Format Bot for CI

[bushrat011899]

What problem does this solve or what need does it fill?

It's very common for either typos, cargo fmt, and/or taplo fmt to fail during CI, requiring a usually trivial commit where the relevant tool is used. This also occurs when a reviewer suggests changes to a PR, since the suggestion may require formatting changes which are either hard or impossible to include in a suggestion block.

What solution would you like?

One or more GitHub Actions which can run the relevant linter and create a suggestion which can be applied to a PR. Crucially, this is not an automatic fix for the PR, it just allows the fix to be done entirely from the web or application UI. This is especially helpful for maintainers who have permission to modify PRs, since they could (for example) upstream main, suggest some minor fixes, and then apply suggested formatting changes all in one sitting without switching context.

What alternative(s) have you considered?

Could do nothing. This is an annoyance at most. But if we can automate the process, why not?

Additional context

• An example of this can be found in the Suggest Changes Action.
• CI permissions can be challenging to get right, so I would encourage any CI experts to weigh in on the potential risks of such a bot.

[mockersf]

there are a few possible ways to do it:

1. CI could just commit directly to the PR. I personally hate this and I've never seen it working in a good way
2. Actions checking for format can suggest changes. tools can give their results in a format understood by GitHub that can be applied directly
3. Actions checking for format can suggest changes (bis). With the Suggest Change Action from the additional context in the issue, the idea is: instead of running cargo fmt --check, run cargo fmt, fail if there's any changes to a committed file, then use the diff as a suggestion
4. Use an action that would be triggered by a comment like "format this" to format
5. Use a bot that would watch PRs and suggest format fixes

3 should be very straightforward and not require any additional permissions

[JMS55]

I would also like to take this opportunity to suggest that we use the unstable rustfmt option to enforce formatting for how imports are grouped. The lack of consistency really bugs me.

[mockersf]

I wouldn't be against using a nightly formatter options in CI if it doesn't go against the stable formatter