feature request: warn if src/test/ is modified without modifying compiler/ or src/librustdoc/

[jyn514]

This would help prevent an ever-growing backlog for rust-lang/rust#76268.

[matklad]

I think are more precise rule would work like this:

• Warn if a PR modifies only library/ and src/test.

This actually should bring false-positive rate to almost zero I think.

[workingjubilee]

Would this be part of x.py test tidy, or something else?

[jyn514]

I don't think it should be part of tidy, because it could have false positives. tidy is a hard error, I want this to be more like submodules where it says 'only do this if you're sure'.

[Mark-Simulacrum]

It would probably be better as a request against triagebot, where most new functionality should go. We are already receiving the webhook needed for this so mostly a matter of hooking up a check for the rule @matklad laid out above. AFAIK, no checks on the diff of PRs exists in triagebot today so we'll probably need to add some logic for that.

[jyn514]

Opened rust-lang/triagebot#831