+	"context"
+	"embed"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/a-h/templ/cmd/templ/generatecmd/run"
+)
+
+//go:embed testprogram/*
+var testprogram embed.FS
+
+func TestGoRun(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping test in short mode.")
+	}
+
+	// Copy testprogram to a temporary directory.
+	dir, err := os.MkdirTemp("", "testprogram")
+	if err != nil {
+		t.Fatalf("failed to make test dir: %v", err)
+	}
+	files, err := testprogram.ReadDir("testprogram")
+	if err != nil {
+		t.Fatalf("failed to read embedded dir: %v", err)
+	}
+	for _, file := range files {
+		srcFileName := "testprogram/" + file.Name()
+		srcData, err := testprogram.ReadFile(srcFileName)
+		if err != nil {
+			t.Fatalf("failed to read src file %q: %v", srcFileName, err)
+		}
+		tgtFileName := filepath.Join(dir, file.Name())
+		tgtFile, err := os.Create(tgtFileName)
+		if err != nil {
+			t.Fatalf("failed to create tgt file %q: %v", tgtFileName, err)
+		}
+		defer tgtFile.Close()
+		if _, err := tgtFile.Write(srcData); err != nil {
+			t.Fatalf("failed to write to tgt file %q: %v", tgtFileName, err)
+		}
+	}
+	// Rename the go.mod.embed file to go.mod.
+	if err := os.Rename(filepath.Join(dir, "go.mod.embed"), filepath.Join(dir, "go.mod")); err != nil {
+		t.Fatalf("failed to rename go.mod.embed: %v", err)
+	}
+
+	tests := []struct {
+		name string
+		cmd  string
+	}{
+		{
+			name: "Well behaved programs get shut down",
+			cmd:  "go run .",
+		},
+		{
+			name: "Badly behaved programs get shut down",
+			cmd:  "go run . -badly-behaved",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.Background()
+			cmd, err := run.Run(ctx, dir, tt.cmd)
+			if err != nil {
+				t.Fatalf("failed to run program: %v", err)
+			}
+
+			time.Sleep(1 * time.Second)
+
+			pid := cmd.Process.Pid
+
+			if err := run.KillAll(); err != nil {
+				t.Fatalf("failed to kill all: %v", err)
+			}
+
+			// Check the parent process is no longer running.
+			if err := cmd.Process.Signal(os.Signal(syscall.Signal(0))); err == nil {
+				t.Fatalf("process %d is still running", pid)
+			}
+			// Check that the child was stopped.
+			body, err := readResponse("http://localhost:7777")
+			if err == nil {
+				t.Fatalf("child process is still running: %s", body)
+			}
+		})
+	}
+}
+
+func readResponse(url string) (body string, err error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return body, err
+	}
+	defer resp.Body.Close()
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return body, err
+	}
+	return string(b), nil
+}

templ/cmd/templ/generatecmd/run/run_unix.go

@@ -0,0 +1,84 @@
+//go:build unix
+
+package run
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+	"syscall"
+	"time"
+)
+
+var (
+	m       = &sync.Mutex{}
+	running = map[string]*exec.Cmd{}
+)
+
+func KillAll() (err error) {
+	m.Lock()
+	defer m.Unlock()
+	var errs []error
+	for _, cmd := range running {
+		if err := kill(cmd); err != nil {
+			errs = append(errs, fmt.Errorf("failed to kill process %d: %w", cmd.Process.Pid, err))
+		}
+	}
+	running = map[string]*exec.Cmd{}
+	return errors.Join(errs...)
+}
+
+func kill(cmd *exec.Cmd) (err error) {
+	errs := make([]error, 4)
+	errs[0] = ignoreExited(cmd.Process.Signal(syscall.SIGINT))
+	errs[1] = ignoreExited(cmd.Process.Signal(syscall.SIGTERM))
+	errs[2] = ignoreExited(cmd.Wait())
+	errs[3] = ignoreExited(syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL))
+	return errors.Join(errs...)
+}
+
+func ignoreExited(err error) error {
+	if errors.Is(err, syscall.ESRCH) {
+		return nil
+	}
+	// Ignore *exec.ExitError
+	if _, ok := err.(*exec.ExitError); ok {
+		return nil
+	}
+	return err
+}
+
+func Run(ctx context.Context, workingDir string, input string) (cmd *exec.Cmd, err error) {
+	m.Lock()
+	defer m.Unlock()
+	cmd, ok := running[input]
+	if ok {
+		if err := kill(cmd); err != nil {
+			return cmd, fmt.Errorf("failed to kill process %d: %w", cmd.Process.Pid, err)
+		}
+
+		delete(running, input)
+	}
+	parts := strings.Fields(input)
+	executable := parts[0]
+	args := []string{}
+	if len(parts) > 1 {
+		args = append(args, parts[1:]...)
+	}
+
+	cmd = exec.CommandContext(ctx, executable, args...)
+	// Wait for the process to finish gracefully before termination.
+	cmd.WaitDelay = time.Second * 3
+	cmd.Env = os.Environ()
+	cmd.Dir = workingDir
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	running[input] = cmd
+	err = cmd.Start()
+	return
+}

templ/cmd/templ/generatecmd/run/run_windows.go

@@ -0,0 +1,69 @@
+//go:build windows
+
+package run
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+var m = &sync.Mutex{}
+var running = map[string]*exec.Cmd{}
+
+func KillAll() (err error) {
+	m.Lock()
+	defer m.Unlock()
+	for _, cmd := range running {
+		kill := exec.Command("TASKKILL", "/T", "/F", "/PID", strconv.Itoa(cmd.Process.Pid))
+		kill.Stderr = os.Stderr
+		kill.Stdout = os.Stdout
+		err := kill.Run()
+		if err != nil {
+			return err
+		}
+	}
+	running = map[string]*exec.Cmd{}
+	return
+}
+
+func Stop(cmd *exec.Cmd) (err error) {
+	kill := exec.Command("TASKKILL", "/T", "/F", "/PID", strconv.Itoa(cmd.Process.Pid))
+	kill.Stderr = os.Stderr
+	kill.Stdout = os.Stdout
+	return kill.Run()
+}
+
+func Run(ctx context.Context, workingDir string, input string) (cmd *exec.Cmd, err error) {
+	m.Lock()
+	defer m.Unlock()
+	cmd, ok := running[input]
+	if ok {
+		kill := exec.Command("TASKKILL", "/T", "/F", "/PID", strconv.Itoa(cmd.Process.Pid))
+		kill.Stderr = os.Stderr
+		kill.Stdout = os.Stdout
+		err := kill.Run()
+		if err != nil {
+			return cmd, err
+		}
+		delete(running, input)
+	}
+	parts := strings.Fields(input)
+	executable := parts[0]
+	args := []string{}
+	if len(parts) > 1 {
+		args = append(args, parts[1:]...)
+	}
+
+	cmd = exec.Command(executable, args...)
+	cmd.Env = os.Environ()
+	cmd.Dir = workingDir
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	running[input] = cmd
+	err = cmd.Start()
+	return
+}

templ/cmd/templ/generatecmd/run/testprogram/go.mod.embed

@@ -0,0 +1,3 @@
+module testprogram
+
+go 1.23

templ/cmd/templ/generatecmd/run/testprogram/main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+// This is a test program. It is used only to test the behaviour of the run package.
+// The run package is supposed to be able to run and stop programs. Those programs may start
+// child processes, which should also be stopped when the parent program is stopped.
+
+// For example, running `go run .` will compile an executable and run it.
+
+// So, this program does nothing. It just waits for a signal to stop.
+
+// In "Well behaved" mode, the program will stop when it receives a signal.
+// In "Badly behaved" mode, the program will ignore the signal and continue running.
+
+// The run package should be able to stop the program in both cases.
+
+var badlyBehavedFlag = flag.Bool("badly-behaved", false, "If set, the program will ignore the stop signal and continue running.")
+
+func main() {
+	flag.Parse()
+
+	mode := "Well behaved"
+	if *badlyBehavedFlag {
+		mode = "Badly behaved"
+	}
+	fmt.Printf("%s process %d started.\n", mode, os.Getpid())
+
+	// Start a web server on a known port so that we can check that this process is
+	// not running, when it's been started as a child process, and we don't know
+	// its pid.
+	go func() {
+		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			fmt.Fprintf(w, "%d", os.Getpid())
+		})
+		err := http.ListenAndServe("127.0.0.1:7777", nil)
+		if err != nil {
+			fmt.Printf("Error running web server: %v\n", err)
+		}
+	}()
+
+	sigs := make(chan os.Signal, 1)
+	if !*badlyBehavedFlag {
+		signal.Notify(sigs, os.Interrupt, syscall.SIGTERM)
+	}
+	for {
+		select {
+		case <-sigs:
+			fmt.Printf("Process %d received signal. Stopping.\n", os.Getpid())
+			return
+		case <-time.After(1 * time.Second):
+			fmt.Printf("Process %d still running...\n", os.Getpid())
+		}
+	}
+}

templ/cmd/templ/generatecmd/sse/server.go

@@ -0,0 +1,84 @@
+package sse
+
+import (
+	_ "embed"
+	"fmt"
+	"net/http"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+func New() *Handler {
+	return &Handler{
+		m:        new(sync.Mutex),
+		requests: map[int64]chan event{},
+	}
+}
+
+type Handler struct {
+	m        *sync.Mutex
+	counter  int64
+	requests map[int64]chan event
+}
+
+type event struct {
+	Type string
+	Data string
+}
+
+// Send an event to all connected clients.
+func (s *Handler) Send(eventType string, data string) {
+	s.m.Lock()
+	defer s.m.Unlock()
+	for _, f := range s.requests {
+		f := f
+		go func(f chan event) {
+			f <- event{
+				Type: eventType,
+				Data: data,
+			}
+		}(f)
+	}
+}
+
+func (s *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Access-Control-Allow-Origin", "*")
+	w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+
+	id := atomic.AddInt64(&s.counter, 1)
+	s.m.Lock()
+	events := make(chan event)
+	s.requests[id] = events
+	s.m.Unlock()
+	defer func() {
+		s.m.Lock()
+		defer s.m.Unlock()
+		delete(s.requests, id)
+		close(events)
+	}()
+
+	timer := time.NewTimer(0)
+loop:
+	for {
+		select {
+		case <-timer.C:
+			if _, err := fmt.Fprintf(w, "event: message\ndata: ping\n\n"); err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+			timer.Reset(time.Second * 5)
+		case e := <-events:
+			if _, err := fmt.Fprintf(w, "event: %s\ndata: %s\n\n", e.Type, e.Data); err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+		case <-r.Context().Done():
+			break loop
+		}
+		w.(http.Flusher).Flush()
+	}
+}

templ/cmd/templ/generatecmd/symlink/symlink_test.go

@@ -0,0 +1,52 @@
+package symlink
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"os"
+	"path"
+	"testing"
+
+	"github.com/a-h/templ/cmd/templ/generatecmd"
+	"github.com/a-h/templ/cmd/templ/testproject"
+)
+
+func TestSymlink(t *testing.T) {
+	log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+	t.Run("can generate if root is symlink", func(t *testing.T) {
+		// templ generate -f templates.templ
+		dir, err := testproject.Create("github.com/a-h/templ/cmd/templ/testproject")
+		if err != nil {
+			t.Fatalf("failed to create test project: %v", err)
+		}
+		defer os.RemoveAll(dir)
+
+		symlinkPath := dir + "-symlink"
+		err = os.Symlink(dir, symlinkPath)
+		if err != nil {
+			t.Fatalf("failed to create dir symlink: %v", err)
+		}
+		defer os.Remove(symlinkPath)
+
+		// Delete the templates_templ.go file to ensure it is generated.
+		err = os.Remove(path.Join(symlinkPath, "templates_templ.go"))
+		if err != nil {
+			t.Fatalf("failed to remove templates_templ.go: %v", err)
+		}
+
+		// Run the generate command.
+		err = generatecmd.Run(context.Background(), log, generatecmd.Arguments{
+			Path: symlinkPath,
+		})
+		if err != nil {
+			t.Fatalf("failed to run generate command: %v", err)
+		}
+
+		// Check the templates_templ.go file was created.
+		_, err = os.Stat(path.Join(symlinkPath, "templates_templ.go"))
+		if err != nil {
+			t.Fatalf("templates_templ.go was not created: %v", err)
+		}
+	})
+}

templ/cmd/templ/generatecmd/test-eventhandler/eventhandler_test.go

@@ -0,0 +1,101 @@
+package testeventhandler
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/scanner"
+	"go/token"
+	"io"
+	"log/slog"
+	"os"
+	"testing"
+
+	"github.com/a-h/templ/cmd/templ/generatecmd"
+	"github.com/a-h/templ/generator"
+	"github.com/fsnotify/fsnotify"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestErrorLocationMapping(t *testing.T) {
+	tests := []struct {
+		name           string
+		rawFileName    string
+		errorPositions []token.Position
+	}{
+		{
+			name:        "single error outputs location in srcFile",
+			rawFileName: "single_error.templ.error",
+			errorPositions: []token.Position{
+				{Offset: 46, Line: 3, Column: 20},
+			},
+		},
+		{
+			name:        "multiple errors all output locations in srcFile",
+			rawFileName: "multiple_errors.templ.error",
+			errorPositions: []token.Position{
+				{Offset: 41, Line: 3, Column: 15},
+				{Offset: 101, Line: 7, Column: 22},
+				{Offset: 126, Line: 10, Column: 1},
+			},
+		},
+	}
+
+	slog := slog.New(slog.NewTextHandler(io.Discard, &slog.HandlerOptions{}))
+	var fw generatecmd.FileWriterFunc
+	fseh := generatecmd.NewFSEventHandler(slog, ".", false, []generator.GenerateOpt{}, false, false, fw, false)
+	for _, test := range tests {
+		// The raw files cannot end in .templ because they will cause the generator to fail. Instead,
+		// we create a tmp file that ends in .templ only for the duration of the test.
+		rawFile, err := os.Open(test.rawFileName)
+		if err != nil {
+			t.Errorf("%s: Failed to open file %s: %v", test.name, test.rawFileName, err)
+			break
+		}
+		file, err := os.CreateTemp("", fmt.Sprintf("*%s.templ", test.rawFileName))
+		if err != nil {
+			t.Errorf("%s: Failed to create a tmp file at %s: %v", test.name, file.Name(), err)
+			break
+		}
+		defer os.Remove(file.Name())
+		if _, err = io.Copy(file, rawFile); err != nil {
+			t.Errorf("%s: Failed to copy contents from raw file %s to tmp %s: %v", test.name, test.rawFileName, file.Name(), err)
+		}
+
+		event := fsnotify.Event{Name: file.Name(), Op: fsnotify.Write}
+		_, err = fseh.HandleEvent(context.Background(), event)
+		if err == nil {
+			t.Errorf("%s: no error was thrown", test.name)
+			break
+		}
+		list, ok := err.(scanner.ErrorList)
+		for !ok {
+			err = errors.Unwrap(err)
+			if err == nil {
+				t.Errorf("%s: reached end of error wrapping before finding an ErrorList", test.name)
+				break
+			} else {
+				list, ok = err.(scanner.ErrorList)
+			}
+		}
+		if !ok {
+			break
+		}
+
+		if len(list) != len(test.errorPositions) {
+			t.Errorf("%s: expected %d errors but got %d", test.name, len(test.errorPositions), len(list))
+			break
+		}
+		for i, err := range list {
+			test.errorPositions[i].Filename = file.Name()
+			diff := cmp.Diff(test.errorPositions[i], err.Pos)
+			if diff != "" {
+				t.Error(diff)
+				t.Error("expected:")
+				t.Error(test.errorPositions[i])
+				t.Error("actual:")
+				t.Error(err.Pos)
+			}
+		}
+	}
+}

templ/cmd/templ/generatecmd/test-eventhandler/multiple_errors.templ.error

@@ -0,0 +1,10 @@
+package testeventhandler
+
+func invalid(a: string) string {
+    return "foo"
+}
+
+templ multipleError(a: string) {
+    <div/>
+}
+l

templ/cmd/templ/generatecmd/test-eventhandler/single_error.templ.error

@@ -0,0 +1,5 @@
+package testeventhandler
+
+templ singleError(a: string) {
+    <div/>
+}

templ/cmd/templ/generatecmd/testwatch/generate_test.go

@@ -0,0 +1,485 @@
+package testwatch
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"embed"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/PuerkitoBio/goquery"
+	"github.com/a-h/templ/cmd/templ/generatecmd"
+	"github.com/a-h/templ/cmd/templ/generatecmd/modcheck"
+)
+
+//go:embed testdata/*
+var testdata embed.FS
+
+func createTestProject(moduleRoot string) (dir string, err error) {
+	dir, err = os.MkdirTemp("", "templ_watch_test_*")
+	if err != nil {
+		return dir, fmt.Errorf("failed to make test dir: %w", err)
+	}
+	files, err := testdata.ReadDir("testdata")
+	if err != nil {
+		return dir, fmt.Errorf("failed to read embedded dir: %w", err)
+	}
+	for _, file := range files {
+		src := filepath.Join("testdata", file.Name())
+		data, err := testdata.ReadFile(src)
+		if err != nil {
+			return dir, fmt.Errorf("failed to read file: %w", err)
+		}
+
+		target := filepath.Join(dir, file.Name())
+		if file.Name() == "go.mod.embed" {
+			data = bytes.ReplaceAll(data, []byte("{moduleRoot}"), []byte(moduleRoot))
+			target = filepath.Join(dir, "go.mod")
+		}
+		err = os.WriteFile(target, data, 0660)
+		if err != nil {
+			return dir, fmt.Errorf("failed to copy file: %w", err)
+		}
+	}
+	return dir, nil
+}
+
+func replaceInFile(name, src, tgt string) error {
+	data, err := os.ReadFile(name)
+	if err != nil {
+		return err
+	}
+	updated := strings.Replace(string(data), src, tgt, -1)
+	return os.WriteFile(name, []byte(updated), 0660)
+}
+
+func getPort() (port int, err error) {
+	var a *net.TCPAddr
+	if a, err = net.ResolveTCPAddr("tcp", "localhost:0"); err == nil {
+		var l *net.TCPListener
+		if l, err = net.ListenTCP("tcp", a); err == nil {
+			defer l.Close()
+			return l.Addr().(*net.TCPAddr).Port, nil
+		}
+	}
+	return
+}
+
+func getHTML(url string) (doc *goquery.Document, err error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get %q: %w", url, err)
+	}
+	return goquery.NewDocumentFromReader(resp.Body)
+}
+
+func TestCanAccessDirect(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+	args, teardown, err := Setup(false)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+
+	// Assert.
+	doc, err := getHTML(args.AppURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	countText := doc.Find(`div[data-testid="count"]`).Text()
+	actualCount, err := strconv.Atoi(countText)
+	if err != nil {
+		t.Fatalf("got count %q instead of integer", countText)
+	}
+	if actualCount < 1 {
+		t.Errorf("expected count >= 1, got %d", actualCount)
+	}
+}
+
+func TestCanAccessViaProxy(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+	args, teardown, err := Setup(false)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+
+	// Assert.
+	doc, err := getHTML(args.ProxyURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	countText := doc.Find(`div[data-testid="count"]`).Text()
+	actualCount, err := strconv.Atoi(countText)
+	if err != nil {
+		t.Fatalf("got count %q instead of integer", countText)
+	}
+	if actualCount < 1 {
+		t.Errorf("expected count >= 1, got %d", actualCount)
+	}
+}
+
+type Event struct {
+	Type string
+	Data string
+}
+
+func readSSE(ctx context.Context, url string, sse chan<- Event) (err error) {
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Cache-Control", "no-cache")
+	req.Header.Set("Accept", "text/event-stream")
+	req.Header.Set("Connection", "keep-alive")
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+
+	var e Event
+	scanner := bufio.NewScanner(resp.Body)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			sse <- e
+			e = Event{}
+			continue
+		}
+		if strings.HasPrefix(line, "event: ") {
+			e.Type = line[len("event: "):]
+		}
+		if strings.HasPrefix(line, "data: ") {
+			e.Data = line[len("data: "):]
+		}
+	}
+	return scanner.Err()
+}
+
+func TestFileModificationsResultInSSEWithGzip(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+	args, teardown, err := Setup(false)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+
+	// Start the SSE check.
+	events := make(chan Event)
+	var eventsErr error
+	go func() {
+		eventsErr = readSSE(context.Background(), fmt.Sprintf("%s/_templ/reload/events", args.ProxyURL), events)
+	}()
+
+	// Assert data is expected.
+	doc, err := getHTML(args.ProxyURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	if text := doc.Find(`div[data-testid="modification"]`).Text(); text != "Original" {
+		t.Errorf("expected %q, got %q", "Original", text)
+	}
+
+	// Change file.
+	templFile := filepath.Join(args.AppDir, "templates.templ")
+	err = replaceInFile(templFile,
+		`<div data-testid="modification">Original</div>`,
+		`<div data-testid="modification">Updated</div>`)
+	if err != nil {
+		t.Errorf("failed to replace text in file: %v", err)
+	}
+
+	// Give the filesystem watcher a few seconds.
+	var reloadCount int
+loop:
+	for {
+		select {
+		case event := <-events:
+			if event.Data == "reload" {
+				reloadCount++
+				break loop
+			}
+		case <-time.After(time.Second * 5):
+			break loop
+		}
+	}
+	if reloadCount == 0 {
+		t.Error("failed to receive SSE about update after 5 seconds")
+	}
+
+	// Check to see if there were any errors.
+	if eventsErr != nil {
+		t.Errorf("error reading events: %v", err)
+	}
+
+	// See results in browser immediately.
+	doc, err = getHTML(args.ProxyURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	if text := doc.Find(`div[data-testid="modification"]`).Text(); text != "Updated" {
+		t.Errorf("expected %q, got %q", "Updated", text)
+	}
+}
+
+func TestFileModificationsResultInSSE(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+	args, teardown, err := Setup(false)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+
+	// Start the SSE check.
+	events := make(chan Event)
+	var eventsErr error
+	go func() {
+		eventsErr = readSSE(context.Background(), fmt.Sprintf("%s/_templ/reload/events", args.ProxyURL), events)
+	}()
+
+	// Assert data is expected.
+	doc, err := getHTML(args.ProxyURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	if text := doc.Find(`div[data-testid="modification"]`).Text(); text != "Original" {
+		t.Errorf("expected %q, got %q", "Original", text)
+	}
+
+	// Change file.
+	templFile := filepath.Join(args.AppDir, "templates.templ")
+	err = replaceInFile(templFile,
+		`<div data-testid="modification">Original</div>`,
+		`<div data-testid="modification">Updated</div>`)
+	if err != nil {
+		t.Errorf("failed to replace text in file: %v", err)
+	}
+
+	// Give the filesystem watcher a few seconds.
+	var reloadCount int
+loop:
+	for {
+		select {
+		case event := <-events:
+			if event.Data == "reload" {
+				reloadCount++
+				break loop
+			}
+		case <-time.After(time.Second * 5):
+			break loop
+		}
+	}
+	if reloadCount == 0 {
+		t.Error("failed to receive SSE about update after 5 seconds")
+	}
+
+	// Check to see if there were any errors.
+	if eventsErr != nil {
+		t.Errorf("error reading events: %v", err)
+	}
+
+	// See results in browser immediately.
+	doc, err = getHTML(args.ProxyURL)
+	if err != nil {
+		t.Fatalf("failed to read HTML: %v", err)
+	}
+	if text := doc.Find(`div[data-testid="modification"]`).Text(); text != "Updated" {
+		t.Errorf("expected %q, got %q", "Updated", text)
+	}
+}
+
+func NewTestArgs(modRoot, appDir string, appPort int, proxyBind string, proxyPort int) TestArgs {
+	return TestArgs{
+		ModRoot:   modRoot,
+		AppDir:    appDir,
+		AppPort:   appPort,
+		AppURL:    fmt.Sprintf("http://localhost:%d", appPort),
+		ProxyBind: proxyBind,
+		ProxyPort: proxyPort,
+		ProxyURL:  fmt.Sprintf("http://%s:%d", proxyBind, proxyPort),
+	}
+}
+
+type TestArgs struct {
+	ModRoot   string
+	AppDir    string
+	AppPort   int
+	AppURL    string
+	ProxyBind string
+	ProxyPort int
+	ProxyURL  string
+}
+
+func Setup(gzipEncoding bool) (args TestArgs, teardown func(t *testing.T), err error) {
+	wd, err := os.Getwd()
+	if err != nil {
+		return args, teardown, fmt.Errorf("could not find working dir: %w", err)
+	}
+	moduleRoot, err := modcheck.WalkUp(wd)
+	if err != nil {
+		return args, teardown, fmt.Errorf("could not find local templ go.mod file: %v", err)
+	}
+
+	appDir, err := createTestProject(moduleRoot)
+	if err != nil {
+		return args, teardown, fmt.Errorf("failed to create test project: %v", err)
+	}
+	appPort, err := getPort()
+	if err != nil {
+		return args, teardown, fmt.Errorf("failed to get available port: %v", err)
+	}
+	proxyPort, err := getPort()
+	if err != nil {
+		return args, teardown, fmt.Errorf("failed to get available port: %v", err)
+	}
+	proxyBind := "localhost"
+
+	args = NewTestArgs(moduleRoot, appDir, appPort, proxyBind, proxyPort)
+	ctx, cancel := context.WithCancel(context.Background())
+
+	var wg sync.WaitGroup
+	var cmdErr error
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		command := fmt.Sprintf("go run . -port %d", args.AppPort)
+		if gzipEncoding {
+			command += " -gzip true"
+		}
+
+		log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+
+		cmdErr = generatecmd.Run(ctx, log, generatecmd.Arguments{
+			Path:                            appDir,
+			Watch:                           true,
+			OpenBrowser:                     false,
+			Command:                         command,
+			ProxyBind:                       proxyBind,
+			ProxyPort:                       proxyPort,
+			Proxy:                           args.AppURL,
+			NotifyProxy:                     false,
+			WorkerCount:                     0,
+			GenerateSourceMapVisualisations: false,
+			IncludeVersion:                  false,
+			IncludeTimestamp:                false,
+			PPROFPort:                       0,
+			KeepOrphanedFiles:               false,
+		})
+	}()
+
+	// Wait for server to start.
+	if err = waitForURL(args.AppURL); err != nil {
+		cancel()
+		wg.Wait()
+		return args, teardown, fmt.Errorf("failed to start app server, command error %v: %w", cmdErr, err)
+	}
+	if err = waitForURL(args.ProxyURL); err != nil {
+		cancel()
+		wg.Wait()
+		return args, teardown, fmt.Errorf("failed to start proxy server, command error %v: %w", cmdErr, err)
+	}
+
+	// Wait for exit.
+	teardown = func(t *testing.T) {
+		cancel()
+		wg.Wait()
+		if cmdErr != nil {
+			t.Errorf("failed to run generate cmd: %v", err)
+		}
+
+		if err = os.RemoveAll(appDir); err != nil {
+			t.Fatalf("failed to remove test dir %q: %v", appDir, err)
+		}
+	}
+	return args, teardown, err
+}
+
+func waitForURL(url string) (err error) {
+	var tries int
+	for {
+		time.Sleep(time.Second)
+		if tries > 20 {
+			return err
+		}
+		tries++
+		var resp *http.Response
+		resp, err = http.Get(url)
+		if err != nil {
+			fmt.Printf("failed to get %q: %v\n", url, err)
+			continue
+		}
+		if resp.StatusCode != http.StatusOK {
+			fmt.Printf("failed to get %q: %v\n", url, err)
+			err = fmt.Errorf("expected status code %d, got %d", http.StatusOK, resp.StatusCode)
+			continue
+		}
+		return nil
+	}
+}
+
+func TestGenerateReturnsErrors(t *testing.T) {
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("could not find working dir: %v", err)
+	}
+	moduleRoot, err := modcheck.WalkUp(wd)
+	if err != nil {
+		t.Fatalf("could not find local templ go.mod file: %v", err)
+	}
+
+	appDir, err := createTestProject(moduleRoot)
+	if err != nil {
+		t.Fatalf("failed to create test project: %v", err)
+	}
+	defer func() {
+		if err = os.RemoveAll(appDir); err != nil {
+			t.Fatalf("failed to remove test dir %q: %v", appDir, err)
+		}
+	}()
+
+	// Break the HTML.
+	templFile := filepath.Join(appDir, "templates.templ")
+	err = replaceInFile(templFile,
+		`<div data-testid="modification">Original</div>`,
+		`<div data-testid="modification" -unclosed div-</div>`)
+	if err != nil {
+		t.Errorf("failed to replace text in file: %v", err)
+	}
+
+	log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+
+	// Run.
+	err = generatecmd.Run(context.Background(), log, generatecmd.Arguments{
+		Path:              appDir,
+		Watch:             false,
+		IncludeVersion:    false,
+		IncludeTimestamp:  false,
+		KeepOrphanedFiles: false,
+	})
+	if err == nil {
+		t.Errorf("expected generation error, got %v", err)
+	}
+}

templ/cmd/templ/generatecmd/testwatch/testdata/go.mod.embed

@@ -0,0 +1,7 @@
+module templ/testproject
+
+go 1.23
+
+require github.com/a-h/templ v0.2.513 // indirect
+
+replace github.com/a-h/templ => {moduleRoot}

templ/cmd/templ/generatecmd/testwatch/testdata/go.sum

@@ -0,0 +1,2 @@
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=

templ/cmd/templ/generatecmd/testwatch/testdata/main.go

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"bytes"
+	"compress/gzip"
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"strconv"
+
+	"github.com/a-h/templ"
+)
+
+type GzipResponseWriter struct {
+	w http.ResponseWriter
+}
+
+func (w *GzipResponseWriter) Header() http.Header {
+	return w.w.Header()
+}
+
+func (w *GzipResponseWriter) Write(b []byte) (int, error) {
+	var buf bytes.Buffer
+	gzw := gzip.NewWriter(&buf)
+	defer gzw.Close()
+
+	_, err := gzw.Write(b)
+	if err != nil {
+		return 0, err
+	}
+	err = gzw.Close()
+	if err != nil {
+		return 0, err
+	}
+
+	w.w.Header().Set("Content-Length", strconv.Itoa(buf.Len()))
+
+	return w.w.Write(buf.Bytes())
+}
+
+func (w *GzipResponseWriter) WriteHeader(statusCode int) {
+	w.w.WriteHeader(statusCode)
+}
+
+var flagPort = flag.Int("port", 0, "Set the HTTP listen port")
+var useGzip = flag.Bool("gzip", false, "Toggle gzip encoding")
+
+func main() {
+	flag.Parse()
+
+	if *flagPort == 0 {
+		fmt.Println("missing port flag")
+		os.Exit(1)
+	}
+
+	var count int
+	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if useGzip != nil && *useGzip {
+			w.Header().Set("Content-Encoding", "gzip")
+			w = &GzipResponseWriter{w: w}
+		}
+
+		count++
+		c := Page(count)
+		h := templ.Handler(c)
+		h.ErrorHandler = func(r *http.Request, err error) http.Handler {
+			slog.Error("failed to render template", slog.Any("error", err))
+			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+			})
+		}
+		h.ServeHTTP(w, r)
+	})
+	err := http.ListenAndServe(fmt.Sprintf("localhost:%d", *flagPort), nil)
+	if err != nil {
+		fmt.Printf("Error listening: %v\n", err)
+		os.Exit(1)
+	}
+}

templ/cmd/templ/generatecmd/testwatch/testdata/templates.templ

@@ -0,0 +1,17 @@
+package main
+
+import "fmt"
+
+templ Page(count int) {
+	<!DOCTYPE html>
+	<html>
+		<head>
+			<title>templ test page</title>
+		</head>
+		<body>
+			<h1>Count</h1>
+			<div data-testid="count">{ fmt.Sprintf("%d", count) }</div>
+			<div data-testid="modification">Original</div>
+		</body>
+	</html>
+}

templ/cmd/templ/generatecmd/testwatch/testdata/templates_templ.go

@@ -0,0 +1,55 @@
+// Code generated by templ - DO NOT EDIT.
+
+// templ: version: v0.3.833
+package main
+
+//lint:file-ignore SA4006 This context is only used if a nested component is present.
+
+import "github.com/a-h/templ"
+import templruntime "github.com/a-h/templ/runtime"
+
+import "fmt"
+
+func Page(count int) templ.Component {
+	return templruntime.GeneratedTemplate(func(templ_7745c5c3_Input templruntime.GeneratedComponentInput) (templ_7745c5c3_Err error) {
+		templ_7745c5c3_W, ctx := templ_7745c5c3_Input.Writer, templ_7745c5c3_Input.Context
+		if templ_7745c5c3_CtxErr := ctx.Err(); templ_7745c5c3_CtxErr != nil {
+			return templ_7745c5c3_CtxErr
+		}
+		templ_7745c5c3_Buffer, templ_7745c5c3_IsBuffer := templruntime.GetBuffer(templ_7745c5c3_W)
+		if !templ_7745c5c3_IsBuffer {
+			defer func() {
+				templ_7745c5c3_BufErr := templruntime.ReleaseBuffer(templ_7745c5c3_Buffer)
+				if templ_7745c5c3_Err == nil {
+					templ_7745c5c3_Err = templ_7745c5c3_BufErr
+				}
+			}()
+		}
+		ctx = templ.InitializeContext(ctx)
+		templ_7745c5c3_Var1 := templ.GetChildren(ctx)
+		if templ_7745c5c3_Var1 == nil {
+			templ_7745c5c3_Var1 = templ.NopComponent
+		}
+		ctx = templ.ClearChildren(ctx)
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 1, "<!doctype html><html><head><title>templ test page</title></head><body><h1>Count</h1><div data-testid=\"count\">")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		var templ_7745c5c3_Var2 string
+		templ_7745c5c3_Var2, templ_7745c5c3_Err = templ.JoinStringErrs(fmt.Sprintf("%d", count))
+		if templ_7745c5c3_Err != nil {
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templ/cmd/templ/generatecmd/testwatch/testdata/templates.templ`, Line: 13, Col: 54}
+		}
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var2))
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 2, "</div><div data-testid=\"modification\">Original</div></body></html>")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		return nil
+	})
+}
+
+var _ = templruntime.GeneratedTemplate

templ/cmd/templ/generatecmd/watcher/watch.go

@@ -0,0 +1,166 @@
+package watcher
+
+import (
+	"context"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+func Recursive(
+	ctx context.Context,
+	path string,
+	watchPattern *regexp.Regexp,
+	out chan fsnotify.Event,
+	errors chan error,
+) (w *RecursiveWatcher, err error) {
+	fsnw, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, err
+	}
+	w = NewRecursiveWatcher(ctx, fsnw, watchPattern, out, errors)
+	go w.loop()
+	return w, w.Add(path)
+}
+
+func NewRecursiveWatcher(ctx context.Context, w *fsnotify.Watcher, watchPattern *regexp.Regexp, events chan fsnotify.Event, errors chan error) *RecursiveWatcher {
+	return &RecursiveWatcher{
+		ctx:          ctx,
+		w:            w,
+		WatchPattern: watchPattern,
+		Events:       events,
+		Errors:       errors,
+		timers:       make(map[timerKey]*time.Timer),
+	}
+}
+
+// WalkFiles walks the file tree rooted at path, sending a Create event for each
+// file it encounters.
+func WalkFiles(ctx context.Context, path string, watchPattern *regexp.Regexp, out chan fsnotify.Event) (err error) {
+	rootPath := path
+	fileSystem := os.DirFS(rootPath)
+	return fs.WalkDir(fileSystem, ".", func(path string, info os.DirEntry, err error) error {
+		if err != nil {
+			return nil
+		}
+		absPath, err := filepath.Abs(filepath.Join(rootPath, path))
+		if err != nil {
+			return nil
+		}
+		if info.IsDir() && shouldSkipDir(absPath) {
+			return filepath.SkipDir
+		}
+		if !watchPattern.MatchString(absPath) {
+			return nil
+		}
+		out <- fsnotify.Event{
+			Name: absPath,
+			Op:   fsnotify.Create,
+		}
+		return nil
+	})
+}
+
+type RecursiveWatcher struct {
+	ctx          context.Context
+	w            *fsnotify.Watcher
+	WatchPattern *regexp.Regexp
+	Events       chan fsnotify.Event
+	Errors       chan error
+	timerMu      sync.Mutex
+	timers       map[timerKey]*time.Timer
+}
+
+type timerKey struct {
+	name string
+	op   fsnotify.Op
+}
+
+func timerKeyFromEvent(event fsnotify.Event) timerKey {
+	return timerKey{
+		name: event.Name,
+		op:   event.Op,
+	}
+}
+
+func (w *RecursiveWatcher) Close() error {
+	return w.w.Close()
+}
+
+func (w *RecursiveWatcher) loop() {
+	for {
+		select {
+		case <-w.ctx.Done():
+			return
+		case event, ok := <-w.w.Events:
+			if !ok {
+				return
+			}
+			if event.Has(fsnotify.Create) {
+				if err := w.Add(event.Name); err != nil {
+					w.Errors <- err
+				}
+			}
+			// Only notify on templ related files.
+			if !w.WatchPattern.MatchString(event.Name) {
+				continue
+			}
+			tk := timerKeyFromEvent(event)
+			w.timerMu.Lock()
+			t, ok := w.timers[tk]
+			w.timerMu.Unlock()
+			if !ok {
+				t = time.AfterFunc(100*time.Millisecond, func() {
+					w.Events <- event
+				})
+				w.timerMu.Lock()
+				w.timers[tk] = t
+				w.timerMu.Unlock()
+				continue
+			}
+			t.Reset(100 * time.Millisecond)
+		case err, ok := <-w.w.Errors:
+			if !ok {
+				return
+			}
+			w.Errors <- err
+		}
+	}
+}
+
+func (w *RecursiveWatcher) Add(dir string) error {
+	return filepath.WalkDir(dir, func(dir string, info os.DirEntry, err error) error {
+		if err != nil {
+			return nil
+		}
+		if !info.IsDir() {
+			return nil
+		}
+		if shouldSkipDir(dir) {
+			return filepath.SkipDir
+		}
+		return w.w.Add(dir)
+	})
+}
+
+func shouldSkipDir(dir string) bool {
+	if dir == "." {
+		return false
+	}
+	if dir == "vendor" || dir == "node_modules" {
+		return true
+	}
+	_, name := path.Split(dir)
+	// These directories are ignored by the Go tool.
+	if strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+		return true
+	}
+	return false
+}

templ/cmd/templ/generatecmd/watcher/watch_test.go

@@ -0,0 +1,133 @@
+package watcher
+
+import (
+	"context"
+	"fmt"
+	"regexp"
+	"testing"
+	"time"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+func TestWatchDebouncesDuplicates(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	w := &fsnotify.Watcher{
+		Events: make(chan fsnotify.Event),
+	}
+	events := make(chan fsnotify.Event, 2)
+	errors := make(chan error)
+	watchPattern, err := regexp.Compile(".*")
+	if err != nil {
+		t.Fatal(fmt.Errorf("failed to compile watch pattern: %w", err))
+	}
+	rw := NewRecursiveWatcher(ctx, w, watchPattern, events, errors)
+	go func() {
+		rw.w.Events <- fsnotify.Event{Name: "test.templ"}
+		rw.w.Events <- fsnotify.Event{Name: "test.templ"}
+		cancel()
+		close(rw.w.Events)
+	}()
+	rw.loop()
+	count := 0
+	exp := time.After(300 * time.Millisecond)
+	for {
+		select {
+		case <-rw.Events:
+			count++
+		case <-exp:
+			if count != 1 {
+				t.Errorf("expected 1 event, got %d", count)
+			}
+			return
+		}
+	}
+}
+
+func TestWatchDoesNotDebounceDifferentEvents(t *testing.T) {
+	tests := []struct {
+		event1 fsnotify.Event
+		event2 fsnotify.Event
+	}{
+		// Different files
+		{fsnotify.Event{Name: "test.templ"}, fsnotify.Event{Name: "test2.templ"}},
+		// Different operations
+		{
+			fsnotify.Event{Name: "test.templ", Op: fsnotify.Create},
+			fsnotify.Event{Name: "test.templ", Op: fsnotify.Write},
+		},
+		// Different operations and files
+		{
+			fsnotify.Event{Name: "test.templ", Op: fsnotify.Create},
+			fsnotify.Event{Name: "test2.templ", Op: fsnotify.Write},
+		},
+	}
+	for _, test := range tests {
+		ctx, cancel := context.WithCancel(context.Background())
+		w := &fsnotify.Watcher{
+			Events: make(chan fsnotify.Event),
+		}
+		events := make(chan fsnotify.Event, 2)
+		errors := make(chan error)
+		watchPattern, err := regexp.Compile(".*")
+		if err != nil {
+			t.Fatal(fmt.Errorf("failed to compile watch pattern: %w", err))
+		}
+		rw := NewRecursiveWatcher(ctx, w, watchPattern, events, errors)
+		go func() {
+			rw.w.Events <- test.event1
+			rw.w.Events <- test.event2
+			cancel()
+			close(rw.w.Events)
+		}()
+		rw.loop()
+		count := 0
+		exp := time.After(300 * time.Millisecond)
+		for {
+			select {
+			case <-rw.Events:
+				count++
+			case <-exp:
+				if count != 2 {
+					t.Errorf("expected 2 event, got %d", count)
+				}
+				return
+			}
+		}
+	}
+}
+
+func TestWatchDoesNotDebounceSeparateEvents(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	w := &fsnotify.Watcher{
+		Events: make(chan fsnotify.Event),
+	}
+	events := make(chan fsnotify.Event, 2)
+	errors := make(chan error)
+	watchPattern, err := regexp.Compile(".*")
+	if err != nil {
+		t.Fatal(fmt.Errorf("failed to compile watch pattern: %w", err))
+	}
+	rw := NewRecursiveWatcher(ctx, w, watchPattern, events, errors)
+	go func() {
+		rw.w.Events <- fsnotify.Event{Name: "test.templ"}
+		<-time.After(200 * time.Millisecond)
+		rw.w.Events <- fsnotify.Event{Name: "test.templ"}
+		cancel()
+		close(rw.w.Events)
+	}()
+	rw.loop()
+	count := 0
+	exp := time.After(500 * time.Millisecond)
+	for {
+		select {
+		case <-rw.Events:
+			count++
+		case <-exp:
+			if count != 2 {
+				t.Errorf("expected 2 event, got %d", count)
+			}
+			return
+		}
+	}
+}

templ/cmd/templ/imports/process.go

@@ -0,0 +1,174 @@
+package imports
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/format"
+	"go/token"
+	"path"
+	"slices"
+	"strconv"
+	"strings"
+
+	goparser "go/parser"
+
+	"golang.org/x/sync/errgroup"
+	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/imports"
+
+	"github.com/a-h/templ/generator"
+	"github.com/a-h/templ/parser/v2"
+)
+
+var internalImports = []string{"github.com/a-h/templ", "github.com/a-h/templ/runtime"}
+
+func convertTemplToGoURI(templURI string) (isTemplFile bool, goURI string) {
+	base, fileName := path.Split(templURI)
+	if !strings.HasSuffix(fileName, ".templ") {
+		return
+	}
+	return true, base + (strings.TrimSuffix(fileName, ".templ") + "_templ.go")
+}
+
+var fset = token.NewFileSet()
+
+func updateImports(name, src string) (updated []*ast.ImportSpec, err error) {
+	// Apply auto imports.
+	updatedGoCode, err := imports.Process(name, []byte(src), nil)
+	if err != nil {
+		return updated, fmt.Errorf("failed to process go code %q: %w", src, err)
+	}
+	// Get updated imports.
+	gofile, err := goparser.ParseFile(fset, name, updatedGoCode, goparser.ImportsOnly)
+	if err != nil {
+		return updated, fmt.Errorf("failed to get imports from updated go code: %w", err)
+	}
+	for _, imp := range gofile.Imports {
+		if !slices.Contains(internalImports, strings.Trim(imp.Path.Value, "\"")) {
+			updated = append(updated, imp)
+		}
+	}
+	return updated, nil
+}
+
+func Process(t parser.TemplateFile) (parser.TemplateFile, error) {
+	if t.Filepath == "" {
+		return t, nil
+	}
+	isTemplFile, fileName := convertTemplToGoURI(t.Filepath)
+	if !isTemplFile {
+		return t, fmt.Errorf("invalid filepath: %s", t.Filepath)
+	}
+
+	// The first node always contains existing imports.
+	// If there isn't one, create it.
+	if len(t.Nodes) == 0 {
+		t.Nodes = append(t.Nodes, parser.TemplateFileGoExpression{})
+	}
+	// If there is one, ensure it is a Go expression.
+	if _, ok := t.Nodes[0].(parser.TemplateFileGoExpression); !ok {
+		t.Nodes = append([]parser.TemplateFileNode{parser.TemplateFileGoExpression{}}, t.Nodes...)
+	}
+
+	// Find all existing imports.
+	importsNode := t.Nodes[0].(parser.TemplateFileGoExpression)
+
+	// Generate code.
+	gw := bytes.NewBuffer(nil)
+	var updatedImports []*ast.ImportSpec
+	var eg errgroup.Group
+	eg.Go(func() (err error) {
+		if _, err := generator.Generate(t, gw); err != nil {
+			return fmt.Errorf("failed to generate go code: %w", err)
+		}
+		updatedImports, err = updateImports(fileName, gw.String())
+		if err != nil {
+			return fmt.Errorf("failed to get imports from generated go code: %w", err)
+		}
+		return nil
+	})
+
+	var firstGoNodeInTemplate *ast.File
+	// Update the template with the imports.
+	// Ensure that there is a Go expression to add the imports to as the first node.
+	eg.Go(func() (err error) {
+		firstGoNodeInTemplate, err = goparser.ParseFile(fset, fileName, t.Package.Expression.Value+"\n"+importsNode.Expression.Value, goparser.AllErrors|goparser.ParseComments)
+		if err != nil {
+			return fmt.Errorf("failed to parse imports section: %w", err)
+		}
+		return nil
+	})
+
+	// Wait for completion of both parts.
+	if err := eg.Wait(); err != nil {
+		return t, err
+	}
+	// Delete unused imports.
+	for _, imp := range firstGoNodeInTemplate.Imports {
+		if !containsImport(updatedImports, imp) {
+			name, path, err := getImportDetails(imp)
+			if err != nil {
+				return t, err
+			}
+			astutil.DeleteNamedImport(fset, firstGoNodeInTemplate, name, path)
+		}
+	}
+	// Add imports, if there are any to add.
+	for _, imp := range updatedImports {
+		if !containsImport(firstGoNodeInTemplate.Imports, imp) {
+			name, path, err := getImportDetails(imp)
+			if err != nil {
+				return t, err
+			}
+			astutil.AddNamedImport(fset, firstGoNodeInTemplate, name, path)
+		}
+	}
+	// Edge case: reinsert the import to use import syntax without parentheses.
+	if len(firstGoNodeInTemplate.Imports) == 1 {
+		name, path, err := getImportDetails(firstGoNodeInTemplate.Imports[0])
+		if err != nil {
+			return t, err
+		}
+		astutil.DeleteNamedImport(fset, firstGoNodeInTemplate, name, path)
+		astutil.AddNamedImport(fset, firstGoNodeInTemplate, name, path)
+	}
+	// Write out the Go code with the imports.
+	updatedGoCode := new(strings.Builder)
+	err := format.Node(updatedGoCode, fset, firstGoNodeInTemplate)
+	if err != nil {
+		return t, fmt.Errorf("failed to write updated go code: %w", err)
+	}
+	// Remove the package statement from the node, by cutting the first line of the file.
+	importsNode.Expression.Value = strings.TrimSpace(strings.SplitN(updatedGoCode.String(), "\n", 2)[1])
+	if len(updatedImports) == 0 && importsNode.Expression.Value == "" {
+		t.Nodes = t.Nodes[1:]
+		return t, nil
+	}
+	t.Nodes[0] = importsNode
+	return t, nil
+}
+
+func getImportDetails(imp *ast.ImportSpec) (name, importPath string, err error) {
+	if imp.Name != nil {
+		name = imp.Name.Name
+	}
+	if imp.Path != nil {
+		importPath, err = strconv.Unquote(imp.Path.Value)
+		if err != nil {
+			err = fmt.Errorf("failed to unquote package path %s: %w", imp.Path.Value, err)
+			return
+		}
+	}
+	return name, importPath, nil
+}
+
+func containsImport(imports []*ast.ImportSpec, spec *ast.ImportSpec) bool {
+	for _, imp := range imports {
+		if imp.Path.Value == spec.Path.Value {
+			return true
+		}
+	}
+
+	return false
+}

templ/cmd/templ/imports/process_test.go

@@ -0,0 +1,154 @@
+package imports
+
+import (
+	"bytes"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/a-h/templ/cmd/templ/testproject"
+	"github.com/a-h/templ/parser/v2"
+	"github.com/google/go-cmp/cmp"
+	"golang.org/x/tools/txtar"
+)
+
+func TestFormatting(t *testing.T) {
+	files, _ := filepath.Glob("testdata/*.txtar")
+	if len(files) == 0 {
+		t.Errorf("no test files found")
+	}
+	for _, file := range files {
+		t.Run(filepath.Base(file), func(t *testing.T) {
+			a, err := txtar.ParseFile(file)
+			if err != nil {
+				t.Fatalf("failed to parse txtar file: %v", err)
+			}
+			if len(a.Files) != 2 {
+				t.Fatalf("expected 2 files, got %d", len(a.Files))
+			}
+			template, err := parser.ParseString(clean(a.Files[0].Data))
+			if err != nil {
+				t.Fatalf("failed to parse %v", err)
+			}
+			template.Filepath = a.Files[0].Name
+			tf, err := Process(template)
+			if err != nil {
+				t.Fatalf("failed to process file: %v", err)
+			}
+			expected := string(a.Files[1].Data)
+			actual := new(strings.Builder)
+			if err := tf.Write(actual); err != nil {
+				t.Fatalf("failed to write template file: %v", err)
+			}
+			if diff := cmp.Diff(expected, actual.String()); diff != "" {
+				t.Errorf("%s:\n%s", file, diff)
+				t.Errorf("expected:\n%s", showWhitespace(expected))
+				t.Errorf("actual:\n%s", showWhitespace(actual.String()))
+			}
+		})
+	}
+}
+
+func showWhitespace(s string) string {
+	s = strings.ReplaceAll(s, "\n", "⏎\n")
+	s = strings.ReplaceAll(s, "\t", "→")
+	s = strings.ReplaceAll(s, " ", "·")
+	return s
+}
+
+func clean(b []byte) string {
+	b = bytes.ReplaceAll(b, []byte("$\n"), []byte("\n"))
+	b = bytes.TrimSuffix(b, []byte("\n"))
+	return string(b)
+}
+
+func TestImport(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+		return
+	}
+
+	tests := []struct {
+		name       string
+		src        string
+		assertions func(t *testing.T, updated string)
+	}{
+		{
+			name: "un-named imports are removed",
+			src: `package main
+
+import "fmt"
+import "github.com/a-h/templ/cmd/templ/testproject/css-classes"
+
+templ Page(count int) {
+	{ fmt.Sprintf("%d", count) }
+	{ cssclasses.Header }
+}
+`,
+			assertions: func(t *testing.T, updated string) {
+				if count := strings.Count(updated, "github.com/a-h/templ/cmd/templ/testproject/css-classes"); count != 0 {
+					t.Errorf("expected un-named import to be removed, but got %d instance of it", count)
+				}
+			},
+		},
+		{
+			name: "named imports are retained",
+			src: `package main
+
+import "fmt"
+import  cssclasses "github.com/a-h/templ/cmd/templ/testproject/css-classes"
+
+templ Page(count int) {
+	{ fmt.Sprintf("%d", count) }
+	{ cssclasses.Header }
+}
+`,
+			assertions: func(t *testing.T, updated string) {
+				if count := strings.Count(updated, "cssclasses \"github.com/a-h/templ/cmd/templ/testproject/css-classes\""); count != 1 {
+					t.Errorf("expected named import to be retained, got %d instances of it", count)
+				}
+				if count := strings.Count(updated, "github.com/a-h/templ/cmd/templ/testproject/css-classes"); count != 1 {
+					t.Errorf("expected one import, got %d", count)
+				}
+			},
+		},
+	}
+
+	for _, test := range tests {
+		// Create test project.
+		dir, err := testproject.Create("github.com/a-h/templ/cmd/templ/testproject")
+		if err != nil {
+			t.Fatalf("failed to create test project: %v", err)
+		}
+		defer os.RemoveAll(dir)
+
+		// Load the templates.templ file.
+		filePath := path.Join(dir, "templates.templ")
+		err = os.WriteFile(filePath, []byte(test.src), 0660)
+		if err != nil {
+			t.Fatalf("failed to write file: %v", err)
+		}
+
+		// Parse the new file.
+		template, err := parser.Parse(filePath)
+		if err != nil {
+			t.Fatalf("failed to parse %v", err)
+		}
+		template.Filepath = filePath
+		tf, err := Process(template)
+		if err != nil {
+			t.Fatalf("failed to process file: %v", err)
+		}
+
+		// Write it back out after processing.
+		buf := new(strings.Builder)
+		if err := tf.Write(buf); err != nil {
+			t.Fatalf("failed to write template file: %v", err)
+		}
+
+		// Assert.
+		test.assertions(t, buf.String())
+	}
+}

templ/cmd/templ/imports/testdata/comments.txtar

@@ -0,0 +1,12 @@
+-- fmt_templ.templ --
+package test
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")
+-- fmt_templ.templ --
+package test
+
+import "fmt"
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")

templ/cmd/templ/imports/testdata/commentsbeforepackage.txtar

@@ -0,0 +1,28 @@
+-- fmt_templ.templ --
+// Comments before.
+/*
+  Some more comments
+*/
+package test
+
+templ test() {
+  <div>Hello</div>
+}
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")
+-- fmt_templ.templ --
+// Comments before.
+/*
+  Some more comments
+*/
+package test
+
+import "fmt"
+
+templ test() {
+	<div>Hello</div>
+}
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")

templ/cmd/templ/imports/testdata/deleteimports.txtar

@@ -0,0 +1,14 @@
+-- fmt.templ --
+package test
+
+import "strconv"
+
+templ Hello() {
+	<div>Hello</div>
+}
+-- fmt.templ --
+package test
+
+templ Hello() {
+	<div>Hello</div>
+}

templ/cmd/templ/imports/testdata/extraspace.txtar

@@ -0,0 +1,15 @@
+-- fmt_templ.templ --
+package test
+
+const x = 123
+
+
+var x = fmt.Sprintf("Hello")
+-- fmt_templ.templ --
+package test
+
+import "fmt"
+
+const x = 123
+
+var x = fmt.Sprintf("Hello")

templ/cmd/templ/imports/testdata/groups.txtar

@@ -0,0 +1,22 @@
+-- fmt.templ --
+package test
+
+import (
+	"strings"
+	"fmt"
+
+	"strconv"
+)
+
+var _, _ = fmt.Print(strings.Contains(strconv.Quote("Hello"), ""))
+-- fmt.templ --
+package test
+
+import (
+	"fmt"
+	"strings"
+
+	"strconv"
+)
+
+var _, _ = fmt.Print(strings.Contains(strconv.Quote("Hello"), ""))

templ/cmd/templ/imports/testdata/groupsmanynewlines.txtar

@@ -0,0 +1,21 @@
+-- fmt.templ --
+package test
+
+import (
+	"fmt"
+
+
+	"strconv"
+)
+
+var _, _ = fmt.Print(strconv.Quote("Hello"))
+-- fmt.templ --
+package test
+
+import (
+	"fmt"
+
+	"strconv"
+)
+
+var _, _ = fmt.Print(strconv.Quote("Hello"))

templ/cmd/templ/imports/testdata/header.txtar

@@ -0,0 +1,10 @@
+-- fmt_templ.templ --
+package test
+
+var x = fmt.Sprintf("Hello")
+-- fmt_templ.templ --
+package test
+
+import "fmt"
+
+var x = fmt.Sprintf("Hello")

templ/cmd/templ/imports/testdata/namedimportsadd.txtar

@@ -0,0 +1,19 @@
+-- fmt_templ.templ --
+package test
+
+import (
+  sconv "strconv"
+)
+
+// Comment on variable or function.
+var x = fmt.Sprintf(sconv.Quote("Hello"))
+-- fmt_templ.templ --
+package test
+
+import (
+	"fmt"
+	sconv "strconv"
+)
+
+// Comment on variable or function.
+var x = fmt.Sprintf(sconv.Quote("Hello"))

templ/cmd/templ/imports/testdata/namedimportsremoved.txtar

@@ -0,0 +1,16 @@
+-- fmt_templ.templ --
+package test
+
+import (
+  sconv "strconv"
+)
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")
+-- fmt_templ.templ --
+package test
+
+import "fmt"
+
+// Comment on variable or function.
+var x = fmt.Sprintf("Hello")

templ/cmd/templ/imports/testdata/noimports.txtar

@@ -0,0 +1,12 @@
+-- fmt.templ --
+package test
+
+templ Hello() {
+	<div>Hello</div>
+}
+-- fmt.templ --
+package test
+
+templ Hello() {
+	<div>Hello</div>
+}

templ/cmd/templ/imports/testdata/noimportscode.txtar

@@ -0,0 +1,20 @@
+-- fmt.templ --
+package test
+
+func test() {
+  // Do nothing.
+}
+
+templ Hello() {
+	<div>Hello</div>
+}
+-- fmt.templ --
+package test
+
+func test() {
+	// Do nothing.
+}
+
+templ Hello() {
+	<div>Hello</div>
+}

templ/cmd/templ/imports/testdata/stringexp.txtar

@@ -0,0 +1,14 @@
+-- fmt.templ --
+package test
+
+templ Hello(name string) {
+	{ fmt.Sprintf("Hello, %s!", name) }
+}
+-- fmt.templ --
+package test
+
+import "fmt"
+
+templ Hello(name string) {
+	{ fmt.Sprintf("Hello, %s!", name) }
+}

templ/cmd/templ/imports/testdata/twoimports.txtar

@@ -0,0 +1,21 @@
+-- fmt.templ --
+package test
+
+templ Hello(name string) {
+	<div id={ strconv.Atoi("123") }>
+		{ fmt.Sprintf("Hello, %s!", name) }
+	</div>
+}
+-- fmt.templ --
+package test
+
+import (
+	"fmt"
+	"strconv"
+)
+
+templ Hello(name string) {
+	<div id={ strconv.Atoi("123") }>
+		{ fmt.Sprintf("Hello, %s!", name) }
+	</div>
+}

templ/cmd/templ/infocmd/main.go

@@ -0,0 +1,157 @@
+package infocmd
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+
+	"github.com/a-h/templ"
+	"github.com/a-h/templ/cmd/templ/lspcmd/pls"
+)
+
+type Arguments struct {
+	JSON bool `flag:"json" help:"Output info as JSON."`
+}
+
+type Info struct {
+	OS struct {
+		GOOS   string `json:"goos"`
+		GOARCH string `json:"goarch"`
+	} `json:"os"`
+	Go    ToolInfo `json:"go"`
+	Gopls ToolInfo `json:"gopls"`
+	Templ ToolInfo `json:"templ"`
+}
+
+type ToolInfo struct {
+	Location string `json:"location"`
+	Version  string `json:"version"`
+	OK       bool   `json:"ok"`
+	Message  string `json:"message,omitempty"`
+}
+
+func getGoInfo() (d ToolInfo) {
+	// Find Go.
+	var err error
+	d.Location, err = exec.LookPath("go")
+	if err != nil {
+		d.Message = fmt.Sprintf("failed to find go: %v", err)
+		return
+	}
+	// Run go to find the version.
+	cmd := exec.Command(d.Location, "version")
+	v, err := cmd.Output()
+	if err != nil {
+		d.Message = fmt.Sprintf("failed to get go version, check that Go is installed: %v", err)
+		return
+	}
+	d.Version = strings.TrimSpace(string(v))
+	d.OK = true
+	return
+}
+
+func getGoplsInfo() (d ToolInfo) {
+	var err error
+	d.Location, err = pls.FindGopls()
+	if err != nil {
+		d.Message = fmt.Sprintf("failed to find gopls: %v", err)
+		return
+	}
+	cmd := exec.Command(d.Location, "version")
+	v, err := cmd.Output()
+	if err != nil {
+		d.Message = fmt.Sprintf("failed to get gopls version: %v", err)
+		return
+	}
+	d.Version = strings.TrimSpace(string(v))
+	d.OK = true
+	return
+}
+
+func getTemplInfo() (d ToolInfo) {
+	// Find templ.
+	var err error
+	d.Location, err = findTempl()
+	if err != nil {
+		d.Message = err.Error()
+		return
+	}
+	// Run templ to find the version.
+	cmd := exec.Command(d.Location, "version")
+	v, err := cmd.Output()
+	if err != nil {
+		d.Message = fmt.Sprintf("failed to get templ version: %v", err)
+		return
+	}
+	d.Version = strings.TrimSpace(string(v))
+	if d.Version != templ.Version() {
+		d.Message = fmt.Sprintf("version mismatch - you're running %q at the command line, but the version in the path is %q", templ.Version(), d.Version)
+		return
+	}
+	d.OK = true
+	return
+}
+
+func findTempl() (location string, err error) {
+	executableName := "templ"
+	if runtime.GOOS == "windows" {
+		executableName = "templ.exe"
+	}
+	executableName, err = exec.LookPath(executableName)
+	if err == nil {
+		// Found on the path.
+		return executableName, nil
+	}
+
+	// Unexpected error.
+	if !errors.Is(err, exec.ErrNotFound) {
+		return "", fmt.Errorf("unexpected error looking for templ: %w", err)
+	}
+
+	return "", fmt.Errorf("templ is not in the path (%q). You can install templ with `go install github.com/a-h/templ/cmd/templ@latest`", os.Getenv("PATH"))
+}
+
+func getInfo() (d Info) {
+	d.OS.GOOS = runtime.GOOS
+	d.OS.GOARCH = runtime.GOARCH
+	d.Go = getGoInfo()
+	d.Gopls = getGoplsInfo()
+	d.Templ = getTemplInfo()
+	return
+}
+
+func Run(ctx context.Context, log *slog.Logger, stdout io.Writer, args Arguments) (err error) {
+	info := getInfo()
+	if args.JSON {
+		enc := json.NewEncoder(stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(info)
+	}
+	log.Info("os", slog.String("goos", info.OS.GOOS), slog.String("goarch", info.OS.GOARCH))
+	logInfo(ctx, log, "go", info.Go)
+	logInfo(ctx, log, "gopls", info.Gopls)
+	logInfo(ctx, log, "templ", info.Templ)
+	return nil
+}
+
+func logInfo(ctx context.Context, log *slog.Logger, name string, ti ToolInfo) {
+	level := slog.LevelInfo
+	if !ti.OK {
+		level = slog.LevelError
+	}
+	args := []any{
+		slog.String("location", ti.Location),
+		slog.String("version", ti.Version),
+	}
+	if ti.Message != "" {
+		args = append(args, slog.String("message", ti.Message))
+	}
+	log.Log(ctx, level, name, args...)
+}

templ/cmd/templ/lspcmd/httpdebug/handler.go

@@ -0,0 +1,130 @@
+package httpdebug
+
+import (
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/url"
+
+	"github.com/a-h/templ"
+	"github.com/a-h/templ/cmd/templ/lspcmd/proxy"
+	"github.com/a-h/templ/cmd/templ/visualize"
+)
+
+var log *slog.Logger
+
+func NewHandler(l *slog.Logger, s *proxy.Server) http.Handler {
+	m := http.NewServeMux()
+	log = l
+	m.HandleFunc("/templ", func(w http.ResponseWriter, r *http.Request) {
+		uri := r.URL.Query().Get("uri")
+		c, ok := s.TemplSource.Get(uri)
+		if !ok {
+			Error(w, "uri not found", http.StatusNotFound)
+			return
+		}
+		String(w, c.String())
+	})
+	m.HandleFunc("/sourcemap", func(w http.ResponseWriter, r *http.Request) {
+		uri := r.URL.Query().Get("uri")
+		sm, ok := s.SourceMapCache.Get(uri)
+		if !ok {
+			Error(w, "uri not found", http.StatusNotFound)
+			return
+		}
+		JSON(w, sm.SourceLinesToTarget)
+	})
+	m.HandleFunc("/go", func(w http.ResponseWriter, r *http.Request) {
+		uri := r.URL.Query().Get("uri")
+		c, ok := s.GoSource[uri]
+		if !ok {
+			Error(w, "uri not found", http.StatusNotFound)
+			return
+		}
+		String(w, c)
+	})
+	m.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		uri := r.URL.Query().Get("uri")
+		if uri == "" {
+			// List all URIs.
+			if err := list(s.TemplSource.URIs()).Render(r.Context(), w); err != nil {
+				Error(w, "failed to list URIs", http.StatusInternalServerError)
+			}
+			return
+		}
+		// Assume we've got a URI.
+		templSource, ok := s.TemplSource.Get(uri)
+		if !ok {
+			if !ok {
+				Error(w, "uri not found in document contents", http.StatusNotFound)
+				return
+			}
+		}
+		goSource, ok := s.GoSource[uri]
+		if !ok {
+			if !ok {
+				Error(w, "uri not found in document contents", http.StatusNotFound)
+				return
+			}
+		}
+		sm, ok := s.SourceMapCache.Get(uri)
+		if !ok {
+			Error(w, "uri not found", http.StatusNotFound)
+			return
+		}
+		if err := visualize.HTML(uri, templSource.String(), goSource, sm).Render(r.Context(), w); err != nil {
+			Error(w, "failed to visualize HTML", http.StatusInternalServerError)
+		}
+	})
+	return m
+}
+
+func getMapURL(uri string) templ.SafeURL {
+	return withQuery("/", uri)
+}
+
+func getSourceMapURL(uri string) templ.SafeURL {
+	return withQuery("/sourcemap", uri)
+}
+
+func getTemplURL(uri string) templ.SafeURL {
+	return withQuery("/templ", uri)
+}
+
+func getGoURL(uri string) templ.SafeURL {
+	return withQuery("/go", uri)
+}
+
+func withQuery(path, uri string) templ.SafeURL {
+	q := make(url.Values)
+	q.Set("uri", uri)
+	u := &url.URL{
+		Path:     path,
+		RawPath:  path,
+		RawQuery: q.Encode(),
+	}
+	return templ.SafeURL(u.String())
+}
+
+func JSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	enc := json.NewEncoder(w)
+	enc.SetIndent("", "  ")
+	if err := enc.Encode(v); err != nil {
+		log.Error("failed to write JSON response", slog.Any("error", err))
+	}
+}
+
+func String(w http.ResponseWriter, s string) {
+	if _, err := io.WriteString(w, s); err != nil {
+		log.Error("failed to write string response", slog.Any("error", err))
+	}
+}
+
+func Error(w http.ResponseWriter, msg string, status int) {
+	w.WriteHeader(status)
+	if _, err := io.WriteString(w, msg); err != nil {
+		log.Error("failed to write error response", slog.Any("error", err))
+	}
+}

templ/cmd/templ/lspcmd/httpdebug/list.templ

@@ -0,0 +1,22 @@
+package httpdebug
+
+templ list(uris []string) {
+	<table>
+		<tr>
+			<th>File</th>
+			<th></th>
+			<th></th>
+			<th></th>
+			<th></th>
+		</tr>
+		for _, uri := range uris {
+			<tr>
+				<td>{ uri }</td>
+				<td><a href={ getMapURL(uri) }>Mapping</a></td>
+				<td><a href={ getSourceMapURL(uri) }>Source Map</a></td>
+				<td><a href={ getTemplURL(uri) }>Templ</a></td>
+				<td><a href={ getGoURL(uri) }>Go</a></td>
+			</tr>
+		}
+	</table>
+}

templ/cmd/templ/lspcmd/httpdebug/list_templ.go

@@ -0,0 +1,99 @@
+// Code generated by templ - DO NOT EDIT.
+
+// templ: version: v0.3.833
+package httpdebug
+
+//lint:file-ignore SA4006 This context is only used if a nested component is present.
+
+import "github.com/a-h/templ"
+import templruntime "github.com/a-h/templ/runtime"
+
+func list(uris []string) templ.Component {
+	return templruntime.GeneratedTemplate(func(templ_7745c5c3_Input templruntime.GeneratedComponentInput) (templ_7745c5c3_Err error) {
+		templ_7745c5c3_W, ctx := templ_7745c5c3_Input.Writer, templ_7745c5c3_Input.Context
+		if templ_7745c5c3_CtxErr := ctx.Err(); templ_7745c5c3_CtxErr != nil {
+			return templ_7745c5c3_CtxErr
+		}
+		templ_7745c5c3_Buffer, templ_7745c5c3_IsBuffer := templruntime.GetBuffer(templ_7745c5c3_W)
+		if !templ_7745c5c3_IsBuffer {
+			defer func() {
+				templ_7745c5c3_BufErr := templruntime.ReleaseBuffer(templ_7745c5c3_Buffer)
+				if templ_7745c5c3_Err == nil {
+					templ_7745c5c3_Err = templ_7745c5c3_BufErr
+				}
+			}()
+		}
+		ctx = templ.InitializeContext(ctx)
+		templ_7745c5c3_Var1 := templ.GetChildren(ctx)
+		if templ_7745c5c3_Var1 == nil {
+			templ_7745c5c3_Var1 = templ.NopComponent
+		}
+		ctx = templ.ClearChildren(ctx)
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 1, "<table><tr><th>File</th><th></th><th></th><th></th><th></th></tr>")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		for _, uri := range uris {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 2, "<tr><td>")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var2 string
+			templ_7745c5c3_Var2, templ_7745c5c3_Err = templ.JoinStringErrs(uri)
+			if templ_7745c5c3_Err != nil {
+				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templ/cmd/templ/lspcmd/httpdebug/list.templ`, Line: 14, Col: 13}
+			}
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var2))
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 3, "</td><td><a href=\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var3 templ.SafeURL = getMapURL(uri)
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(string(templ_7745c5c3_Var3)))
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 4, "\">Mapping</a></td><td><a href=\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var4 templ.SafeURL = getSourceMapURL(uri)
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(string(templ_7745c5c3_Var4)))
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 5, "\">Source Map</a></td><td><a href=\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var5 templ.SafeURL = getTemplURL(uri)
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(string(templ_7745c5c3_Var5)))
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 6, "\">Templ</a></td><td><a href=\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var6 templ.SafeURL = getGoURL(uri)
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(string(templ_7745c5c3_Var6)))
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 7, "\">Go</a></td></tr>")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+		}
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 8, "</table>")
+		if templ_7745c5c3_Err != nil {
+			return templ_7745c5c3_Err
+		}
+		return nil
+	})
+}
+
+var _ = templruntime.GeneratedTemplate

templ/cmd/templ/lspcmd/lsp_test.go

@@ -0,0 +1,957 @@
+package lspcmd
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"sync"
+	"testing"
+	"time"
+	"unicode/utf8"
+
+	"github.com/a-h/templ/cmd/templ/generatecmd/modcheck"
+	"github.com/a-h/templ/cmd/templ/lspcmd/lspdiff"
+	"github.com/a-h/templ/cmd/templ/testproject"
+	"github.com/a-h/templ/lsp/jsonrpc2"
+	"github.com/a-h/templ/lsp/protocol"
+	"github.com/a-h/templ/lsp/uri"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestCompletion(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	log := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+
+	ctx, appDir, _, server, teardown, err := Setup(ctx, log)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+	defer cancel()
+
+	templFile, err := os.ReadFile(appDir + "/templates.templ")
+	if err != nil {
+		t.Errorf("failed to read file %q: %v", appDir+"/templates.templ", err)
+		return
+	}
+	err = server.DidOpen(ctx, &protocol.DidOpenTextDocumentParams{
+		TextDocument: protocol.TextDocumentItem{
+			URI:        uri.URI("file://" + appDir + "/templates.templ"),
+			LanguageID: "templ",
+			Version:    1,
+			Text:       string(templFile),
+		},
+	})
+	if err != nil {
+		t.Errorf("failed to register open file: %v", err)
+		return
+	}
+	log.Info("Calling completion")
+
+	globalSnippetsLen := 1
+
+	// Edit the file.
+	// Replace:
+	// <div data-testid="count">{ fmt.Sprintf("%d", count) }</div>
+	// With various tests:
+	// <div data-testid="count">{ f
+	tests := []struct {
+		line        int
+		replacement string
+		cursor      string
+		assert      func(t *testing.T, cl *protocol.CompletionList) (msg string, ok bool)
+	}{
+		{
+			line:        13,
+			replacement: ` <div data-testid="count">{  `,
+			cursor:      `                            ^`,
+			assert: func(t *testing.T, actual *protocol.CompletionList) (msg string, ok bool) {
+				if actual != nil && len(actual.Items) != globalSnippetsLen {
+					return "expected completion list to be empty", false
+				}
+				return "", true
+			},
+		},
+		{
+			line:        13,
+			replacement: ` <div data-testid="count">{ fmt.`,
+			cursor:      `                               ^`,
+			assert: func(t *testing.T, actual *protocol.CompletionList) (msg string, ok bool) {
+				if !lspdiff.CompletionListContainsText(actual, "fmt.Sprintf") {
+					return fmt.Sprintf("expected fmt.Sprintf to be in the completion list, but got %#v", actual), false
+				}
+				return "", true
+			},
+		},
+		{
+			line:        13,
+			replacement: ` <div data-testid="count">{ fmt.Sprintf("%d",`,
+			cursor:      `                                            ^`,
+			assert: func(t *testing.T, actual *protocol.CompletionList) (msg string, ok bool) {
+				if actual != nil && len(actual.Items) != globalSnippetsLen {
+					return "expected completion list to be empty", false
+				}
+				return "", true
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("test-%d", i), func(t *testing.T) {
+			// Edit the file.
+			updated := testproject.MustReplaceLine(string(templFile), test.line, test.replacement)
+			err = server.DidChange(ctx, &protocol.DidChangeTextDocumentParams{
+				TextDocument: protocol.VersionedTextDocumentIdentifier{
+					TextDocumentIdentifier: protocol.TextDocumentIdentifier{
+						URI: uri.URI("file://" + appDir + "/templates.templ"),
+					},
+					Version: int32(i + 2),
+				},
+				ContentChanges: []protocol.TextDocumentContentChangeEvent{
+					{
+						Range: nil,
+						Text:  updated,
+					},
+				},
+			})
+			if err != nil {
+				t.Errorf("failed to change file: %v", err)
+				return
+			}
+
+			// Give CI/CD pipeline executors some time because they're often quite slow.
+			var ok bool
+			var msg string
+			for i := 0; i < 3; i++ {
+				actual, err := server.Completion(ctx, &protocol.CompletionParams{
+					Context: &protocol.CompletionContext{
+						TriggerCharacter: ".",
+						TriggerKind:      protocol.CompletionTriggerKindTriggerCharacter,
+					},
+					TextDocumentPositionParams: protocol.TextDocumentPositionParams{
+						TextDocument: protocol.TextDocumentIdentifier{
+							URI: uri.URI("file://" + appDir + "/templates.templ"),
+						},
+						// Positions are zero indexed.
+						Position: protocol.Position{
+							Line:      uint32(test.line - 1),
+							Character: uint32(len(test.cursor) - 1),
+						},
+					},
+				})
+				if err != nil {
+					t.Errorf("failed to get completion: %v", err)
+					return
+				}
+				msg, ok = test.assert(t, actual)
+				if !ok {
+					break
+				}
+				time.Sleep(time.Millisecond * 500)
+			}
+			if !ok {
+				t.Error(msg)
+			}
+		})
+	}
+	log.Info("Completed test")
+}
+
+func TestHover(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	log := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+
+	ctx, appDir, _, server, teardown, err := Setup(ctx, log)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+	defer cancel()
+
+	templFile, err := os.ReadFile(appDir + "/templates.templ")
+	if err != nil {
+		t.Fatalf("failed to read file %q: %v", appDir+"/templates.templ", err)
+	}
+	err = server.DidOpen(ctx, &protocol.DidOpenTextDocumentParams{
+		TextDocument: protocol.TextDocumentItem{
+			URI:        uri.URI("file://" + appDir + "/templates.templ"),
+			LanguageID: "templ",
+			Version:    1,
+			Text:       string(templFile),
+		},
+	})
+	if err != nil {
+		t.Errorf("failed to register open file: %v", err)
+		return
+	}
+	log.Info("Calling hover")
+
+	// Edit the file.
+	// Replace:
+	// <div data-testid="count">{ fmt.Sprintf("%d", count) }</div>
+	// With various tests:
+	// <div data-testid="count">{ f
+	tests := []struct {
+		line        int
+		replacement string
+		cursor      string
+		assert      func(t *testing.T, hr *protocol.Hover) (msg string, ok bool)
+	}{
+		{
+			line:        13,
+			replacement: `			<div data-testid="count">{ fmt.Sprintf("%d", count) }</div>`,
+			cursor:      `                                 ^`,
+			assert: func(t *testing.T, actual *protocol.Hover) (msg string, ok bool) {
+				expectedHover := protocol.Hover{
+					Contents: protocol.MarkupContent{
+						Kind:  "markdown",
+						Value: "```go\npackage fmt\n```\n\n---\n\n[`fmt` on pkg.go.dev](https://pkg.go.dev/fmt)",
+					},
+				}
+				if diff := lspdiff.Hover(expectedHover, *actual); diff != "" {
+					return fmt.Sprintf("unexpected hover: %v\n\n: markdown: %#v", diff, actual.Contents.Value), false
+				}
+				return "", true
+			},
+		},
+		{
+			line:        13,
+			replacement: `			<div data-testid="count">{ fmt.Sprintf("%d", count) }</div>`,
+			cursor:      `                                     ^`,
+			assert: func(t *testing.T, actual *protocol.Hover) (msg string, ok bool) {
+				expectedHover := protocol.Hover{
+					Contents: protocol.MarkupContent{
+						Kind:  "markdown",
+						Value: "```go\nfunc fmt.Sprintf(format string, a ...any) string\n```\n\n---\n\nSprintf formats according to a format specifier and returns the resulting string.\n\n\n---\n\n[`fmt.Sprintf` on pkg.go.dev](https://pkg.go.dev/fmt#Sprintf)",
+					},
+				}
+				if actual == nil {
+					return "expected hover to be non-nil", false
+				}
+				if diff := lspdiff.Hover(expectedHover, *actual); diff != "" {
+					return fmt.Sprintf("unexpected hover: %v", diff), false
+				}
+				return "", true
+			},
+		},
+		{
+			line:        19,
+			replacement: `var nihao = "你好"`,
+			cursor:      `             ^`,
+			assert: func(t *testing.T, actual *protocol.Hover) (msg string, ok bool) {
+				// There's nothing to hover, just want to make sure it doesn't panic.
+				return "", true
+			},
+		},
+		{
+			line:        19,
+			replacement: `var nihao = "你好"`,
+			cursor:      `              ^`, // Your text editor might not render this well, but it's the hao.
+			assert: func(t *testing.T, actual *protocol.Hover) (msg string, ok bool) {
+				// There's nothing to hover, just want to make sure it doesn't panic.
+				return "", true
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("test-%d", i), func(t *testing.T) {
+			// Put the file back to the initial point.
+			err = server.DidChange(ctx, &protocol.DidChangeTextDocumentParams{
+				TextDocument: protocol.VersionedTextDocumentIdentifier{
+					TextDocumentIdentifier: protocol.TextDocumentIdentifier{
+						URI: uri.URI("file://" + appDir + "/templates.templ"),
+					},
+					Version: int32(i + 2),
+				},
+				ContentChanges: []protocol.TextDocumentContentChangeEvent{
+					{
+						Range: nil,
+						Text:  string(templFile),
+					},
+				},
+			})
+			if err != nil {
+				t.Errorf("failed to change file: %v", err)
+				return
+			}
+
+			// Give CI/CD pipeline executors some time because they're often quite slow.
+			var ok bool
+			var msg string
+			for i := 0; i < 3; i++ {
+				lspCharIndex, err := runeIndexToUTF8ByteIndex(test.replacement, len(test.cursor)-1)
+				if err != nil {
+					t.Error(err)
+				}
+				actual, err := server.Hover(ctx, &protocol.HoverParams{
+					TextDocumentPositionParams: protocol.TextDocumentPositionParams{
+						TextDocument: protocol.TextDocumentIdentifier{
+							URI: uri.URI("file://" + appDir + "/templates.templ"),
+						},
+						// Positions are zero indexed.
+						Position: protocol.Position{
+							Line:      uint32(test.line - 1),
+							Character: lspCharIndex,
+						},
+					},
+				})
+				if err != nil {
+					t.Errorf("failed to hover: %v", err)
+					return
+				}
+				msg, ok = test.assert(t, actual)
+				if !ok {
+					break
+				}
+				time.Sleep(time.Millisecond * 500)
+			}
+			if !ok {
+				t.Error(msg)
+			}
+		})
+	}
+}
+
+func TestReferences(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	log := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+
+	ctx, appDir, _, server, teardown, err := Setup(ctx, log)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+		return
+	}
+	defer teardown(t)
+	defer cancel()
+
+	log.Info("Calling References")
+
+	tests := []struct {
+		line      int
+		character int
+		filename  string
+		assert    func(t *testing.T, l []protocol.Location) (msg string, ok bool)
+	}{
+		{
+			// this is the definition of the templ function in the templates.templ file.
+			line:      5,
+			character: 9,
+			filename:  "/templates.templ",
+			assert: func(t *testing.T, actual []protocol.Location) (msg string, ok bool) {
+				expectedReference := []protocol.Location{
+					{
+						// This is the usage of the templ function in the main.go file.
+						URI: uri.URI("file://" + appDir + "/main.go"),
+						Range: protocol.Range{
+							Start: protocol.Position{
+								Line:      uint32(24),
+								Character: uint32(7),
+							},
+							End: protocol.Position{
+								Line:      uint32(24),
+								Character: uint32(11),
+							},
+						},
+					},
+				}
+				if diff := lspdiff.References(expectedReference, actual); diff != "" {
+					return fmt.Sprintf("Expected: %+v\nActual: %+v", expectedReference, actual), false
+				}
+				return "", true
+			},
+		},
+		{
+			// this is the definition of the struct in the templates.templ file.
+			line:      21,
+			character: 9,
+			filename:  "/templates.templ",
+			assert: func(t *testing.T, actual []protocol.Location) (msg string, ok bool) {
+				expectedReference := []protocol.Location{
+					{
+						// This is the usage of the struct in the templates.templ file.
+						URI: uri.URI("file://" + appDir + "/templates.templ"),
+						Range: protocol.Range{
+							Start: protocol.Position{
+								Line:      uint32(24),
+								Character: uint32(8),
+							},
+							End: protocol.Position{
+								Line:      uint32(24),
+								Character: uint32(14),
+							},
+						},
+					},
+				}
+				if diff := lspdiff.References(expectedReference, actual); diff != "" {
+					return fmt.Sprintf("Expected: %+v\nActual: %+v", expectedReference, actual), false
+				}
+				return "", true
+			},
+		},
+		{
+			// this test is for inclusions from a remote file that has not been explicitly called with didOpen
+			line:      3,
+			character: 9,
+			filename:  "/remotechild.templ",
+			assert: func(t *testing.T, actual []protocol.Location) (msg string, ok bool) {
+				expectedReference := []protocol.Location{
+					{
+						URI: uri.URI("file://" + appDir + "/remoteparent.templ"),
+						Range: protocol.Range{
+							Start: protocol.Position{
+								Line:      uint32(3),
+								Character: uint32(2),
+							},
+							End: protocol.Position{
+								Line:      uint32(3),
+								Character: uint32(8),
+							},
+						},
+					},
+					{
+						URI: uri.URI("file://" + appDir + "/remoteparent.templ"),
+						Range: protocol.Range{
+							Start: protocol.Position{
+								Line:      uint32(7),
+								Character: uint32(2),
+							},
+							End: protocol.Position{
+								Line:      uint32(7),
+								Character: uint32(8),
+							},
+						},
+					},
+				}
+				if diff := lspdiff.References(expectedReference, actual); diff != "" {
+					return fmt.Sprintf("Expected: %+v\nActual: %+v", expectedReference, actual), false
+				}
+				return "", true
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("test-%d", i), func(t *testing.T) {
+			// Give CI/CD pipeline executors some time because they're often quite slow.
+			var ok bool
+			var msg string
+			for i := 0; i < 3; i++ {
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				actual, err := server.References(ctx, &protocol.ReferenceParams{
+					TextDocumentPositionParams: protocol.TextDocumentPositionParams{
+						TextDocument: protocol.TextDocumentIdentifier{
+							URI: uri.URI("file://" + appDir + test.filename),
+						},
+						// Positions are zero indexed.
+						Position: protocol.Position{
+							Line:      uint32(test.line - 1),
+							Character: uint32(test.character - 1),
+						},
+					},
+				})
+				if err != nil {
+					t.Errorf("failed to get references: %v", err)
+					return
+				}
+				msg, ok = test.assert(t, actual)
+				if !ok {
+					break
+				}
+				time.Sleep(time.Millisecond * 500)
+			}
+			if !ok {
+				t.Error(msg)
+			}
+		})
+	}
+}
+
+func TestCodeAction(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	log := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+
+	ctx, appDir, _, server, teardown, err := Setup(ctx, log)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+	defer cancel()
+
+	templFile, err := os.ReadFile(appDir + "/templates.templ")
+	if err != nil {
+		t.Fatalf("failed to read file %q: %v", appDir+"/templates.templ", err)
+	}
+	err = server.DidOpen(ctx, &protocol.DidOpenTextDocumentParams{
+		TextDocument: protocol.TextDocumentItem{
+			URI:        uri.URI("file://" + appDir + "/templates.templ"),
+			LanguageID: "templ",
+			Version:    1,
+			Text:       string(templFile),
+		},
+	})
+	if err != nil {
+		t.Errorf("failed to register open file: %v", err)
+		return
+	}
+	log.Info("Calling codeAction")
+
+	tests := []struct {
+		line        int
+		replacement string
+		cursor      string
+		assert      func(t *testing.T, hr []protocol.CodeAction) (msg string, ok bool)
+	}{
+		{
+			line:        25,
+			replacement: `var s = Struct{}`,
+			cursor:      `              ^`,
+			assert: func(t *testing.T, actual []protocol.CodeAction) (msg string, ok bool) {
+				var expected []protocol.CodeAction
+				// To support code actions, update cmd/templ/lspcmd/proxy/server.go and add the
+				// Title (e.g. Organize Imports, or Fill Struct) to the supportedCodeActions map.
+
+				// Some Code Actions are simple edits, so all that is needed is for the server
+				// to remap the source code positions.
+
+				// However, other Code Actions are commands, where the arguments must be rewritten
+				// and will need to be handled individually.
+				if diff := lspdiff.CodeAction(expected, actual); diff != "" {
+					return fmt.Sprintf("unexpected codeAction: %v", diff), false
+				}
+				return "", true
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("test-%d", i), func(t *testing.T) {
+			// Put the file back to the initial point.
+			err = server.DidChange(ctx, &protocol.DidChangeTextDocumentParams{
+				TextDocument: protocol.VersionedTextDocumentIdentifier{
+					TextDocumentIdentifier: protocol.TextDocumentIdentifier{
+						URI: uri.URI("file://" + appDir + "/templates.templ"),
+					},
+					Version: int32(i + 2),
+				},
+				ContentChanges: []protocol.TextDocumentContentChangeEvent{
+					{
+						Range: nil,
+						Text:  string(templFile),
+					},
+				},
+			})
+			if err != nil {
+				t.Errorf("failed to change file: %v", err)
+				return
+			}
+
+			// Give CI/CD pipeline executors some time because they're often quite slow.
+			var ok bool
+			var msg string
+			for i := 0; i < 3; i++ {
+				lspCharIndex, err := runeIndexToUTF8ByteIndex(test.replacement, len(test.cursor)-1)
+				if err != nil {
+					t.Error(err)
+				}
+				actual, err := server.CodeAction(ctx, &protocol.CodeActionParams{
+					TextDocument: protocol.TextDocumentIdentifier{
+						URI: uri.URI("file://" + appDir + "/templates.templ"),
+					},
+					Range: protocol.Range{
+						Start: protocol.Position{
+							Line:      uint32(test.line - 1),
+							Character: lspCharIndex,
+						},
+						End: protocol.Position{
+							Line:      uint32(test.line - 1),
+							Character: lspCharIndex + 1,
+						},
+					},
+				})
+				if err != nil {
+					t.Errorf("failed code action: %v", err)
+					return
+				}
+				msg, ok = test.assert(t, actual)
+				if !ok {
+					break
+				}
+				time.Sleep(time.Millisecond * 500)
+			}
+			if !ok {
+				t.Error(msg)
+			}
+		})
+	}
+}
+
+func TestDocumentSymbol(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	log := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+
+	ctx, appDir, _, server, teardown, err := Setup(ctx, log)
+	if err != nil {
+		t.Fatalf("failed to setup test: %v", err)
+	}
+	defer teardown(t)
+	defer cancel()
+
+	tests := []struct {
+		uri    string
+		expect []protocol.SymbolInformationOrDocumentSymbol
+	}{
+		{
+			uri: "file://" + appDir + "/templates.templ",
+			expect: []protocol.SymbolInformationOrDocumentSymbol{
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "Page",
+						Kind: protocol.SymbolKindFunction,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 11, Character: 0},
+								End:   protocol.Position{Line: 50, Character: 1},
+							},
+						},
+					},
+				},
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "nihao",
+						Kind: protocol.SymbolKindVariable,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 18, Character: 4},
+								End:   protocol.Position{Line: 18, Character: 16},
+							},
+						},
+					},
+				},
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "Struct",
+						Kind: protocol.SymbolKindStruct,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 20, Character: 5},
+								End:   protocol.Position{Line: 22, Character: 1},
+							},
+						},
+					},
+				},
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "s",
+						Kind: protocol.SymbolKindVariable,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 24, Character: 4},
+								End:   protocol.Position{Line: 24, Character: 16},
+							},
+						},
+					},
+				},
+			},
+		},
+		{
+			uri: "file://" + appDir + "/remoteparent.templ",
+			expect: []protocol.SymbolInformationOrDocumentSymbol{
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "RemoteInclusionTest",
+						Kind: protocol.SymbolKindFunction,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 9, Character: 0},
+								End:   protocol.Position{Line: 35, Character: 1},
+							},
+						},
+					},
+				},
+				{
+					SymbolInformation: &protocol.SymbolInformation{
+						Name: "Remote2",
+						Kind: protocol.SymbolKindFunction,
+						Location: protocol.Location{
+							Range: protocol.Range{
+								Start: protocol.Position{Line: 37, Character: 0},
+								End:   protocol.Position{Line: 63, Character: 1},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("test-%d", i), func(t *testing.T) {
+			actual, err := server.DocumentSymbol(ctx, &protocol.DocumentSymbolParams{
+				TextDocument: protocol.TextDocumentIdentifier{
+					URI: uri.URI(test.uri),
+				},
+			})
+			if err != nil {
+				t.Errorf("failed to get document symbol: %v", err)
+			}
+
+			// Set expected URI.
+			for i, v := range test.expect {
+				if v.SymbolInformation != nil {
+					v.SymbolInformation.Location.URI = uri.URI(test.uri)
+					test.expect[i] = v
+				}
+			}
+
+			if err != nil {
+				t.Errorf("failed to convert expect to any slice: %v", err)
+			}
+			diff := cmp.Diff(test.expect, actual)
+			if diff != "" {
+				t.Errorf("unexpected document symbol: %v", diff)
+			}
+		})
+	}
+}
+
+func runeIndexToUTF8ByteIndex(s string, runeIndex int) (lspChar uint32, err error) {
+	for i, r := range []rune(s) {
+		if i == runeIndex {
+			break
+		}
+		l := utf8.RuneLen(r)
+		if l < 0 {
+			return 0, fmt.Errorf("invalid rune in string at index %d", runeIndex)
+		}
+		lspChar += uint32(l)
+	}
+	return lspChar, nil
+}
+
+func NewTestClient(log *slog.Logger) TestClient {
+	return TestClient{
+		log: log,
+	}
+}
+
+type TestClient struct {
+	log *slog.Logger
+}
+
+func (tc TestClient) Progress(ctx context.Context, params *protocol.ProgressParams) (err error) {
+	tc.log.Info("client: Received Progress", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) WorkDoneProgressCreate(ctx context.Context, params *protocol.WorkDoneProgressCreateParams) (err error) {
+	tc.log.Info("client: Received WorkDoneProgressCreate", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) LogMessage(ctx context.Context, params *protocol.LogMessageParams) (err error) {
+	tc.log.Info("client: Received LogMessage", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) PublishDiagnostics(ctx context.Context, params *protocol.PublishDiagnosticsParams) (err error) {
+	tc.log.Info("client: Received PublishDiagnostics", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) ShowMessage(ctx context.Context, params *protocol.ShowMessageParams) (err error) {
+	tc.log.Info("client: Received ShowMessage", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) ShowMessageRequest(ctx context.Context, params *protocol.ShowMessageRequestParams) (result *protocol.MessageActionItem, err error) {
+	return nil, nil
+}
+
+func (tc TestClient) Telemetry(ctx context.Context, params any) (err error) {
+	tc.log.Info("client: Received Telemetry", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) RegisterCapability(ctx context.Context, params *protocol.RegistrationParams,
+) (err error) {
+	tc.log.Info("client: Received RegisterCapability", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) UnregisterCapability(ctx context.Context, params *protocol.UnregistrationParams) (err error) {
+	tc.log.Info("client: Received UnregisterCapability", slog.Any("params", params))
+	return nil
+}
+
+func (tc TestClient) ApplyEdit(ctx context.Context, params *protocol.ApplyWorkspaceEditParams) (result *protocol.ApplyWorkspaceEditResponse, err error) {
+	tc.log.Info("client: Received ApplyEdit", slog.Any("params", params))
+	return nil, nil
+}
+
+func (tc TestClient) Configuration(ctx context.Context, params *protocol.ConfigurationParams) (result []any, err error) {
+	tc.log.Info("client: Received Configuration", slog.Any("params", params))
+	return nil, nil
+}
+
+func (tc TestClient) WorkspaceFolders(ctx context.Context) (result []protocol.WorkspaceFolder, err error) {
+	tc.log.Info("client: Received WorkspaceFolders")
+	return nil, nil
+}
+
+func Setup(ctx context.Context, log *slog.Logger) (clientCtx context.Context, appDir string, client protocol.Client, server protocol.Server, teardown func(t *testing.T), err error) {
+	wd, err := os.Getwd()
+	if err != nil {
+		return ctx, appDir, client, server, teardown, fmt.Errorf("could not find working dir: %w", err)
+	}
+	moduleRoot, err := modcheck.WalkUp(wd)
+	if err != nil {
+		return ctx, appDir, client, server, teardown, fmt.Errorf("could not find local templ go.mod file: %v", err)
+	}
+
+	appDir, err = testproject.Create(moduleRoot)
+	if err != nil {
+		return ctx, appDir, client, server, teardown, fmt.Errorf("failed to create test project: %v", err)
+	}
+
+	var wg sync.WaitGroup
+	var cmdErr error
+
+	// Copy from the LSP to the Client, and vice versa.
+	fromClient, toLSP := io.Pipe()
+	fromLSP, toClient := io.Pipe()
+	clientStream := jsonrpc2.NewStream(newStdRwc(log, "clientStream", toLSP, fromLSP))
+	serverStream := jsonrpc2.NewStream(newStdRwc(log, "serverStream", toClient, fromClient))
+
+	// Create the client that the server needs.
+	client = NewTestClient(log)
+	ctx, _, server = protocol.NewClient(ctx, client, clientStream, log)
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		log.Info("Running")
+		// Create the server that the client needs.
+		cmdErr = run(ctx, log, serverStream, Arguments{})
+		if cmdErr != nil {
+			log.Error("Failed to run", slog.Any("error", cmdErr))
+		}
+		log.Info("Stopped")
+	}()
+
+	// Initialize.
+	ir, err := server.Initialize(ctx, &protocol.InitializeParams{
+		ClientInfo: &protocol.ClientInfo{},
+		Capabilities: protocol.ClientCapabilities{
+			Workspace: &protocol.WorkspaceClientCapabilities{
+				ApplyEdit: true,
+				WorkspaceEdit: &protocol.WorkspaceClientCapabilitiesWorkspaceEdit{
+					DocumentChanges: true,
+				},
+				WorkspaceFolders: true,
+				FileOperations: &protocol.WorkspaceClientCapabilitiesFileOperations{
+					DidCreate:  true,
+					WillCreate: true,
+					DidRename:  true,
+					WillRename: true,
+					DidDelete:  true,
+					WillDelete: true,
+				},
+			},
+			TextDocument: &protocol.TextDocumentClientCapabilities{
+				Synchronization: &protocol.TextDocumentSyncClientCapabilities{
+					DidSave: true,
+				},
+				Completion: &protocol.CompletionTextDocumentClientCapabilities{
+					CompletionItem: &protocol.CompletionTextDocumentClientCapabilitiesItem{
+						SnippetSupport:       true,
+						DeprecatedSupport:    true,
+						InsertReplaceSupport: true,
+					},
+				},
+				Hover:              &protocol.HoverTextDocumentClientCapabilities{},
+				SignatureHelp:      &protocol.SignatureHelpTextDocumentClientCapabilities{},
+				Declaration:        &protocol.DeclarationTextDocumentClientCapabilities{},
+				Definition:         &protocol.DefinitionTextDocumentClientCapabilities{},
+				TypeDefinition:     &protocol.TypeDefinitionTextDocumentClientCapabilities{},
+				Implementation:     &protocol.ImplementationTextDocumentClientCapabilities{},
+				References:         &protocol.ReferencesTextDocumentClientCapabilities{},
+				DocumentHighlight:  &protocol.DocumentHighlightClientCapabilities{},
+				DocumentSymbol:     &protocol.DocumentSymbolClientCapabilities{},
+				CodeAction:         &protocol.CodeActionClientCapabilities{},
+				CodeLens:           &protocol.CodeLensClientCapabilities{},
+				Formatting:         &protocol.DocumentFormattingClientCapabilities{},
+				RangeFormatting:    &protocol.DocumentRangeFormattingClientCapabilities{},
+				OnTypeFormatting:   &protocol.DocumentOnTypeFormattingClientCapabilities{},
+				PublishDiagnostics: &protocol.PublishDiagnosticsClientCapabilities{},
+				Rename:             &protocol.RenameClientCapabilities{},
+				FoldingRange:       &protocol.FoldingRangeClientCapabilities{},
+				SelectionRange:     &protocol.SelectionRangeClientCapabilities{},
+				CallHierarchy:      &protocol.CallHierarchyClientCapabilities{},
+				SemanticTokens:     &protocol.SemanticTokensClientCapabilities{},
+				LinkedEditingRange: &protocol.LinkedEditingRangeClientCapabilities{},
+			},
+			Window:       &protocol.WindowClientCapabilities{},
+			General:      &protocol.GeneralClientCapabilities{},
+			Experimental: nil,
+		},
+		WorkspaceFolders: []protocol.WorkspaceFolder{
+			{
+				URI:  "file://" + appDir,
+				Name: "templ-test",
+			},
+		},
+	})
+	if err != nil {
+		log.Error("Failed to init", slog.Any("error", err))
+	}
+	if ir.ServerInfo.Name != "templ-lsp" {
+		return ctx, appDir, client, server, teardown, fmt.Errorf("expected server name to be templ-lsp, got %q", ir.ServerInfo.Name)
+	}
+
+	// Confirm initialization.
+	log.Info("Confirming initialization...")
+	if err = server.Initialized(ctx, &protocol.InitializedParams{}); err != nil {
+		return ctx, appDir, client, server, teardown, fmt.Errorf("failed to confirm initialization: %v", err)
+	}
+	log.Info("Initialized")
+
+	// Wait for exit.
+	teardown = func(t *testing.T) {
+		log.Info("Tearing down LSP")
+		wg.Wait()
+		if cmdErr != nil {
+			t.Errorf("failed to run lsp cmd: %v", err)
+		}
+
+		if err = os.RemoveAll(appDir); err != nil {
+			t.Errorf("failed to remove test dir %q: %v", appDir, err)
+		}
+	}
+	return ctx, appDir, client, server, teardown, err
+}

templ/cmd/templ/lspcmd/lspdiff/lspdiff.go

@@ -0,0 +1,42 @@
+package lspdiff
+
+import (
+	"github.com/a-h/templ/lsp/protocol"
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+)
+
+// This package provides a way to compare LSP	protocol messages, ignoring irrelevant fields.
+
+func Hover(expected, actual protocol.Hover) string {
+	return cmp.Diff(expected, actual,
+		cmpopts.IgnoreFields(protocol.Hover{}, "Range"),
+		cmpopts.IgnoreFields(protocol.MarkupContent{}, "Kind"),
+	)
+}
+
+func CodeAction(expected, actual []protocol.CodeAction) string {
+	return cmp.Diff(expected, actual)
+}
+
+func CompletionList(expected, actual *protocol.CompletionList) string {
+	return cmp.Diff(expected, actual,
+		cmpopts.IgnoreFields(protocol.CompletionList{}, "IsIncomplete"),
+	)
+}
+
+func References(expected, actual []protocol.Location) string {
+	return cmp.Diff(expected, actual)
+}
+
+func CompletionListContainsText(cl *protocol.CompletionList, text string) bool {
+	if cl == nil {
+		return false
+	}
+	for _, item := range cl.Items {
+		if item.Label == text {
+			return true
+		}
+	}
+	return false
+}

templ/cmd/templ/lspcmd/main.go

@@ -0,0 +1,131 @@
+package lspcmd
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+
+	"github.com/a-h/templ/cmd/templ/lspcmd/httpdebug"
+	"github.com/a-h/templ/cmd/templ/lspcmd/pls"
+	"github.com/a-h/templ/cmd/templ/lspcmd/proxy"
+	"github.com/a-h/templ/lsp/jsonrpc2"
+	"github.com/a-h/templ/lsp/protocol"
+
+	_ "net/http/pprof"
+)
+
+type Arguments struct {
+	Log           string
+	GoplsLog      string
+	GoplsRPCTrace bool
+	// PPROF sets whether to start a profiling server on localhost:9999
+	PPROF bool
+	// HTTPDebug sets the HTTP endpoint to listen on. Leave empty for no web debug.
+	HTTPDebug string
+}
+
+func Run(stdin io.Reader, stdout, stderr io.Writer, args Arguments) (err error) {
+	ctx := context.Background()
+	ctx, cancel := context.WithCancel(ctx)
+	signalChan := make(chan os.Signal, 1)
+	signal.Notify(signalChan, os.Interrupt)
+	defer func() {
+		signal.Stop(signalChan)
+		cancel()
+	}()
+	if args.PPROF {
+		go func() {
+			_ = http.ListenAndServe("localhost:9999", nil)
+		}()
+	}
+	go func() {
+		select {
+		case <-signalChan: // First signal, cancel context.
+			cancel()
+		case <-ctx.Done():
+		}
+		<-signalChan // Second signal, hard exit.
+		os.Exit(2)
+	}()
+	log := slog.New(slog.NewJSONHandler(io.Discard, nil))
+	if args.Log != "" {
+		file, err := os.OpenFile(args.Log, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
+		if err != nil {
+			return fmt.Errorf("failed to open log file: %w", err)
+		}
+		defer file.Close()
+
+		// Create a new logger with a file writer
+		log = slog.New(slog.NewJSONHandler(file, nil))
+		log.Debug("Logging to file", slog.String("file", args.Log))
+	}
+	templStream := jsonrpc2.NewStream(newStdRwc(log, "templStream", stdout, stdin))
+	return run(ctx, log, templStream, args)
+}
+
+func run(ctx context.Context, log *slog.Logger, templStream jsonrpc2.Stream, args Arguments) (err error) {
+	log.Info("lsp: starting up...")
+	defer func() {
+		if r := recover(); r != nil {
+			log.Error("handled panic", slog.Any("recovered", r))
+		}
+	}()
+
+	log.Info("lsp: starting gopls...")
+	rwc, err := pls.NewGopls(ctx, log, pls.Options{
+		Log:      args.GoplsLog,
+		RPCTrace: args.GoplsRPCTrace,
+	})
+	if err != nil {
+		log.Error("failed to start gopls", slog.Any("error", err))
+		os.Exit(1)
+	}
+
+	cache := proxy.NewSourceMapCache()
+	diagnosticCache := proxy.NewDiagnosticCache()
+
+	log.Info("creating gopls client")
+	clientProxy, clientInit := proxy.NewClient(log, cache, diagnosticCache)
+	_, goplsConn, goplsServer := protocol.NewClient(ctx, clientProxy, jsonrpc2.NewStream(rwc), log)
+	defer goplsConn.Close()
+
+	log.Info("creating proxy")
+	// Create the proxy to sit between.
+	serverProxy := proxy.NewServer(log, goplsServer, cache, diagnosticCache)
+
+	// Create templ server.
+	log.Info("creating templ server")
+	_, templConn, templClient := protocol.NewServer(context.Background(), serverProxy, templStream, log)
+	defer templConn.Close()
+
+	// Allow both the server and the client to initiate outbound requests.
+	clientInit(templClient)
+
+	// Start the web server if required.
+	if args.HTTPDebug != "" {
+		log.Info("starting debug http server", slog.String("addr", args.HTTPDebug))
+		h := httpdebug.NewHandler(log, serverProxy)
+		go func() {
+			if err := http.ListenAndServe(args.HTTPDebug, h); err != nil {
+				log.Error("web server failed", slog.Any("error", err))
+			}
+		}()
+	}
+
+	log.Info("listening")
+
+	select {
+	case <-ctx.Done():
+		log.Info("context closed")
+	case <-templConn.Done():
+		log.Info("templConn closed")
+	case <-goplsConn.Done():
+		log.Info("goplsConn closed")
+	}
+	log.Info("shutdown complete")
+	return
+}

templ/cmd/templ/lspcmd/pls/main.go

@@ -0,0 +1,124 @@
+package pls
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"os/exec"
+	"path"
+	"runtime"
+)
+
+// Options for the gopls client.
+type Options struct {
+	Log      string
+	RPCTrace bool
+}
+
+// AsArguments converts the options into command line arguments for gopls.
+func (opts Options) AsArguments() []string {
+	var args []string
+	if opts.Log != "" {
+		args = append(args, "-logfile", opts.Log)
+	}
+	if opts.RPCTrace {
+		args = append(args, "-rpc.trace")
+	}
+	return args
+}
+
+func FindGopls() (location string, err error) {
+	executableName := "gopls"
+	if runtime.GOOS == "windows" {
+		executableName = "gopls.exe"
+	}
+
+	pathLocation, err := exec.LookPath(executableName)
+	if err == nil {
+		// Found on the path.
+		return pathLocation, nil
+	}
+	// Unexpected error.
+	if !errors.Is(err, exec.ErrNotFound) {
+		return "", fmt.Errorf("unexpected error looking for gopls: %w", err)
+	}
+
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("unexpected error looking for gopls: %w", err)
+	}
+
+	// Probe standard locations.
+	locations := []string{
+		path.Join(home, "go", "bin", executableName),
+		path.Join(home, ".local", "bin", executableName),
+	}
+	for _, location := range locations {
+		_, err = os.Stat(location)
+		if err != nil {
+			continue
+		}
+		// Found in a standard location.
+		return location, nil
+	}
+
+	return "", fmt.Errorf("cannot find gopls on the path (%q), in $HOME/go/bin or $HOME/.local/bin/gopls. You can install gopls with `go install golang.org/x/tools/gopls@latest`", os.Getenv("PATH"))
+}
+
+// NewGopls starts gopls and opens up a jsonrpc2 connection to it.
+func NewGopls(ctx context.Context, log *slog.Logger, opts Options) (rwc io.ReadWriteCloser, err error) {
+	location, err := FindGopls()
+	if err != nil {
+		return nil, err
+	}
+	cmd := exec.Command(location, opts.AsArguments()...)
+	return newProcessReadWriteCloser(log, cmd)
+}
+
+// newProcessReadWriteCloser creates a processReadWriteCloser to allow stdin/stdout to be used as
+// a JSON RPC 2.0 transport.
+func newProcessReadWriteCloser(logger *slog.Logger, cmd *exec.Cmd) (rwc processReadWriteCloser, err error) {
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		return
+	}
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return
+	}
+	rwc = processReadWriteCloser{
+		in:  stdin,
+		out: stdout,
+	}
+	go func() {
+		if err := cmd.Run(); err != nil {
+			logger.Error("gopls command error", slog.Any("error", err))
+		}
+	}()
+	return
+}
+
+type processReadWriteCloser struct {
+	in  io.WriteCloser
+	out io.ReadCloser
+}
+
+func (prwc processReadWriteCloser) Read(p []byte) (n int, err error) {
+	return prwc.out.Read(p)
+}
+
+func (prwc processReadWriteCloser) Write(p []byte) (n int, err error) {
+	return prwc.in.Write(p)
+}
+
+func (prwc processReadWriteCloser) Close() error {
+	errInClose := prwc.in.Close()
+	errOutClose := prwc.out.Close()
+	if errInClose != nil || errOutClose != nil {
+		return fmt.Errorf("error closing process - in: %v, out: %v", errInClose, errOutClose)
+	}
+	return nil
+}

templ/cmd/templ/lspcmd/proxy/client.go

@@ -0,0 +1,143 @@
+package proxy
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"strings"
+
+	lsp "github.com/a-h/templ/lsp/protocol"
+)
+
+// Client is responsible for rewriting messages that are
+// originated from gopls, and are sent to the client.
+//
+// Since `gopls` is working on Go files, and this is the `templ` LSP,
+// the job of this code is to rewrite incoming requests to adjust the
+// file name from `*_templ.go` to `*.templ`, and to remap the char
+// positions where required.
+type Client struct {
+	Log             *slog.Logger
+	Target          lsp.Client
+	SourceMapCache  *SourceMapCache
+	DiagnosticCache *DiagnosticCache
+}
+
+func NewClient(log *slog.Logger, cache *SourceMapCache, diagnosticCache *DiagnosticCache) (c *Client, init func(lsp.Client)) {
+	c = &Client{
+		Log:             log,
+		SourceMapCache:  cache,
+		DiagnosticCache: diagnosticCache,
+	}
+	return c, func(target lsp.Client) {
+		c.Target = target
+	}
+}
+
+func (p Client) Progress(ctx context.Context, params *lsp.ProgressParams) (err error) {
+	p.Log.Info("client <- server: Progress")
+	return p.Target.Progress(ctx, params)
+}
+func (p Client) WorkDoneProgressCreate(ctx context.Context, params *lsp.WorkDoneProgressCreateParams) (err error) {
+	p.Log.Info("client <- server: WorkDoneProgressCreate")
+	return p.Target.WorkDoneProgressCreate(ctx, params)
+}
+
+func (p Client) LogMessage(ctx context.Context, params *lsp.LogMessageParams) (err error) {
+	p.Log.Info("client <- server: LogMessage", slog.String("message", params.Message))
+	return p.Target.LogMessage(ctx, params)
+}
+
+func (p Client) PublishDiagnostics(ctx context.Context, params *lsp.PublishDiagnosticsParams) (err error) {
+	p.Log.Info("client <- server: PublishDiagnostics")
+	if strings.HasSuffix(string(params.URI), "go.mod") {
+		p.Log.Info("client <- server: PublishDiagnostics: skipping go.mod diagnostics")
+		return nil
+	}
+	// Log diagnostics.
+	for i, diagnostic := range params.Diagnostics {
+		p.Log.Info(fmt.Sprintf("client <- server: PublishDiagnostics: [%d]", i), slog.Any("diagnostic", diagnostic))
+	}
+	// Get the sourcemap from the cache.
+	uri := strings.TrimSuffix(string(params.URI), "_templ.go") + ".templ"
+	sourceMap, ok := p.SourceMapCache.Get(uri)
+	if !ok {
+		p.Log.Error("unable to complete because the sourcemap for the URI doesn't exist in the cache", slog.String("uri", uri))
+		return fmt.Errorf("unable to complete because the sourcemap for %q doesn't exist in the cache, has the didOpen notification been sent yet?", uri)
+	}
+	params.URI = lsp.DocumentURI(uri)
+	// Rewrite the positions.
+	for i := 0; i < len(params.Diagnostics); i++ {
+		item := params.Diagnostics[i]
+		start, ok := sourceMap.SourcePositionFromTarget(item.Range.Start.Line, item.Range.Start.Character)
+		if !ok {
+			continue
+		}
+		if item.Range.Start.Line == item.Range.End.Line {
+			length := item.Range.End.Character - item.Range.Start.Character
+			item.Range.Start.Line = start.Line
+			item.Range.Start.Character = start.Col
+			item.Range.End.Line = start.Line
+			item.Range.End.Character = start.Col + length
+			params.Diagnostics[i] = item
+			p.Log.Info(fmt.Sprintf("diagnostic [%d] rewritten", i), slog.Any("diagnostic", item))
+			continue
+		}
+		end, ok := sourceMap.SourcePositionFromTarget(item.Range.End.Line, item.Range.End.Character)
+		if !ok {
+			continue
+		}
+		item.Range.Start.Line = start.Line
+		item.Range.Start.Character = start.Col
+		item.Range.End.Line = end.Line
+		item.Range.End.Character = end.Col
+		params.Diagnostics[i] = item
+		p.Log.Info(fmt.Sprintf("diagnostic [%d] rewritten", i), slog.Any("diagnostic", item))
+	}
+	params.Diagnostics = p.DiagnosticCache.AddTemplDiagnostics(uri, params.Diagnostics)
+	err = p.Target.PublishDiagnostics(ctx, params)
+	return err
+}
+
+func (p Client) ShowMessage(ctx context.Context, params *lsp.ShowMessageParams) (err error) {
+	p.Log.Info("client <- server: ShowMessage", slog.String("message", params.Message))
+	if strings.HasPrefix(params.Message, "Do not edit this file!") {
+		return
+	}
+	return p.Target.ShowMessage(ctx, params)
+}
+
+func (p Client) ShowMessageRequest(ctx context.Context, params *lsp.ShowMessageRequestParams) (result *lsp.MessageActionItem, err error) {
+	p.Log.Info("client <- server: ShowMessageRequest", slog.String("message", params.Message))
+	return p.Target.ShowMessageRequest(ctx, params)
+}
+
+func (p Client) Telemetry(ctx context.Context, params any) (err error) {
+	p.Log.Info("client <- server: Telemetry")
+	return p.Target.Telemetry(ctx, params)
+}
+
+func (p Client) RegisterCapability(ctx context.Context, params *lsp.RegistrationParams) (err error) {
+	p.Log.Info("client <- server: RegisterCapability")
+	return p.Target.RegisterCapability(ctx, params)
+}
+
+func (p Client) UnregisterCapability(ctx context.Context, params *lsp.UnregistrationParams) (err error) {
+	p.Log.Info("client <- server: UnregisterCapability")
+	return p.Target.UnregisterCapability(ctx, params)
+}
+
+func (p Client) ApplyEdit(ctx context.Context, params *lsp.ApplyWorkspaceEditParams) (result *lsp.ApplyWorkspaceEditResponse, err error) {
+	p.Log.Info("client <- server: ApplyEdit")
+	return p.Target.ApplyEdit(ctx, params)
+}
+
+func (p Client) Configuration(ctx context.Context, params *lsp.ConfigurationParams) (result []any, err error) {
+	p.Log.Info("client <- server: Configuration")
+	return p.Target.Configuration(ctx, params)
+}
+
+func (p Client) WorkspaceFolders(ctx context.Context) (result []lsp.WorkspaceFolder, err error) {
+	p.Log.Info("client <- server: WorkspaceFolders")
+	return p.Target.WorkspaceFolders(ctx)
+}

templ/cmd/templ/lspcmd/proxy/diagnosticcache.go

@@ -0,0 +1,61 @@
+package proxy
+
+import (
+	"sync"
+
+	lsp "github.com/a-h/templ/lsp/protocol"
+)
+
+func NewDiagnosticCache() *DiagnosticCache {
+	return &DiagnosticCache{
+		m:     &sync.Mutex{},
+		cache: make(map[string]fileDiagnostic),
+	}
+}
+
+type fileDiagnostic struct {
+	templDiagnostics []lsp.Diagnostic
+	goplsDiagnostics []lsp.Diagnostic
+}
+
+type DiagnosticCache struct {
+	m     *sync.Mutex
+	cache map[string]fileDiagnostic
+}
+
+func zeroLengthSliceIfNil(diags []lsp.Diagnostic) []lsp.Diagnostic {
+	if diags == nil {
+		return make([]lsp.Diagnostic, 0)
+	}
+	return diags
+}
+
+func (dc *DiagnosticCache) AddTemplDiagnostics(uri string, goDiagnostics []lsp.Diagnostic) []lsp.Diagnostic {
+	goDiagnostics = zeroLengthSliceIfNil(goDiagnostics)
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	diag := dc.cache[uri]
+	diag.goplsDiagnostics = goDiagnostics
+	diag.templDiagnostics = zeroLengthSliceIfNil(diag.templDiagnostics)
+	dc.cache[uri] = diag
+	return append(diag.templDiagnostics, goDiagnostics...)
+}
+
+func (dc *DiagnosticCache) ClearTemplDiagnostics(uri string) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	diag := dc.cache[uri]
+	diag.templDiagnostics = make([]lsp.Diagnostic, 0)
+	dc.cache[uri] = diag
+}
+
+func (dc *DiagnosticCache) AddGoDiagnostics(uri string, templDiagnostics []lsp.Diagnostic) []lsp.Diagnostic {
+	templDiagnostics = zeroLengthSliceIfNil(templDiagnostics)
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	diag := dc.cache[uri]
+	diag.templDiagnostics = templDiagnostics
+	diag.goplsDiagnostics = zeroLengthSliceIfNil(diag.goplsDiagnostics)
+	dc.cache[uri] = diag
+	return append(diag.goplsDiagnostics, templDiagnostics...)
+}

templ/cmd/templ/lspcmd/proxy/documentcontents.go

@@ -0,0 +1,215 @@
+package proxy
+
+import (
+	"fmt"
+	"log/slog"
+	"strings"
+	"sync"
+
+	lsp "github.com/a-h/templ/lsp/protocol"
+)
+
+// newDocumentContents creates a document content processing tool.
+func newDocumentContents(log *slog.Logger) *DocumentContents {
+	return &DocumentContents{
+		m:             new(sync.Mutex),
+		uriToContents: make(map[string]*Document),
+		log:           log,
+	}
+}
+
+type DocumentContents struct {
+	m             *sync.Mutex
+	uriToContents map[string]*Document
+	log           *slog.Logger
+}
+
+// Set the contents of a document.
+func (dc *DocumentContents) Set(uri string, d *Document) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	dc.uriToContents[uri] = d
+}
+
+// Get the contents of a document.
+func (dc *DocumentContents) Get(uri string) (d *Document, ok bool) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	d, ok = dc.uriToContents[uri]
+	return
+}
+
+// Delete a document from memory.
+func (dc *DocumentContents) Delete(uri string) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	delete(dc.uriToContents, uri)
+}
+
+func (dc *DocumentContents) URIs() (uris []string) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	uris = make([]string, len(dc.uriToContents))
+	var i int
+	for k := range dc.uriToContents {
+		uris[i] = k
+		i++
+	}
+	return uris
+}
+
+// Apply changes to the document from the client, and return a list of change requests to send back to the client.
+func (dc *DocumentContents) Apply(uri string, changes []lsp.TextDocumentContentChangeEvent) (d *Document, err error) {
+	dc.m.Lock()
+	defer dc.m.Unlock()
+	var ok bool
+	d, ok = dc.uriToContents[uri]
+	if !ok {
+		err = fmt.Errorf("document not found")
+		return
+	}
+	for _, change := range changes {
+		d.Apply(change.Range, change.Text)
+	}
+	return
+}
+
+func NewDocument(log *slog.Logger, s string) *Document {
+	return &Document{
+		Log:   log,
+		Lines: strings.Split(s, "\n"),
+	}
+}
+
+type Document struct {
+	Log   *slog.Logger
+	Lines []string
+}
+
+func (d *Document) LineLengths() (lens []int) {
+	lens = make([]int, len(d.Lines))
+	for i, l := range d.Lines {
+		lens[i] = len(l)
+	}
+	return
+}
+
+func (d *Document) Len() (line, col int) {
+	line = len(d.Lines)
+	col = len(d.Lines[len(d.Lines)-1])
+	return
+}
+
+func (d *Document) Overwrite(fromLine, fromCol, toLine, toCol int, lines []string) {
+	suffix := d.Lines[toLine][toCol:]
+	toLen := d.LineLengths()[toLine]
+	d.Delete(fromLine, fromCol, toLine, toLen)
+	lines[len(lines)-1] = lines[len(lines)-1] + suffix
+	d.Insert(fromLine, fromCol, lines)
+}
+
+func (d *Document) Insert(line, col int, lines []string) {
+	prefix := d.Lines[line][:col]
+	suffix := d.Lines[line][col:]
+	lines[0] = prefix + lines[0]
+	d.Lines[line] = lines[0]
+
+	if len(lines) > 1 {
+		d.InsertLines(line+1, lines[1:])
+	}
+
+	d.Lines[line+len(lines)-1] = lines[len(lines)-1] + suffix
+}
+
+func (d *Document) InsertLines(i int, withLines []string) {
+	d.Lines = append(d.Lines[:i], append(withLines, d.Lines[i:]...)...)
+}
+
+func (d *Document) Delete(fromLine, fromCol, toLine, toCol int) {
+	prefix := d.Lines[fromLine][:fromCol]
+	suffix := d.Lines[toLine][toCol:]
+
+	// Delete intermediate lines.
+	deleteFrom := fromLine
+	deleteTo := fromLine + (toLine - fromLine)
+	d.DeleteLines(deleteFrom, deleteTo)
+
+	// Merge the contents of the final line.
+	d.Lines[fromLine] = prefix + suffix
+}
+
+func (d *Document) DeleteLines(i, j int) {
+	d.Lines = append(d.Lines[:i], d.Lines[j:]...)
+}
+
+func (d *Document) String() string {
+	return strings.Join(d.Lines, "\n")
+}
+
+func (d *Document) Replace(with string) {
+	d.Lines = strings.Split(with, "\n")
+}
+
+func (d *Document) Apply(r *lsp.Range, with string) {
+	withLines := strings.Split(with, "\n")
+	d.normalize(r)
+	if d.isWholeDocument(r) {
+		d.Lines = withLines
+		return
+	}
+	if d.isInsert(r, with) {
+		d.Insert(int(r.Start.Line), int(r.Start.Character), withLines)
+		return
+	}
+	if d.isDelete(r, with) {
+		d.Delete(int(r.Start.Line), int(r.Start.Character), int(r.End.Line), int(r.End.Character))
+		return
+	}
+	if d.isOverwrite(r, with) {
+		d.Overwrite(int(r.Start.Line), int(r.Start.Character), int(r.End.Line), int(r.End.Character), withLines)
+	}
+}
+
+func (d *Document) normalize(r *lsp.Range) {
+	if r == nil {
+		return
+	}
+	lens := d.LineLengths()
+	if r.Start.Line >= uint32(len(lens)) {
+		r.Start.Line = uint32(len(lens) - 1)
+		r.Start.Character = uint32(lens[r.Start.Line])
+	}
+	if r.Start.Character > uint32(lens[r.Start.Line]) {
+		r.Start.Character = uint32(lens[r.Start.Line])
+	}
+	if r.End.Line >= uint32(len(lens)) {
+		r.End.Line = uint32(len(lens) - 1)
+		r.End.Character = uint32(lens[r.End.Line])
+	}
+	if r.End.Character > uint32(lens[r.End.Line]) {
+		r.End.Character = uint32(lens[r.End.Line])
+	}
+}
+
+func (d *Document) isOverwrite(r *lsp.Range, with string) bool {
+	return (r.End.Line != r.Start.Line || r.Start.Character != r.End.Character) && with != ""
+}
+
+func (d *Document) isInsert(r *lsp.Range, with string) bool {
+	return r.End.Line == r.Start.Line && r.Start.Character == r.End.Character && with != ""
+}
+
+func (d *Document) isDelete(r *lsp.Range, with string) bool {
+	return (r.End.Line != r.Start.Line || r.Start.Character != r.End.Character) && with == ""
+}
+
+func (d *Document) isWholeDocument(r *lsp.Range) bool {
+	if r == nil {
+		return true
+	}
+	if r.Start.Line != 0 || r.Start.Character != 0 {
+		return false
+	}
+	l, c := d.Len()
+	return r.End.Line == uint32(l) || r.End.Character == uint32(c)
+}

templ/cmd/templ/lspcmd/proxy/documentcontents_test.go

@@ -0,0 +1,571 @@
+package proxy
+
+import (
+	"log/slog"
+	"os"
+	"testing"
+
+	lsp "github.com/a-h/templ/lsp/protocol"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestDocument(t *testing.T) {
+	tests := []struct {
+		name       string
+		start      string
+		operations []func(d *Document)
+		expected   string
+	}{
+		{
+			name:  "Replace all content if the range is nil",
+			start: "0\n1\n2",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(nil, "replaced")
+				},
+			},
+			expected: "replaced",
+		},
+		{
+			name:  "If the range matches the length of the file, all of it is replaced",
+			start: "0\n1\n2",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 1,
+						},
+					}, "replaced")
+				},
+			},
+			expected: "replaced",
+		},
+		{
+			name:  "Can insert new text",
+			start: ``,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+					}, "abc")
+				},
+			},
+			expected: "abc",
+		},
+		{
+			name:  "Can insert new text that ends with a newline",
+			start: ``,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+					}, "abc\n")
+				},
+			},
+			expected: `abc
+`,
+		},
+		{
+			name: "Can insert a new line at the end of existing text",
+			start: `abc
+`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 3,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 3,
+						},
+					}, "\n")
+				},
+			},
+			expected: `abc
+
+`,
+		},
+		{
+			name:  "Can insert a word at the start of existing text",
+			start: `bc`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+					}, "a")
+				},
+			},
+			expected: `abc`,
+		},
+		{
+			name:  "Can remove whole line",
+			start: "0\n1\n2",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 0,
+						},
+					}, "")
+				},
+			},
+			expected: "0\n2",
+		},
+		{
+			name:  "Can remove line prefix",
+			start: "abcdef",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 3,
+						},
+					}, "")
+				},
+			},
+			expected: "def",
+		},
+		{
+			name:  "Can remove line substring",
+			start: "abcdef",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 2,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 3,
+						},
+					}, "")
+				},
+			},
+			expected: "abdef",
+		},
+		{
+			name:  "Can remove line suffix",
+			start: "abcdef",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 4,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 6,
+						},
+					}, "")
+				},
+			},
+			expected: "abcd",
+		},
+		{
+			name:  "Can remove across lines",
+			start: "0\n1\n22",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 1,
+						},
+					}, "")
+				},
+			},
+			expected: "0\n2",
+		},
+		{
+			name:  "Can remove part of two lines",
+			start: "Line one\nLine two\nLine three",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 4,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 4,
+						},
+					}, "")
+				},
+			},
+			expected: "Line three",
+		},
+		{
+			name:  "Can remove all lines",
+			start: "0\n1\n2",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 1,
+						},
+					}, "")
+				},
+			},
+			expected: "",
+		},
+		{
+			name:  "Can replace line prefix",
+			start: "012345",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 3,
+						},
+					}, "ABCDEFG")
+				},
+			},
+			expected: "ABCDEFG345",
+		},
+		{
+			name:  "Can replace text across line boundaries",
+			start: "Line one\nLine two\nLine three",
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 4,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 4,
+						},
+					}, " one test\nNew Line 2\nNew line")
+				},
+			},
+			expected: "Line one test\nNew Line 2\nNew line three",
+		},
+		{
+			name:  "Can add new line to end of single line",
+			start: `a`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 1,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 1,
+						},
+					}, "\nb")
+				},
+			},
+			expected: "a\nb",
+		},
+		{
+			name:  "Exceeding the col and line count rounds down to the end of the file",
+			start: `a`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      200,
+							Character: 600,
+						},
+						End: lsp.Position{
+							Line:      300,
+							Character: 1200,
+						},
+					}, "\nb")
+				},
+			},
+			expected: "a\nb",
+		},
+		{
+			name:  "Can remove a line and add it back from the end of the previous line (insert)",
+			start: "a\nb\nc",
+			operations: []func(d *Document){
+				func(d *Document) {
+					// Delete.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 0,
+						},
+					}, "")
+					// Put it back.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 1,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 1,
+						},
+					}, "\nb")
+				},
+			},
+			expected: "a\nb\nc",
+		},
+		{
+			name:  "Can remove a line and add it back from the end of the previous line (overwrite)",
+			start: "a\nb\nc",
+			operations: []func(d *Document){
+				func(d *Document) {
+					// Delete.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 0,
+						},
+					}, "")
+					// Put it back.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 1,
+						},
+						End: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+					}, "\nb\n")
+				},
+			},
+			expected: "a\nb\nc",
+		},
+		{
+			name: "Add new line with indent to the end of the line",
+			// Based on log entry.
+			// {"level":"info","ts":"2022-06-04T20:55:15+01:00","caller":"proxy/server.go:391","msg":"client -> server: DidChange","params":{"textDocument":{"uri":"file:///Users/adrian/github.com/a-h/templ/generator/test-call/template.templ","version":2},"contentChanges":[{"range":{"start":{"line":4,"character":21},"end":{"line":4,"character":21}},"text":"\n\t\t"}]}}
+			start: `package testcall
+
+templ personTemplate(p person) {
+	<div>
+		<h1>{ p.name }</h1>
+	</div>
+}
+`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      4,
+							Character: 21,
+						},
+						End: lsp.Position{
+							Line:      4,
+							Character: 21,
+						},
+					}, "\n\t\t")
+				},
+			},
+			expected: `package testcall
+
+templ personTemplate(p person) {
+	<div>
+		<h1>{ p.name }</h1>
+		
+	</div>
+}
+`,
+		},
+		{
+			name: "Recreate error smaller",
+			// Based on log entry.
+			// {"level":"info","ts":"2022-06-04T20:55:15+01:00","caller":"proxy/server.go:391","msg":"client -> server: DidChange","params":{"textDocument":{"uri":"file:///Users/adrian/github.com/a-h/templ/generator/test-call/template.templ","version":2},"contentChanges":[{"range":{"start":{"line":4,"character":21},"end":{"line":4,"character":21}},"text":"\n\t\t"}]}}
+			start: "line1\n\t\tline2\nline3",
+			operations: []func(d *Document){
+				func(d *Document) {
+					// Remove \t\tline2
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 0,
+						},
+					}, "")
+					// Put it back.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 5,
+						},
+						End: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+					},
+						"\n\t\tline2\n")
+				},
+			},
+			expected: "line1\n\t\tline2\nline3",
+		},
+		{
+			name: "Recreate error",
+			// Based on log entry.
+			// {"level":"info","ts":"2022-06-04T20:55:15+01:00","caller":"proxy/server.go:391","msg":"client -> server: DidChange","params":{"textDocument":{"uri":"file:///Users/adrian/github.com/a-h/templ/generator/test-call/template.templ","version":2},"contentChanges":[{"range":{"start":{"line":4,"character":21},"end":{"line":4,"character":21}},"text":"\n\t\t"}]}}
+			start: ` <footer data-testid="footerTemplate">
+		<div>&copy; { fmt.Sprintf("%d", time.Now().Year()) }</div>
+	</footer>
+}
+`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					// Remove <div>&copy; { fmt.Sprintf("%d", time.Now().Year()) }</div>
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      2,
+							Character: 0,
+						},
+					}, "")
+					// Put it back.
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 38,
+						},
+						End: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+					},
+						"\n\t\t<div>&copy; { fmt.Sprintf(\"%d\", time.Now().Year()) }</div>\n")
+				},
+			},
+			expected: ` <footer data-testid="footerTemplate">
+		<div>&copy; { fmt.Sprintf("%d", time.Now().Year()) }</div>
+	</footer>
+}
+`,
+		},
+		{
+			name: "Insert at start of line",
+			// Based on log entry.
+			// {"level":"info","ts":"2023-03-25T17:17:38Z","caller":"proxy/server.go:393","msg":"client -> server: DidChange","params":{"textDocument":{"uri":"file:///Users/adrian/github.com/a-h/templ/generator/test-call/template.templ","version":5},"contentChanges":[{"range":{"start":{"line":6,"character":0},"end":{"line":6,"character":0}},"text":"a"}]}}
+			start: `b`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      0,
+							Character: 0,
+						},
+					}, "a")
+				},
+			},
+			expected: `ab`,
+		},
+		{
+			name: "Insert full new line",
+			start: `a
+c
+d`,
+			operations: []func(d *Document){
+				func(d *Document) {
+					d.Apply(&lsp.Range{
+						Start: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+						End: lsp.Position{
+							Line:      1,
+							Character: 0,
+						},
+					}, "b\n")
+				},
+			},
+			expected: `a
+b
+c
+d`,
+		},
+	}
+
+	for _, tt := range tests {
+		logger := slog.New(slog.NewJSONHandler(os.Stderr, nil))
+		t.Run(tt.name, func(t *testing.T) {
+			d := NewDocument(logger, tt.start)
+			for _, f := range tt.operations {
+				f(d)
+			}
+			actual := d.String()
+			if diff := cmp.Diff(tt.expected, actual); diff != "" {
+				t.Error(diff)
+			}
+		})
+	}
+}

templ/cmd/templ/lspcmd/proxy/import_test.go

@@ -0,0 +1,293 @@
+package proxy
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestFindLastImport(t *testing.T) {
+	tests := []struct {
+		name          string
+		templContents string
+		packageName   string
+		expected      string
+	}{
+		{
+			name: "if there are no imports, add a single line import",
+			templContents: `package main
+
+templ example() {
+}
+`,
+			packageName: "strings",
+			expected: `package main
+
+import "strings"
+
+templ example() {
+}
+`,
+		},
+		{
+			name: "if there is an existing single-line imports, add one at the end",
+			templContents: `package main
+
+import "strings"
+
+templ example() {
+}
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+templ example() {
+}
+`,
+		},
+		{
+			name: "if there are multiple existing single-line imports, add one at the end",
+			templContents: `package main
+
+import "strings"
+import "fmt"
+
+templ example() {
+}
+`,
+			packageName: "time",
+			expected: `package main
+
+import "strings"
+import "fmt"
+import "time"
+
+templ example() {
+}
+`,
+		},
+		{
+			name: "if there are existing multi-line imports, add one at the end",
+			templContents: `package main
+
+import (
+	"strings"
+)
+
+templ example() {
+}
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import (
+	"strings"
+	"fmt"
+)
+
+templ example() {
+}
+`,
+		},
+		{
+			name: "ignore imports that happen after templates",
+			templContents: `package main
+
+import "strings"
+
+templ example() {
+}
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+templ example() {
+}
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after funcs in the file",
+			templContents: `package main
+
+import "strings"
+
+func example() {
+}
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+func example() {
+}
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after css expressions in the file",
+			templContents: `package main
+
+import "strings"
+
+css example() {
+}
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+css example() {
+}
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after script expressions in the file",
+			templContents: `package main
+
+import "strings"
+
+script example() {
+}
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+script example() {
+}
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after var expressions in the file",
+			templContents: `package main
+
+import "strings"
+
+var s string
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+var s string
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after const expressions in the file",
+			templContents: `package main
+
+import "strings"
+
+const s = "test"
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+const s = "test"
+
+import "other"
+`,
+		},
+		{
+			name: "ignore imports that happen after type expressions in the file",
+			templContents: `package main
+
+import "strings"
+
+type Value int
+
+import "other"
+`,
+			packageName: "fmt",
+			expected: `package main
+
+import "strings"
+import "fmt"
+
+type Value int
+
+import "other"
+`,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			lines := strings.Split(test.templContents, "\n")
+			imp := addImport(lines, fmt.Sprintf("%q", test.packageName))
+			textWithoutNewline := strings.TrimSuffix(imp.Text, "\n")
+			actualLines := append(lines[:imp.LineIndex], append([]string{textWithoutNewline}, lines[imp.LineIndex:]...)...)
+			actual := strings.Join(actualLines, "\n")
+			if diff := cmp.Diff(test.expected, actual); diff != "" {
+				t.Error(diff)
+			}
+		})
+	}
+}
+
+func TestGetPackageFromItemDetail(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{
+			input:    `"fmt"`,
+			expected: `"fmt"`,
+		},
+		{
+			input:    `func(state fmt.State, verb rune) string (from "fmt")`,
+			expected: `"fmt"`,
+		},
+		{
+			input:    `non matching`,
+			expected: `non matching`,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.input, func(t *testing.T) {
+			actual := getPackageFromItemDetail(test.input)
+			if test.expected != actual {
+				t.Errorf("expected %q, got %q", test.expected, actual)
+			}
+		})
+	}
+}

templ/cmd/templ/lspcmd/proxy/rewrite.go

@@ -0,0 +1,24 @@
+package proxy
+
+import (
+	"path"
+	"strings"
+
+	lsp "github.com/a-h/templ/lsp/protocol"
+)
+
+func convertTemplToGoURI(templURI lsp.DocumentURI) (isTemplFile bool, goURI lsp.DocumentURI) {
+	base, fileName := path.Split(string(templURI))
+	if !strings.HasSuffix(fileName, ".templ") {
+		return
+	}
+	return true, lsp.DocumentURI(base + (strings.TrimSuffix(fileName, ".templ") + "_templ.go"))
+}
+
+func convertTemplGoToTemplURI(goURI lsp.DocumentURI) (isTemplGoFile bool, templURI lsp.DocumentURI) {
+	base, fileName := path.Split(string(goURI))
+	if !strings.HasSuffix(fileName, "_templ.go") {
+		return
+	}
+	return true, lsp.DocumentURI(base + (strings.TrimSuffix(fileName, "_templ.go") + ".templ"))
+}

templ/cmd/templ/lspcmd/proxy/snippets.go

@@ -0,0 +1,111 @@
+package proxy
+
+import lsp "github.com/a-h/templ/lsp/protocol"
+
+var htmlSnippets = []lsp.CompletionItem{
+	{
+		Label: "<?>",
+		InsertText: `${1}>
+	${0}
+</${1}>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "a",
+		InsertText:       `a href="${1:}">${2:}</a>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "button",
+		InsertText:       `button type="button" ${1:}>${2:}</button>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label: "div",
+		InsertText: `div>
+	${0}
+</div>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label: "p",
+		InsertText: `p>
+    ${0}
+</p>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label: "head",
+		InsertText: `head>
+    ${0}
+</head>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label: "body",
+		InsertText: `body>
+    ${0}
+</body>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "title",
+		InsertText:       `title>${0}</title>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h1",
+		InsertText:       `h1>${0}</h1>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h2",
+		InsertText:       `h2>${0}</h2>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h3",
+		InsertText:       `h3>${0}</h3>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h4",
+		InsertText:       `h4>${0}</h4>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h5",
+		InsertText:       `h5>${0}</h5>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+	{
+		Label:            "h6",
+		InsertText:       `h6>${0}</h6>`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+}
+
+var snippet = []lsp.CompletionItem{
+	{
+		Label: "templ",
+		InsertText: `templ ${2:TemplateName}() {
+	${0}
+}`,
+		Kind:             lsp.CompletionItemKind(lsp.CompletionItemKindSnippet),
+		InsertTextFormat: lsp.InsertTextFormatSnippet,
+	},
+}

templ/cmd/templ/lspcmd/proxy/sourcemapcache.go

@@ -0,0 +1,52 @@
+package proxy
+
+import (
+	"sync"
+
+	"github.com/a-h/templ/parser/v2"
+)
+
+// NewSourceMapCache creates a cache of .templ file URIs to the source map.
+func NewSourceMapCache() *SourceMapCache {
+	return &SourceMapCache{
+		m:              new(sync.Mutex),
+		uriToSourceMap: make(map[string]*parser.SourceMap),
+	}
+}
+
+// SourceMapCache is a cache of .templ file URIs to the source map.
+type SourceMapCache struct {
+	m              *sync.Mutex
+	uriToSourceMap map[string]*parser.SourceMap
+}
+
+func (fc *SourceMapCache) Set(uri string, m *parser.SourceMap) {
+	fc.m.Lock()
+	defer fc.m.Unlock()
+	fc.uriToSourceMap[uri] = m
+}
+
+func (fc *SourceMapCache) Get(uri string) (m *parser.SourceMap, ok bool) {
+	fc.m.Lock()
+	defer fc.m.Unlock()
+	m, ok = fc.uriToSourceMap[uri]
+	return
+}
+
+func (fc *SourceMapCache) Delete(uri string) {
+	fc.m.Lock()
+	defer fc.m.Unlock()
+	delete(fc.uriToSourceMap, uri)
+}
+
+func (fc *SourceMapCache) URIs() (uris []string) {
+	fc.m.Lock()
+	defer fc.m.Unlock()
+	uris = make([]string, len(fc.uriToSourceMap))
+	var i int
+	for k := range fc.uriToSourceMap {
+		uris[i] = k
+		i++
+	}
+	return uris
+}

templ/cmd/templ/lspcmd/stdrwc.go

@@ -0,0 +1,50 @@
+package lspcmd
+
+import (
+	"errors"
+	"io"
+	"log/slog"
+)
+
+// stdrwc (standard read/write closer) reads from stdin, and writes to stdout.
+func newStdRwc(log *slog.Logger, name string, w io.Writer, r io.Reader) stdrwc {
+	return stdrwc{
+		log:  log,
+		name: name,
+		w:    w,
+		r:    r,
+	}
+}
+
+type stdrwc struct {
+	log  *slog.Logger
+	name string
+	w    io.Writer
+	r    io.Reader
+}
+
+func (s stdrwc) Read(p []byte) (int, error) {
+	return s.r.Read(p)
+}
+
+func (s stdrwc) Write(p []byte) (int, error) {
+	return s.w.Write(p)
+}
+
+func (s stdrwc) Close() error {
+	s.log.Info("rwc: closing", slog.String("name", s.name))
+	var errs []error
+	if closer, isCloser := s.r.(io.Closer); isCloser {
+		if err := closer.Close(); err != nil {
+			s.log.Error("rwc: error closing reader", slog.String("name", s.name), slog.Any("error", err))
+			errs = append(errs, err)
+		}
+	}
+	if closer, isCloser := s.w.(io.Closer); isCloser {
+		if err := closer.Close(); err != nil {
+			s.log.Error("rwc: error closing writer", slog.String("name", s.name), slog.Any("error", err))
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}

templ/cmd/templ/main.go

@@ -0,0 +1,394 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"os/signal"
+	"runtime"
+
+	"github.com/a-h/templ"
+	"github.com/a-h/templ/cmd/templ/fmtcmd"
+	"github.com/a-h/templ/cmd/templ/generatecmd"
+	"github.com/a-h/templ/cmd/templ/infocmd"
+	"github.com/a-h/templ/cmd/templ/lspcmd"
+	"github.com/a-h/templ/cmd/templ/sloghandler"
+	"github.com/fatih/color"
+)
+
+func main() {
+	code := run(os.Stdin, os.Stdout, os.Stderr, os.Args)
+	if code != 0 {
+		os.Exit(code)
+	}
+}
+
+const usageText = `usage: templ <command> [<args>...]
+
+templ - build HTML UIs with Go
+
+See docs at https://templ.guide
+
+commands:
+  generate   Generates Go code from templ files
+  fmt        Formats templ files
+  lsp        Starts a language server for templ files
+  info       Displays information about the templ environment
+  version    Prints the version
+`
+
+func run(stdin io.Reader, stdout, stderr io.Writer, args []string) (code int) {
+	if len(args) < 2 {
+		fmt.Fprint(stderr, usageText)
+		return 64 // EX_USAGE
+	}
+	switch args[1] {
+	case "info":
+		return infoCmd(stdout, stderr, args[2:])
+	case "generate":
+		return generateCmd(stdout, stderr, args[2:])
+	case "fmt":
+		return fmtCmd(stdin, stdout, stderr, args[2:])
+	case "lsp":
+		return lspCmd(stdin, stdout, stderr, args[2:])
+	case "version", "--version":
+		fmt.Fprintln(stdout, templ.Version())
+		return 0
+	case "help", "-help", "--help", "-h":
+		fmt.Fprint(stdout, usageText)
+		return 0
+	}
+	fmt.Fprint(stderr, usageText)
+	return 64 // EX_USAGE
+}
+
+func newLogger(logLevel string, verbose bool, stderr io.Writer) *slog.Logger {
+	if verbose {
+		logLevel = "debug"
+	}
+	level := slog.LevelInfo.Level()
+	switch logLevel {
+	case "debug":
+		level = slog.LevelDebug.Level()
+	case "warn":
+		level = slog.LevelWarn.Level()
+	case "error":
+		level = slog.LevelError.Level()
+	}
+	return slog.New(sloghandler.NewHandler(stderr, &slog.HandlerOptions{
+		AddSource: logLevel == "debug",
+		Level:     level,
+	}))
+}
+
+const infoUsageText = `usage: templ info [<args>...]
+
+Displays information about the templ environment.
+
+Args:
+  -json
+    Output information in JSON format to stdout. (default false)
+  -v
+    Set log verbosity level to "debug". (default "info")
+  -log-level
+    Set log verbosity level. (default "info", options: "debug", "info", "warn", "error")
+  -help
+    Print help and exit.
+`
+
+func infoCmd(stdout, stderr io.Writer, args []string) (code int) {
+	cmd := flag.NewFlagSet("diagnose", flag.ExitOnError)
+	jsonFlag := cmd.Bool("json", false, "")
+	verboseFlag := cmd.Bool("v", false, "")
+	logLevelFlag := cmd.String("log-level", "info", "")
+	helpFlag := cmd.Bool("help", false, "")
+	err := cmd.Parse(args)
+	if err != nil {
+		fmt.Fprint(stderr, infoUsageText)
+		return 64 // EX_USAGE
+	}
+	if *helpFlag {
+		fmt.Fprint(stdout, infoUsageText)
+		return
+	}
+
+	log := newLogger(*logLevelFlag, *verboseFlag, stderr)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	signalChan := make(chan os.Signal, 1)
+	signal.Notify(signalChan, os.Interrupt)
+	go func() {
+		<-signalChan
+		fmt.Fprintln(stderr, "Stopping...")
+		cancel()
+	}()
+
+	err = infocmd.Run(ctx, log, stdout, infocmd.Arguments{
+		JSON: *jsonFlag,
+	})
+	if err != nil {
+		color.New(color.FgRed).Fprint(stderr, "(✗) ")
+		fmt.Fprintln(stderr, "Command failed: "+err.Error())
+		return 1
+	}
+	return 0
+}
+
+const generateUsageText = `usage: templ generate [<args>...]
+
+Generates Go code from templ files.
+
+Args:
+  -path <path>
+    Generates code for all files in path. (default .)
+  -f <file>
+    Optionally generates code for a single file, e.g. -f header.templ
+  -stdout
+    Prints to stdout instead of writing generated files to the filesystem.
+    Only applicable when -f is used.
+  -source-map-visualisations
+    Set to true to generate HTML files to visualise the templ code and its corresponding Go code.
+  -include-version
+    Set to false to skip inclusion of the templ version in the generated code. (default true)
+  -include-timestamp
+    Set to true to include the current time in the generated code.
+  -watch
+    Set to true to watch the path for changes and regenerate code.
+  -watch-pattern <regexp>
+    Set the regexp pattern of files that will be watched for changes. (default: '(.+\.go$)|(.+\.templ$)|(.+_templ\.txt$)')
+  -cmd <cmd>
+    Set the command to run after generating code.
+  -proxy
+    Set the URL to proxy after generating code and executing the command.
+  -proxyport
+    The port the proxy will listen on. (default 7331)
+  -proxybind
+    The address the proxy will listen on. (default 127.0.0.1)
+  -notify-proxy
+    If present, the command will issue a reload event to the proxy 127.0.0.1:7331, or use proxyport and proxybind to specify a different address.
+  -w
+    Number of workers to use when generating code. (default runtime.NumCPUs)
+  -lazy
+    Only generate .go files if the source .templ file is newer.	
+  -pprof
+    Port to run the pprof server on.
+  -keep-orphaned-files
+    Keeps orphaned generated templ files. (default false)
+  -v
+    Set log verbosity level to "debug". (default "info")
+  -log-level
+    Set log verbosity level. (default "info", options: "debug", "info", "warn", "error")
+  -help
+    Print help and exit.
+
+Examples:
+
+  Generate code for all files in the current directory and subdirectories:
+
+    templ generate
+
+  Generate code for a single file:
+
+    templ generate -f header.templ
+
+  Watch the current directory and subdirectories for changes and regenerate code:
+
+    templ generate -watch
+`
+
+func generateCmd(stdout, stderr io.Writer, args []string) (code int) {
+	cmd := flag.NewFlagSet("generate", flag.ExitOnError)
+	fileNameFlag := cmd.String("f", "", "")
+	pathFlag := cmd.String("path", ".", "")
+	toStdoutFlag := cmd.Bool("stdout", false, "")
+	sourceMapVisualisationsFlag := cmd.Bool("source-map-visualisations", false, "")
+	includeVersionFlag := cmd.Bool("include-version", true, "")
+	includeTimestampFlag := cmd.Bool("include-timestamp", false, "")
+	watchFlag := cmd.Bool("watch", false, "")
+	watchPatternFlag := cmd.String("watch-pattern", "(.+\\.go$)|(.+\\.templ$)|(.+_templ\\.txt$)", "")
+	openBrowserFlag := cmd.Bool("open-browser", true, "")
+	cmdFlag := cmd.String("cmd", "", "")
+	proxyFlag := cmd.String("proxy", "", "")
+	proxyPortFlag := cmd.Int("proxyport", 7331, "")
+	proxyBindFlag := cmd.String("proxybind", "127.0.0.1", "")
+	notifyProxyFlag := cmd.Bool("notify-proxy", false, "")
+	workerCountFlag := cmd.Int("w", runtime.NumCPU(), "")
+	pprofPortFlag := cmd.Int("pprof", 0, "")
+	keepOrphanedFilesFlag := cmd.Bool("keep-orphaned-files", false, "")
+	verboseFlag := cmd.Bool("v", false, "")
+	logLevelFlag := cmd.String("log-level", "info", "")
+	lazyFlag := cmd.Bool("lazy", false, "")
+	helpFlag := cmd.Bool("help", false, "")
+	err := cmd.Parse(args)
+	if err != nil {
+		fmt.Fprint(stderr, generateUsageText)
+		return 64 // EX_USAGE
+	}
+	if *helpFlag {
+		fmt.Fprint(stdout, generateUsageText)
+		return
+	}
+
+	log := newLogger(*logLevelFlag, *verboseFlag, stderr)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	signalChan := make(chan os.Signal, 1)
+	signal.Notify(signalChan, os.Interrupt)
+	go func() {
+		<-signalChan
+		fmt.Fprintln(stderr, "Stopping...")
+		cancel()
+	}()
+
+	var fw generatecmd.FileWriterFunc
+	if *toStdoutFlag {
+		fw = generatecmd.WriterFileWriter(stdout)
+	}
+
+	err = generatecmd.Run(ctx, log, generatecmd.Arguments{
+		FileName:                        *fileNameFlag,
+		Path:                            *pathFlag,
+		FileWriter:                      fw,
+		Watch:                           *watchFlag,
+		WatchPattern:                    *watchPatternFlag,
+		OpenBrowser:                     *openBrowserFlag,
+		Command:                         *cmdFlag,
+		Proxy:                           *proxyFlag,
+		ProxyPort:                       *proxyPortFlag,
+		ProxyBind:                       *proxyBindFlag,
+		NotifyProxy:                     *notifyProxyFlag,
+		WorkerCount:                     *workerCountFlag,
+		GenerateSourceMapVisualisations: *sourceMapVisualisationsFlag,
+		IncludeVersion:                  *includeVersionFlag,
+		IncludeTimestamp:                *includeTimestampFlag,
+		PPROFPort:                       *pprofPortFlag,
+		KeepOrphanedFiles:               *keepOrphanedFilesFlag,
+		Lazy:                            *lazyFlag,
+	})
+	if err != nil {
+		color.New(color.FgRed).Fprint(stderr, "(✗) ")
+		fmt.Fprintln(stderr, "Command failed: "+err.Error())
+		return 1
+	}
+	return 0
+}
+
+const fmtUsageText = `usage: templ fmt [<args> ...]
+
+Format all files in directory:
+
+  templ fmt .
+
+Format stdin to stdout:
+
+  templ fmt < header.templ
+
+Format file or directory to stdout:
+
+  templ fmt -stdout FILE
+
+Args:
+  -stdout
+    Prints to stdout instead of in-place format
+  -stdin-filepath
+    Provides the formatter with filepath context when using -stdout.
+    Required for organising imports.
+  -v
+    Set log verbosity level to "debug". (default "info")
+  -log-level
+    Set log verbosity level. (default "info", options: "debug", "info", "warn", "error")
+  -w
+    Number of workers to use when formatting code. (default runtime.NumCPUs).
+  -fail
+    Fails with exit code 1 if files are changed. (e.g. in CI)
+  -help
+    Print help and exit.
+`
+
+func fmtCmd(stdin io.Reader, stdout, stderr io.Writer, args []string) (code int) {
+	cmd := flag.NewFlagSet("fmt", flag.ExitOnError)
+	helpFlag := cmd.Bool("help", false, "")
+	workerCountFlag := cmd.Int("w", runtime.NumCPU(), "")
+	verboseFlag := cmd.Bool("v", false, "")
+	logLevelFlag := cmd.String("log-level", "info", "")
+	failIfChanged := cmd.Bool("fail", false, "")
+	stdoutFlag := cmd.Bool("stdout", false, "")
+	stdinFilepath := cmd.String("stdin-filepath", "", "")
+	err := cmd.Parse(args)
+	if err != nil {
+		fmt.Fprint(stderr, fmtUsageText)
+		return 64 // EX_USAGE
+	}
+	if *helpFlag {
+		fmt.Fprint(stdout, fmtUsageText)
+		return
+	}
+
+	log := newLogger(*logLevelFlag, *verboseFlag, stderr)
+
+	err = fmtcmd.Run(log, stdin, stdout, fmtcmd.Arguments{
+		ToStdout:      *stdoutFlag,
+		Files:         cmd.Args(),
+		WorkerCount:   *workerCountFlag,
+		StdinFilepath: *stdinFilepath,
+		FailIfChanged: *failIfChanged,
+	})
+	if err != nil {
+		return 1
+	}
+	return 0
+}
+
+const lspUsageText = `usage: templ lsp [<args> ...]
+
+Starts a language server for templ.
+
+Args:
+  -log string
+    The file to log templ LSP output to, or leave empty to disable logging.
+  -goplsLog string
+    The file to log gopls output, or leave empty to disable logging.
+  -goplsRPCTrace
+    Set gopls to log input and output messages.
+  -help
+    Print help and exit.
+  -pprof
+    Enable pprof web server (default address is localhost:9999)
+  -http string
+    Enable http debug server by setting a listen address (e.g. localhost:7474)
+`
+
+func lspCmd(stdin io.Reader, stdout, stderr io.Writer, args []string) (code int) {
+	cmd := flag.NewFlagSet("lsp", flag.ExitOnError)
+	logFlag := cmd.String("log", "", "")
+	goplsLog := cmd.String("goplsLog", "", "")
+	goplsRPCTrace := cmd.Bool("goplsRPCTrace", false, "")
+	helpFlag := cmd.Bool("help", false, "")
+	pprofFlag := cmd.Bool("pprof", false, "")
+	httpDebugFlag := cmd.String("http", "", "")
+	err := cmd.Parse(args)
+	if err != nil {
+		fmt.Fprint(stderr, lspUsageText)
+		return 64 // EX_USAGE
+	}
+	if *helpFlag {
+		fmt.Fprint(stdout, lspUsageText)
+		return
+	}
+
+	err = lspcmd.Run(stdin, stdout, stderr, lspcmd.Arguments{
+		Log:           *logFlag,
+		GoplsLog:      *goplsLog,
+		GoplsRPCTrace: *goplsRPCTrace,
+		PPROF:         *pprofFlag,
+		HTTPDebug:     *httpDebugFlag,
+	})
+	if err != nil {
+		fmt.Fprintln(stderr, err.Error())
+		return 1
+	}
+	return 0
+}

templ/cmd/templ/main_test.go

@@ -0,0 +1,102 @@
+package main
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/a-h/templ"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestMain(t *testing.T) {
+	tests := []struct {
+		name           string
+		args           []string
+		expectedStdout string
+		expectedStderr string
+		expectedCode   int
+	}{
+		{
+			name:           "no args prints usage",
+			args:           []string{},
+			expectedStderr: usageText,
+			expectedCode:   64, // EX_USAGE
+		},
+		{
+			name:           `"templ help" prints help`,
+			args:           []string{"templ", "help"},
+			expectedStdout: usageText,
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ --help" prints help`,
+			args:           []string{"templ", "--help"},
+			expectedStdout: usageText,
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ version" prints version`,
+			args:           []string{"templ", "version"},
+			expectedStdout: templ.Version() + "\n",
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ --version" prints version`,
+			args:           []string{"templ", "--version"},
+			expectedStdout: templ.Version() + "\n",
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ fmt --help" prints usage`,
+			args:           []string{"templ", "fmt", "--help"},
+			expectedStdout: fmtUsageText,
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ generate --help" prints usage`,
+			args:           []string{"templ", "generate", "--help"},
+			expectedStdout: generateUsageText,
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ lsp --help" prints usage`,
+			args:           []string{"templ", "lsp", "--help"},
+			expectedStdout: lspUsageText,
+			expectedCode:   0,
+		},
+		{
+			name:           `"templ info --help" prints usage`,
+			args:           []string{"templ", "info", "--help"},
+			expectedStdout: infoUsageText,
+			expectedCode:   0,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			stdin := strings.NewReader("")
+			stdout := bytes.NewBuffer(nil)
+			stderr := bytes.NewBuffer(nil)
+			actualCode := run(stdin, stdout, stderr, test.args)
+
+			if actualCode != test.expectedCode {
+				t.Errorf("expected code %v, got %v", test.expectedCode, actualCode)
+			}
+			if diff := cmp.Diff(test.expectedStdout, stdout.String()); diff != "" {
+				t.Error(diff)
+				t.Error("expected stdout:")
+				t.Error(test.expectedStdout)
+				t.Error("actual stdout:")
+				t.Error(stdout.String())
+			}
+			if diff := cmp.Diff(test.expectedStderr, stderr.String()); diff != "" {
+				t.Error(diff)
+				t.Error("expected stderr:")
+				t.Error(test.expectedStderr)
+				t.Error("actual stderr:")
+				t.Error(stderr.String())
+			}
+		})
+	}
+}

templ/cmd/templ/processor/processor.go

@@ -0,0 +1,80 @@
+package processor
+
+import (
+	"io/fs"
+	"path"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+)
+
+type Result struct {
+	FileName    string
+	Duration    time.Duration
+	Error       error
+	ChangesMade bool
+}
+
+func Process(dir string, f func(fileName string) (error, bool), workerCount int, results chan<- Result) {
+	templates := make(chan string)
+	go func() {
+		defer close(templates)
+		if err := FindTemplates(dir, templates); err != nil {
+			results <- Result{Error: err}
+		}
+	}()
+	ProcessChannel(templates, dir, f, workerCount, results)
+}
+
+func shouldSkipDir(dir string) bool {
+	if dir == "." {
+		return false
+	}
+	if dir == "vendor" || dir == "node_modules" {
+		return true
+	}
+	_, name := path.Split(dir)
+	// These directories are ignored by the Go tool.
+	if strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+		return true
+	}
+	return false
+}
+
+func FindTemplates(srcPath string, output chan<- string) (err error) {
+	return filepath.WalkDir(srcPath, func(currentPath string, info fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if info.IsDir() && shouldSkipDir(currentPath) {
+			return filepath.SkipDir
+		}
+		if !info.IsDir() && strings.HasSuffix(currentPath, ".templ") {
+			output <- currentPath
+		}
+		return nil
+	})
+}
+
+func ProcessChannel(templates <-chan string, dir string, f func(fileName string) (error, bool), workerCount int, results chan<- Result) {
+	defer close(results)
+	var wg sync.WaitGroup
+	wg.Add(workerCount)
+	for i := 0; i < workerCount; i++ {
+		go func() {
+			defer wg.Done()
+			for sourceFileName := range templates {
+				start := time.Now()
+				outErr, outChanged := f(sourceFileName)
+				results <- Result{
+					FileName:    sourceFileName,
+					Error:       outErr,
+					Duration:    time.Since(start),
+					ChangesMade: outChanged,
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}

templ/cmd/templ/processor/processor_test.go

@@ -0,0 +1,19 @@
+package processor
+
+import (
+	"os"
+	"testing"
+)
+
+func TestFindTemplates(t *testing.T) {
+	t.Run("returns an error if the directory does not exist", func(t *testing.T) {
+		output := make(chan string)
+		err := FindTemplates("nonexistent", output)
+		if err == nil {
+			t.Fatal("expected error, but got nil")
+		}
+		if !os.IsNotExist(err) {
+			t.Fatalf("expected os.IsNotExist(err) to be true, but got: %v", err)
+		}
+	})
+}

templ/cmd/templ/sloghandler/handler.go

@@ -0,0 +1,101 @@
+package sloghandler
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"strings"
+	"sync"
+
+	"github.com/fatih/color"
+)
+
+var _ slog.Handler = &Handler{}
+
+type Handler struct {
+	h slog.Handler
+	m *sync.Mutex
+	w io.Writer
+}
+
+var levelToIcon = map[slog.Level]string{
+	slog.LevelDebug: "(✓)",
+	slog.LevelInfo:  "(✓)",
+	slog.LevelWarn:  "(!)",
+	slog.LevelError: "(✗)",
+}
+var levelToColor = map[slog.Level]*color.Color{
+	slog.LevelDebug: color.New(color.FgCyan),
+	slog.LevelInfo:  color.New(color.FgGreen),
+	slog.LevelWarn:  color.New(color.FgYellow),
+	slog.LevelError: color.New(color.FgRed),
+}
+
+func NewHandler(w io.Writer, opts *slog.HandlerOptions) *Handler {
+	if opts == nil {
+		opts = &slog.HandlerOptions{}
+	}
+	return &Handler{
+		w: w,
+		h: slog.NewTextHandler(w, &slog.HandlerOptions{
+			Level:     opts.Level,
+			AddSource: opts.AddSource,
+			ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
+				if opts.ReplaceAttr != nil {
+					a = opts.ReplaceAttr(groups, a)
+				}
+				if a.Key == slog.LevelKey {
+					level, ok := levelToIcon[a.Value.Any().(slog.Level)]
+					if !ok {
+						level = a.Value.Any().(slog.Level).String()
+					}
+					a.Value = slog.StringValue(level)
+					return a
+				}
+				if a.Key == slog.TimeKey {
+					return slog.Attr{}
+				}
+				return a
+			},
+		}),
+		m: &sync.Mutex{},
+	}
+}
+
+func (h *Handler) Enabled(ctx context.Context, level slog.Level) bool {
+	return h.h.Enabled(ctx, level)
+}
+
+func (h *Handler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return &Handler{h: h.h.WithAttrs(attrs), w: h.w, m: h.m}
+}
+
+func (h *Handler) WithGroup(name string) slog.Handler {
+	return &Handler{h: h.h.WithGroup(name), w: h.w, m: h.m}
+}
+
+var keyValueColor = color.New(color.Faint & color.FgBlack)
+
+func (h *Handler) Handle(ctx context.Context, r slog.Record) (err error) {
+	var sb strings.Builder
+
+	sb.WriteString(levelToColor[r.Level].Sprint(levelToIcon[r.Level]))
+	sb.WriteString(" ")
+	sb.WriteString(r.Message)
+
+	if r.NumAttrs() != 0 {
+		sb.WriteString(" [")
+		r.Attrs(func(a slog.Attr) bool {
+			sb.WriteString(keyValueColor.Sprintf(" %s=%s", a.Key, a.Value.String()))
+			return true
+		})
+		sb.WriteString(" ]")
+	}
+
+	sb.WriteString("\n")
+
+	h.m.Lock()
+	defer h.m.Unlock()
+	_, err = io.WriteString(h.w, sb.String())
+	return err
+}

templ/cmd/templ/testproject/testdata/css-classes/classes.go

@@ -0,0 +1,3 @@
+package cssclasses
+
+const Header = "header"

templ/cmd/templ/testproject/testdata/go.mod.embed

@@ -0,0 +1,7 @@
+module templ/testproject
+
+go 1.23
+
+require github.com/a-h/templ v0.2.513 // indirect
+
+replace github.com/a-h/templ => {moduleRoot}

templ/cmd/templ/testproject/testdata/go.sum

@@ -0,0 +1,2 @@
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=