compiletest: Detect unused auxiliary crates?

[ehuss]

There are some auxiliary crates that are unused in the tests directory. I think it would be nice if compiletest could detect these and complain about them in some way.

This might be difficult due to ignored tests, or tests that are otherwise not run, which don't load enough information to know which auxiliary tests are used.

If it ends up being technically challenging, perhaps someone could just manually go through and clean up the orphans?
I've been intending to do that, but just haven't been able to make the time.

Example: https://github.com/rust-lang/rust/blob/master/tests/ui/auxiliary/default-ty-param-cross-crate-crate.rs

[jieyouxu]

AFAIK this is possible to detect (easy or not is a different question), because compiletest should have sufficient information to build the aux dep graph as "early props", which is unrelated to determining when a test should or should not run. I think there might be a limitation where currently compiletest will scan the entire test suite for early prop processing (which would establish the aux build relationships), but this might potentially be too much work if you just passed a specific subdirectory.