Do not cache ParamEnv candidates globally

[compiler-errors]

Fixes #94903

The issue here is that we're globally caching a trait selection result we get from the ParamEnv, specifically TraitPredicate(<SaveUser<ReErased> as StorageRequestReturnType>, polarity:Positive). Not sure where that erased bound comes from.

Not sure how to trigger this with a more structured test, but changing check-pass to build-pass triggers the case that #94903 detected.

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

r? @Aaron1011 (you previously did something with this caching, IIRC)

[Aaron1011]

This seems wrong - we include the ParamEnv in the global cache key, so it should be fine to store a ParamCandidate in the global cache.

[compiler-errors]

Actually, you're right, this is wrong.

So the issue is that we're caching the predicate TraitPredicate(<SaveUser<ReErased> as StorageRequestReturnType>, polarity:Positive) with an empty RevealAll ParamEnv. Maybe we're erasing the regions from the predicate too early, and then it looks global when paired with ParamEnv::and (so it gets paired with an empty ParamEnv).

[compiler-errors]

Anywho, I will take another look at this.

@rustbot author

[compiler-errors]

Actually worked out a (possible?) bug in trait selection that fixes this issue instead.