Create a tool which suggests tests to run based on modified files

[jyn514]

@digama0 came up with an excellent idea: it should be possible to, given the files modified since your git checkout last branched from master, generate suggestions for the tests you should run (i.e. those most likely to fail because of your changes). Some examples:

• library/std/src/lib.rs => x test --stage 0 tidy library/std
• compiler/rustc_data_structures => x check && x test --stage 0 tidy compiler/rustc_data_structures && x test --stage 1 src/test/ui src/test/run-make
• src/librustdoc/src/lib.rs => x test --stage 1 tidy rustdoc

You can determine the files modified with git diff --name-only $(git merge-base HEAD origin/master).

This tool doesn't necessarily need to live in tree - it could just be some crate in a public git repo that people install, and we can move it in-tree later if it's useful. I was imagining it could be called suggest-tests or something like that.

@rustbot label +E-medium +E-mentor +E-help-wanted +A-testsuite +A-contributor-roadblock

This issue has been assigned to @Ezrashaw via this comment.

[lionellloh]

@rustbot claim

[jyn514]

@the8472 had a really great idea: we can also use this to determine what tests to run this in PR CI, so you can get feedback faster without waiting for the PR to be approved.

[jyn514]

@lionellloh it's been a while - have you had a chance to work on this? do you know how to make progress?

[Noratrieb]

@rustbot assign @Ezrashaw