Remove unused features

[mu001999]

Detected by #152164.

Only allow unused_features if there are complex platform-specific features enabled.

[rustbot]

Some changes occurred in exhaustiveness checking

cc @Nadrieril

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

The Miri subtree was changed

cc @rust-lang/miri

[rustbot]

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[Nadrieril]

Hi, thanks for your contribution!

We should not remove these feature gates from tests. Features in tests sometimes test very subtle interactions and we don't want to break that. Also for tools and rustdoc we generally don't touch these directories unless strictly necessary; we're not running their whole test suite in CI so we could be removing a feature they actually use.

I'm also of the opinion that replacing a feature with cfg_attr(..., feature(...)) is noise and not worth the precision. E.g. all the platform-specific stuff in core.

The rest looks good to me, that's some nice cleanups.

[mu001999]

@Nadrieril Make sense, I will keep the removal in compiler and replace the rest with just allowing unused_features