Auto merge of #95031 - compiler-errors:param-env-cache, r=Aaron1011

Do not use `ParamEnv::and` when building a cache key from a param-env and trait eval candidate

Do not use `ParamEnv::and` to cache a param-env with a selection/evaluation candidate.

This is because if the param-env is `RevealAll` mode, and the candidate looks global (i.e. it has erased regions, which can show up when we normalize a projection type under a binder<sup>1</sup>), then when we use `ParamEnv::and` to pair the candidate and the param-env for use as a cache key, we will throw away the param-env's caller bounds, and we'll end up caching a candidate that we inferred from the param-env with a empty param-env, which may cause cache-hit later when we have an empty param-env, and possibly mess with normalization like we see in the referenced issue during codegen.

Not sure how to trigger this with a more structured test, but changing `check-pass` to `build-pass` triggers the case that https://github.com/rust-lang/rust/issues/94903 detected.

<sup>1.</sup> That is, we will replace the late-bound region with a placeholder, which gets canonicalized and turned into an infererence variable, which gets erased during region freshening right before we cache the result. Sorry, it's quite a few steps.

Fixes #94903
r? `@Aaron1011` (or reassign as you see fit)

1 files changed, 10 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/traits/select.rs b/compiler/rustc_middle/src/traits/select.rs
index 5297825a92f..ffa70cddbd5 100644
--- a/compiler/rustc_middle/src/traits/select.rs
+++ b/compiler/rustc_middle/src/traits/select.rs
@@ -13,12 +13,19 @@ use rustc_hir::def_id::DefId;
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    ty::ParamEnvAnd<'tcx, ty::TraitPredicate<'tcx>>,
+    // This cache does not use `ParamEnvAnd` in its keys because `ParamEnv::and` can replace
+    // caller bounds with an empty list if the `TraitPredicate` looks global, which may happen
+    // after erasing lifetimes from the predicate.
+    (ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
-pub type EvaluationCache<'tcx> =
-    Cache<ty::ParamEnvAnd<'tcx, ty::PolyTraitPredicate<'tcx>>, EvaluationResult>;
+pub type EvaluationCache<'tcx> = Cache<
+    // See above: this cache does not use `ParamEnvAnd` in its keys due to sometimes incorrectly
+    // caching with the wrong `ParamEnv`.
+    (ty::ParamEnv<'tcx>, ty::PolyTraitPredicate<'tcx>),
+    EvaluationResult,
+>;
 
 /// The selection process begins by considering all impls, where
 /// clauses, and so forth that might resolve an obligation. Sometimes