feat: add pre-commit hooks

[danthorpe]

I took a shot at adding a pre-commit hooks files, according to this guide.

Fixes #676.

---

Once this is merged, it can be used in your other swift packages, by adding the following to the .pre-commit-config.yaml under the repos section, according to the guide.

  - repo: https://github.com/apple/swift-format
    rev: <commit hash / tag>
    hooks:
      - id: swift-format

[Feuermurmel]

I'm test-driving the hook from this MR in one of my projects using this configuration:

repos:
  - repo: https://github.com/danthorpe/swift-format
    rev: danthorpe/add_pre_commit_hooks
    hooks:
      - id: swift-format

There are two things I noticed:

• I think the hook should have require_serial: true set, because swift-format already processes the input files in parallel.

  When require_serial is set to true, only a single process is started for the hook, with all file names passed to it.

• IMHO it would be nice if swift-format printed the names of files that have been changed. It gives a bit more context as to why the hook failed as well as how many files have been changed.

  I'm mainly a Python dev and I'm using black and isort in most of my projects. Compare their output to that of swift-format:

  

  AFACIT, swift-format has no option to enable that right now.

In any case, happy to see this moving forward! 😊

[danthorpe]

Thanks for the comments @Feuermurmel - I wasn't familiar with the require_serial option, I'll take a look into and update the branch/PR.

[danthorpe]

👋 @ahoppen @MaxDesiatov any update on this one? Would be really handy to get merged so that people can use swift-format in their pre-commit hooks. 🙏

[ahoppen]

Sorry for taking so long to get back to you on this @danthorpe.

Thanks you for providing the integration. The change looks good to me. Just a very minor comments:

• Could you add a trailing newline to the YAML file?
• Could you rebase your changes so they are a single commit without a merge from main into your branch? That generally makes for a nicer Git history

Also, pushing changes should trigger the new GitHub Actions CI, which is required to merge the changes.

[danthorpe]

@ahoppen - done, thanks! 🎉

[ahoppen]

The trailing newline has two spaces, could you remove them to make the YAML lint pass?

[danthorpe]

Okay, done! 🤞

[ahoppen]

It looks like the file still has a space here on the last line (GitHub is showing the indicator that the file doesn’t have a trailing newline).

[danthorpe]

Okay, yep I've removed the space. Not sure if there is a new trailing line now though.

[ahoppen]

Great. Looks good now.

[ahoppen]

@danthorpe Your changes look good, I’m investigating the unrelated CI failures.

[danthorpe]

@ahoppen - just following the discussion in swiftlang/github-workflows#61 - and I'm happy to rebase my branch to get the latest changes from upstream.

I don't think it's an unreasonable request with OSS contributions. 😀

[ahoppen]

Thanks for rebasing, @danthorpe. swiftlang/github-workflows#61 diverged a little more than I expected 😄