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Frequently Asked Questions

Build System and CMake Configuration

How do I add a new file to CMake?

If you forget to add a new file to the CMake configuration, you may end up with undefined symbol errors at link time.

There should be a CMakeLists.txt in the directory where you added the new file, which has the list of different .h/.cpp files to be included in the build. Add your new file to that list.

How do I speed up iterating on changes to the build system?

The general idea is to build as little as you can.

  • Use sccache or ccache if you aren't doing so already.
  • Use build-script's various --skip-* flags to skip configuring for platforms that you do not care about.
  • If you're on macOS, use --swift-darwin-supported-archs "$(uname -m)".
  • Build the release variant without assertions (--release --no-assertions). While debug info and assertions are valuable to enable when working on the toolchain itself, they are not so useful if you are working only on changes to the build system.

Using a Locally Built Toolchain

How do I use a locally built compiler to build X?

You can use the SWIFT_EXEC environment variable to use a locally built compiler to compile both packages and Xcode projects.

  1. For SwiftPM packages, pass the environment variable when invoking SwiftPM.

    # Assuming the current working directory contains the package, build the
# package using a custom compiler.
SWIFT_EXEC=/path/to/swiftc swift build

  2. For Xcode projects, select the project in the Project Navigator. In the Build Settings tab, click '+' and then 'Add User-Defined Setting'. Create a build setting SWIFT_EXEC with the value set to /path/to/swiftc. If you now do a clean build, your locally built compiler will be used.

    At the time of writing, in the latest Xcode 12.2 beta 3, SWIFT_EXEC does not work for SwiftPM integration inside Xcode, so this will not work for Xcode projects that depend on SwiftPM packages.

Note: Even thought the variable says 'SWIFT', it needs to point to 'swiftc', not 'swift'. The extra 'c' is not a typo.

Testing and CI

How do I reproduce/fix a test that fails in CI but passes for me locally?

TODO: Write some tips here, point to Testing.md for simulator setup.

Documentation

Where can I find documentation on X?

This very depends on what X is, but some broad guidelines are:

  1. Do a case-insensitive recursive string search.
    • Use a specialized tool like ripgrep or ag.
    • Use git grep --ignore-case "mypattern". git grep also supports helpful flags which provide more context:
      • --show-function: Tries to print the function name that a match was found in.
      • --function-context: Tries to print the entire surrounding function containing the match.
    • Use 'Find in Workspace' in Xcode (⌘+⇧+F).
    • Use grep --ignore-case --recursive "mypattern" ..
  2. Go through the Documentation Index.

How do I build the documentation as HTML?

You can build the ReST formatted documentation as HTML using Sphinx. Follow Sphinx's installation instructions and check that sphinx-build is available on your PATH:

sphinx-build --version

If that succeeds, you can build the documentation using make

make -C docs

(Tested with sphinx-build version 3.2.1.)

This compiles the .rst files in the docs directory into HTML in the docs/_build/html directory.

For the Markdown documentation, you can view the rendered HTML directly on GitHub. For example, this file is rendered on GitHub at https://github.com/apple/swift/tree/main/docs/HowToGuides/FAQ.md .

HTML documentation for the standard library on Darwin platforms is hosted on the Apple Developer website.

Pull Request Workflow

How do I format my changes?

First, install clang-format using your system's package manager. This should also install the git-clang-format script (try git-clang-format --help). In case it doesn't, you can replace git-clang-format in the following commands with ../llvm-project/clang/tools/clang-format/git-clang-format.

Start out at the tip of the branch where you want to reformat the commits.

# If there is only one commit that needs to be reformatted.
git-clang-format HEAD~1
git add .
git commit --amend --no-edit

# Say the last N commits need to be reformatted.
# Mark them as 'edit' instead of 'pick'.
git rebase -i HEAD~N
# Re-run N times, reformatting each commit.
git-clang-format HEAD~1
git add .
git commit --amend --no-edit
git rebase --continue

How do I clean up my git history?

Git's history can sometimes become cluttered with many small commits. Fortunately, Git has a feature called rebase that allows you to clean up your commit history. If you want to learn more, GitHub - About Git rebase provides a comprehensive overview of rebase.

Warning We suggest considering to rebase only those commits that haven't been pushed to a public branch. Rebasing existing commits would block merging because we don't allow force pushes to the repository. If you need to tidy up commits that have already been pushed, it's generally better to use git revert for the sake of avoid causing confusion for other developers.

Here's a small gist that goes through the basics on how to use it:

  1. Begin an interactive rebase: Use git rebase -i HEAD~N, where N is the number of commits from the latest one you want to edit. This will open a text editor, listing the last N commits with the word "pick" next to each one.

    git rebase -i HEAD~N

  2. Edit the commits: Replace "pick" with the operation you want to perform on the commit:

    • reword: Change the commit message.
    • edit: Amend the commit.
    • squash: Combine the commit with the previous one.
    • fixup: Similar to squash, but discard this commit's log message.
    • drop: Remove the commit.

  3. Save and exit: After saving and closing the file, git will execute each operation. If you selected reword, edit, or squash, git will pause and give you a chance to alter the commit message or the commit itself.

    git commit --amend

  4. Continue the rebase: Once you're done with each commit, you can continue the rebase using git rebase --continue. If you want to abort the rebase at any point, you can use git rebase --abort.

    git rebase --continue