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CONTRIBUTING.md

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Contribution Guidelines

Tutorial

If you'd like to write a SwiftLint rule but aren't sure how to start, please watch and follow along with this video tutorial.

Pull Requests

All changes, no matter how trivial, must be done via pull requests. Commits should never be made directly on the main branch. If possible, avoid mixing different aspects in one pull request. Prefer squashing if there are commits that are not reasonable alone. To update your PR branch and resolve conflicts, use rebasing instead of merging main.

Important

If you have commit access to SwiftLint and believe your change to be trivial and not worth waiting for review, you may open a pull request and merge it immediately, but this should be the exception, not the norm.

Building and Running Locally

The first step is to clone the repository. We recommend Xcode or Visual Studio Code with the awesome Swift extension installed from the Visual Studio Marketplace or the Open VSX Registry for development.

Using Xcode

  1. xed .
  2. Select the "swiftlint" scheme.
  3. Press ⌘ Cmd ⌥ Option R to open the scheme options.
  4. Set the "Arguments Passed On Launch" you want in the "Arguments" tab. See available arguments in the README.
  5. Set the "Working Directory" in the "Options" tab to the path where you would like to execute SwiftLint — a folder that contains Swift source files.
  6. Hit "Run".
Arguments Options
image image

Then you can use the full power of Xcode/LLDB/Instruments to develop and debug your changes to SwiftLint.

Using Visual Studio Code

  1. code .
  2. Wait for the setup to complete.
  3. Press ⌘ Cmd ⇧ Shift P to open the command palette.
  4. With the Swift extension installed search for and select "Task: Run Build Task".
  5. Decide to build the swiftlint binary only or to build everything including tests.
  6. The extension allows you to debug the binary and run tests.

Using the Command Line

  1. swift build [-c release]
  2. Use the produced swiftlint binary from the command line, either by running swift run [-c release] [swiftlint] [arguments] or by invoking the binary directly at .build/[release|debug]/swiftlint.
  3. For debugging, attach LLDB: lldb -- .build/[release|debug]/swiftlint [arguments].

Code Generation

If rules are added/removed/renamed, you'll need to run make sourcery, which requires that Bazel is installed on your machine (brew install bazelisk). This will update source files to reflect these changes.

If you'd rather like to avoid installing Bazel, you can run Sourcery manually. Make sure to use the same version of Sourcery as defined in WORKSPACE.

Tests

SwiftLint supports building via Xcode and Swift Package Manager on macOS, and with Swift Package Manager on Linux. When contributing code changes, please ensure that all four supported build methods continue to work and pass tests:

xcodebuild -scheme swiftlint test -destination 'platform=macOS'
swift test
make bazel_test
make docker_test

If you find it too much effort to installed all the tooling required for the different build/test methods, just open a pull request and watch the CI results carefully. They include all the necessary builds and checks.

Rules

New rules should be added in the Source/SwiftLintBuiltInRules/Rules directory.

Prefer implementing new rules with the help of SwiftSyntax. Look for the @SwiftSyntaxRule attribute for examples and use the same on your own rule.

All new rules or changes to existing rules should be accompanied by unit tests.

Whenever possible, prefer adding tests via the triggeringExamples and nonTriggeringExamples properties of a rule's description rather than adding those test cases in unit tests directly. This makes it easier to understand what rules do by reading their source, and simplifies adding more test cases over time. With make sourcery, you ensure that all test cases are automatically checked in unit tests. Moreover, the examples added to a rule will appear in the rule's rendered documentation accessible from the Rule Directory.

For debugging purposes, examples can be marked as focused. If there are any focused examples found, then only those will be run when executing all tests for that rule.

nonTriggeringExamples: [
    Example("let x: [Int]"),
    Example("let x: [Int: String]").focused()   // Only this one will be run in tests.
],
triggeringExamples: [
    Example("let x: ↓Array<String>"),
    Example("let x: ↓Dictionary<Int, String>")
]

Configuration

Every rule is configurable via .swiftlint.yml, even if only by settings its default severity. This is done by setting the configuration property of a rule as:

var configuration = SeverityConfiguration<Self>(.warning)

If a rule requires more options, a specific configuration can be implemented and associated with the rule via its configuration property. Check for rules providing their own configurations as extensive examples or check out

Configuring them in .swiftlint.yml looks like:

force_cast: warning

file_length:
  warning: 800
  error: 1200

identifier_name:
  min_length:
    warning: 3
    error: 2
  max_length: 20
  excluded: id

Tracking Changes

All changes should be made via pull requests on GitHub.

When issuing a pull request with user-facing changes, please add a summary of your changes to the CHANGELOG.md file.

We follow the same syntax as CocoaPods' CHANGELOG.md:

  1. One Markdown unnumbered list item describing the change.
  2. 2 trailing spaces on the last line describing the change (so that Markdown renders each change on its own line).
  3. A list of Markdown hyperlinks to the contributors to the change. Usually just one.
  4. A list of Markdown hyperlinks to the issues the change addresses. Usually just one or even none. Mentioning the pull request number is not necessary, as GitHub automatically adds it to the commit message upon squash-merge.
  5. All CHANGELOG.md content is hard-wrapped at 80 characters.

Cutting a Release

The release workflows require two tokens specific to your GitHub user account to be set as Action secrets in the SwiftLint repository. Make sure you have the following steps completed once before cutting your first release:

  1. Navigate to Action secrets and variables in the repository settings.
  2. Add a new secret named PERSONAL_GITHUB_TOKEN_<USERNAME> where <USERNAME> is your GitHub username in all uppercase. The value must be a personal access token with the read:user, repo, user:email and workflow scopes.
  3. Follow these instructions to set the variable COCOAPODS_TRUNK_TOKEN_<USERNAME> to your CocoaPods trunk token.

SwiftLint maintainers follow these steps to cut a release:

  1. Come up with a witty washer- or dryer-themed release name. Past names include:

    • Tumble Dry
    • FabricSoftenerRule
    • Top Loading
    • Fresh Out Of The Dryer

    You may ask your favorite AI chat bot for suggestions. 🤖

  2. In the GitHub UI press "Run workflow". Enter the release version and the title. Start the workflow and wait for it to create a release branch, build the most important artifacts and prepare a draft release.

  3. Review the draft release thoroughly making sure that the artifacts have been attached to it and the release notes are correct.

  4. If everything looks good and the release branch has not diverged from main in the meantime, publish the release. 🚀

  5. A few "post-release" jobs will get started to complete the list of artifacts on the release page. One of them will also fast-forward merge the release branch into main. All jobs fail if that's not possible.

  6. Celebrate! 🎉

In case the CocoaPods release fails, you can try to publish it manually:

  1. Make sure you have the latest stable Xcode version installed and xcode-selected.
  2. Make sure that the selected Xcode has the latest SDKs of all supported platforms installed. This is required to build the CocoaPods release.
  3. Run make pod_publish.

CI

SwiftLint uses Azure Pipelines for most of its CI jobs, primarily because they're the only CI provider to have a free tier with 10x concurrency on macOS.

Some CI jobs run as GitHub Actions (e.g. Docker build, linting, release workflows).

The most important CI jobs run on Buildkite using Macs provided by MacStadium. These are jobs that benefit from being run on the latest Xcode & macOS versions on bare metal. This is important for performance comparisons and caching in Bazel builds.

Buildkite Setup

To bring up a new Buildkite worker from MacStadium:

  1. Change account password.
  2. Update macOS to the latest version.
  3. Install Homebrew.
  4. Install the Buildkite agent and other tools via Homebrew: brew install aria2 bazelisk htop buildkite/buildkite/buildkite-agent robotsandpencils/made/xcodes
  5. Install latest Xcode version: xcodes update && xcodes install 14.0.0
  6. Add DANGER_GITHUB_API_TOKEN and HOME to /opt/homebrew/etc/buildkite-agent/hooks/environment
  7. Configure and launch buildkite agent as described in brew info buildkite-agent or on https://buildkite.com/organizations/swiftlint/agents#setup-macos.