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trait_sel: sizedness goals prefer alias candidates

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For sizedness, default and auto trait predicates, now prefer non-param
candidates if any exist. As these traits do not have generic parameters,
it never makes sense to prefer an non-alias candidate, as there can
never be a more permissive candidate.
This commit is contained in:
David Wood
2025-07-17 09:21:51 +00:00
parent efaeacfc96
commit 321a47eb2c
19 changed files with 722 additions and 81 deletions
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29
compiler/rustc_trait_selection/src/traits/select/mod.rs
View File

@@ -28,8 +28,9 @@ use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::error::TypeErrorToStringExt;
use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
use rustc_middle::ty::{
self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, SizedTraitKind, Ty, TyCtxt,
TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate, may_use_unstable_feature,
self, CandidatePreferenceMode, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate,
SizedTraitKind, Ty, TyCtxt, TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate,
may_use_unstable_feature,
};
use rustc_span::{Symbol, sym};
use tracing::{debug, instrument, trace};
@@ -474,7 +475,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
} else {
let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) {
let candidate_preference_mode =
CandidatePreferenceMode::compute(self.tcx(), stack.obligation.predicate.def_id());
if let Some(candidate) =
self.winnow_candidates(has_non_region_infer, candidate_preference_mode, candidates)
{
self.filter_reservation_impls(candidate)
} else {
Ok(None)
@@ -1821,6 +1826,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
fn winnow_candidates(
&mut self,
has_non_region_infer: bool,
candidate_preference_mode: CandidatePreferenceMode,
mut candidates: Vec<EvaluatedCandidate<'tcx>>,
) -> Option<SelectionCandidate<'tcx>> {
if candidates.len() == 1 {
@@ -1874,6 +1880,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
break;
}
let alias_bound = candidates
.iter()
.filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
.try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
if matches!(candidate_preference_mode, CandidatePreferenceMode::Marker) {
match alias_bound {
Some(Some(index)) => return Some(ProjectionCandidate(index)),
Some(None) => {}
None => return None,
}
}
// The next highest priority is for non-global where-bounds. However, while we don't
// prefer global where-clauses here, we do bail with ambiguity when encountering both
// a global and a non-global where-clause.
@@ -1907,10 +1926,6 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
// fairly arbitrary but once again necessary for backwards compatibility.
// If there are multiple applicable candidates which don't affect type inference,
// choose the one with the lowest index.
let alias_bound = candidates
.iter()
.filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
.try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
match alias_bound {
Some(Some(index)) => return Some(ProjectionCandidate(index)),
Some(None) => {}