Add github action workflow check that enforces 'cargo fmt'

[Nukesor]

Hey there :)

This PR adds a cargo fmt check to the rust Github action.
That should enforce/remind people to use cargo fmt in their PRs.

I'm also thinking about adding another PR, which resolves all open cargo clippy issues and adds a cargo clippy check to the action. If you don't mind, I'll start working on that as well momentarily.

This also adds a rule, which actually runs the checks on PRs.

[Nukesor]

I'm not exactly sure why this test is failing. This doesn't look related to any changes I made. The other run in the PR that builds upon this one looks like it's working.

Maybe you have some non-deterministically failing tests?

[eminence]

Yeah, some of the tests are a little racey (a few milliseconds go by between when we get a process listing and when we try to get info about it -- in that time, the process might have exited). I've re-run the tests and they are passing now.

This same comment applies to both of your pull requests: I don't really have an objection to adding clippy or rustfmt to CI (as you'll see several commits in the git history were I apply clippy or fmt fixes), but I'm a little hesitant to block PRs on them. It's a bit of a barrier/annoyance that I'm OK dealing with myself, but I don't necessarily want all contributors to deal with it if they don't want to.

Can we try to mark the rustfmt and clippy jobs as "continue-on-error" (and then introduce a "fake" clippy or fmt error to see what it looks like)? I've not used this github action setting before, but I'm hoping it might be like gitlab's "allow-failure" where it turns the job a yellow color.

[Nukesor]

I looked around and those configs aren't an option yet.

This allows workflows to not fail when some specific steps fail, but there's no indication that anything went wrong.
actions/runner#2347

A workaround would be to set the outcome status manually, but this isn't possible yet either -.-
actions/runner#662

I'll remove the checks and simply fix all formatting and clippy issues, plus add clippy exceptions for stuff that would break backward compatibility

[eminence]

Thanks for researching this a bit. It's a bummer that github doesn't support this, though hopefully one day they will! Something like Gitlab's "allow_failure: true" would fit well here, I think.

So regarding these two pull requests, let's close them, since github actions can't really do what we want.

If you wanted to open another PR with some clippy/fmt fixes, I'd be happy to accept that :)