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Rollup merge of #139798 - lcnr:where-bounds-gt-alias-bound, r=compiler-errors

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normalize: prefer `ParamEnv` over `AliasBound` candidates

cc https://github.com/rust-lang/trait-system-refactor-initiative/issues/175 not the only issue affecting bevy sadly

r? ``@compiler-errors``
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Stuart Cook 2025-04-15 15:47:30 +10:00 • committed by GitHub
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2 changed files with 59 additions and 21 deletions
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51
compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
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@ -792,37 +792,46 @@ where
};
match proven_via {
// Even when a trait bound has been proven using a where-bound, we
// still need to consider alias-bounds for normalization, see
// tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
//
// FIXME(const_trait_impl): should this behavior also be used by
// constness checking. Doing so is *at least theoretically* breaking,
// see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => {
let mut candidates_from_env_and_bounds: Vec<_> = candidates
.iter()
.filter(|c| {
matches!(
c.source,
CandidateSource::AliasBound | CandidateSource::ParamEnv(_)
)
})
.map(|c| c.result)
.collect();
let mut considered_candidates = Vec::new();
considered_candidates.extend(
candidates
.iter()
.filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
.map(|c| c.result),
);
// Even when a trait bound has been proven using a where-bound, we
// still need to consider alias-bounds for normalization, see
// tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
//
// We still need to prefer where-bounds over alias-bounds however.
// See tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs.
//
// FIXME(const_trait_impl): should this behavior also be used by
// constness checking. Doing so is *at least theoretically* breaking,
// see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
if considered_candidates.is_empty() {
considered_candidates.extend(
candidates
.iter()
.filter(|c| matches!(c.source, CandidateSource::AliasBound))
.map(|c| c.result),
);
}
// If the trait goal has been proven by using the environment, we want to treat
// aliases as rigid if there are no applicable projection bounds in the environment.
if candidates_from_env_and_bounds.is_empty() {
if considered_candidates.is_empty() {
if let Ok(response) = inject_normalize_to_rigid_candidate(self) {
candidates_from_env_and_bounds.push(response);
considered_candidates.push(response);
}
}
if let Some(response) = self.try_merge_responses(&candidates_from_env_and_bounds) {
if let Some(response) = self.try_merge_responses(&considered_candidates) {
Ok(response)
} else {
self.flounder(&candidates_from_env_and_bounds)
self.flounder(&considered_candidates)
}
}
TraitGoalProvenVia::Misc => {