core::compiler: remove Executor trait

[nrc]

What does this PR try to resolve?

This PR remove the Executor trait and thus simplifies some code. I added this trait for the RLS and it was never advertised as a publicly available API. The RLS is dead now and I think this API is not the best way to achieve its goals, therefore I think it should be removed.

How should we test and review this PR?

Change is trivial refactoring, so existing tests should be fine. Eyeball review should be fine.

Additional information

r? @arlosi

[rustbot]

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[weihanglo]

Hello Nick! I am happy that RLS retired gloriously and reducing code complexity sounds great to me! However, I am not sure i there are projects depend on Executor trait heavily. From my naive search result on GitHub, there are at least three projects depending on it:

• https://github.com/defuse/siderophile/blob/955a714468439330ae7627cadb86b1851acfc270/src/deps.rs#L386
• https://github.com/not-fl3/cargo-quad-apk/blob/672249200d1054b8a609cd1220d869f652e251ee/src/ops/build/compile.rs#L110
• https://github.com/lu-zero/cargo-c/blob/76a66cd72eb4271501557eebea7060821e63b702/src/build.rs#L797

I didn't look into them. Perhaps we can provide them alternatives (if those tools are still in maintenance).

[nrc]

The first project hasn't been touched for four years, but the other two seem to be maintained. I guess we need to keep supporting this API unless we can provide an alternative or there is really good reason not to.

[bors]

☔ The latest upstream changes (presumably #11430) made this pull request unmergeable. Please resolve the merge conflicts.

[weihanglo]

Just posted comments for those projects. I'll take over this PR if either they agree to this or no reply for a while.

• Switch to some alternatives for impl Executor for Exec lu-zero/cargo-c#324
• not-fl3/cargo-quad-apk@e062b67#r115110954

[weihanglo]

Okay. Got a reply from the author of cargo-c lu-zero/cargo-c#324 (comment). Given cargo-c is a relatively popular project, I don't think we can merge this pull request in any near future. I'll close it for now. Thank you.