chore: move cargo fmt to pre-commit script

[SteveLauC]

What does this PR do

1. Move cargo fmt task to a pre-commit script, and add a doc for it in CONTRIBUTING.md
2. Add a doc on how to skip the CI (Cirrus CI, will add the GitHub Action version when we finish our migration) in CONTRIBUTING.md

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[skip ci]

[SteveLauC]

Kinda weird, the Cirrus CI still runs even though I explicitly [skip ci]

[Jan561]

It seems you have to include [skip ci] in the title of the PR.

If you open a PR, PR title will be checked for [skip ci] or [ci skip]
instead of the last commit message on the PR branch.

https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution

Still weird though as it worked in #2158.

[asomers]

I don't know about you, but personally I would find it surprising if the pre-commit script actually changed my code, as opposed to merely checking it. Also, I don't think that the addition of a pre-commit script absolves us from running rustfmt in CI. Because what if some contributor doesn't run the pre-commit script? We have no way to force them to.

[SteveLauC]

Also, I don't think that the addition of a pre-commit script absolves us from running rustfmt in CI. Because what if some contributor doesn't run the pre-commit script? We have no way to force them to.

Then it seems that we have to add rustfmt to our CI, because AFAIK, there is no way to make this check compulsory with a pre-commit script

[SteveLauC]

It seems you have to include [skip ci] in the title of the PR.

If you open a PR, PR title will be checked for [skip ci] or [ci skip]
instead of the last commit message on the PR branch.

https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution

Still weird though as it worked in #2158.

Yeah, it is indeed weird :<

[asomers]

The function is bash syntax, not sh.

Suggested change

	function red() {
	red() {

[asomers]

This command doesn't actually work. I just tested it by screwing up the format in src/sys/aio.rs, and this command didn't complain. I think it's caused by this bug: rust-lang/rustfmt#3253 . That's why we need to do cargo fmt --all -- --check **/*.rs

[SteveLauC]

For the format check, are we gonna use a pre-commit script or do it in CI?

#2166 has migrated it to GHA, so if we choose the latter, then we can simply close this PR

[asomers]

Yeah, I think we can close this PR if we merge the other one too. The pre-commit.sh script could be useful too, but maybe not useful enough to justify its ongoing maintenance.

[SteveLauC]

Close this PR