Add GitHub Action to test master rustfmt formatting vs a feature branch

[ytmimi]

This new action is intended to help us maintainers determine when feature branches cause breaking formatting changes by running rustfmt (master) and the feature branch on various rust repositories.

Over time I expect the list of checked projects to increase.

With this action in place we can more easily test that a new feature or bug fix doesn't introduce breaking changes. Although this action needs to be manually triggered right now, we might consider adding it to our CI runs in the future.

To run the new action we can head over to the actions tab and click Run workflow to supply the inputs that the script expects.

For example to test out the proposed fn_call_layout changes (#5337) you could supply the following form arguments.

https://github.com/ytmimi/rustfmt.git
issue_5218

I'm far from a bash pro, so if you have any suggestions for how to simplify the script those would be greatly appreciated 🙏🏼

[calebcartwright]

I like the sounds of this! Haven't had a chance to look yet, but curious if/how readily it could be used to test a branch against currently released versions of rustfmt (e.g. branch vs stable or nightly)

[ytmimi]

At the moment the script doesn't support that, but I think it should be pretty easy to add an optional input to specify a git tag to checkout after we do a git clone on the rustfmt repo!

Just as a heads up, GitHub actions that are triggered on workflow_dispatch can only be run if they're on the default branch for that repo. To test this action on my feature branch I temporarily set my diff_check_script as my default branch for my fork of rustfmt.

Here are some links to workflow runs that I already tested so you can get a feel for the output:

• this run failed because there were diffs in the r-l/rust repo and the deno repo when testing the fn_call_layout changes before adding the Foo case.

• this run passed because no diffs were detected when testing the fn_call_layout changes after adding the Foo case.

For anyone reading this that has no ideas what I'm talking about when I say Foo case, check out the discussion in #5337 (comment)

If you want to test the script manually you should be able to run the ci/check_diff.sh script directly and pass it the expected arguments.

For example:

ci/check_diff.sh https://github.com/ytmimi/rustfmt.git issue_5218

[calebcartwright]

I've a few thoughts as we move forward, but view this as a rather significant value add and would rather go ahead and get it merged.

Thank you! Excited about this