Tracking issue: check write mode follow-up

[nrc]

Described in #1977 (comment) This would be used mainly by CI and would emit exit code 0 if no formatting was necessary and 1 if formatting is necessary. The output to stdout would be the same as diff mode.

Mentoring instructions:

Implementation

• add check to the rustfmt bin options
• implement check like diff mode, but with different exit codes (will require refactoring or exit code handling, I think)
• document check mode
• add a test - I think this would have to be an ad hoc unit test in the binary.

Follow ups

• change exit codes as described in Audit exit codes #1977 (comment) (it might be easier to do this whilst refactoring the exit code handling, since I think it will lead to some code being removed).
• make the newline character in diffs optional (i.e., it requires a command line arg)
• use check mode for our tests (I'm not sure if this is possible, but might work)
• use check mode in the CI for an official Rust project - one of the RLS helper crates might be a good place to start. Document what is necessary in the Rustfmt repo.
• align cargo fmt exit codes with rustfmt ones (this might be trivial once this issue is fixed)
• implement/verify other parts of the proposal in Audit exit codes #1977 (comment)

[toolness]

Hey, I'd like to help with this--should I just start working on a PR?

[toolness]

Oh whoops, never mind, just noticed #2539!

[davidbarsky]

@toolness There are lot of low-hanging fruit in my PR! If you've got the time/bandwidth, I'd love to get feedback on some of my design decisions, particularly how I pass around WriteMode. It feels a bit messy, and I'm not too experienced in OSS.

(Apologies if I broke some social rule I'm unaware of...)

[nrc]

Hi @toolness thanks for your interest, but yeah it looks like someone already started on this. If you want another way to get stuck into rustfmt, #2531 might be good - it is not small or easy, but it doesn't require much knowledge of rustfmt - it is essentially refactoring to remove code dup, but in a non-trivial way. If that sounds interesting, I can provide instructions over on that issue.

[toolness]

Sounds good @davidbarsky! I still consider myself fairly inexperienced in Rust, but I'll take a look at your PR and see if there's any feedback I can provide.

@nrc, thanks for the tip about #2531, I just replied in that issue!