feature request: warn if a PR modifies only src/test/ and library/

[jyn514]

From rust-lang/highfive#286.
cc rust-lang/rust#76268

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'.

[workingjubilee]

-#[inline(details)]
+#[doc(inline)]

AFAIK, no checks on the diff of PRs exists in triagebot today so we'll probably need to add some logic for that.

Originally posted by @Mark-Simulacrum in rust-lang/highfive#286 (comment)

Currently, the norm is to add UI tests because by default they're useful and comprehensive, but this has led to many "should be a unit test" tests getting added as UI tests, slowing compile/test iteration massively. So, we should

1. Add the ability to respond to PR diffs.
2. Enable a simple rule to check against this, likely

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

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

Originally posted by @matklad in rust-lang/highfive#286 (comment)

[jyn514]

I think you meant #[doc(inline)] ;)