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Rollup merge of #93892 - compiler-errors:issue-92917, r=jackh726,nikomatsakis

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Only mark projection as ambiguous if GAT substs are constrained

A slightly more targeted version of #92917, where we only give up with ambiguity if we infer something about the GATs substs when probing for a projection candidate.

fixes #93874
also note (but like the previous PR, does not fix) #91762

r? `@jackh726`
cc `@nikomatsakis` who reviewed #92917
This commit is contained in:
Matthias Krüger
2022-02-18 23:23:09 +01:00
committed by GitHub
parent cb35370557 879e4f8131
commit 1e2f63de0a
6 changed files with 100 additions and 43 deletions
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38
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@@ -1494,12 +1494,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
})
}
/// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
/// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
/// and applying this env_predicate constrains any of the obligation's GAT substitutions.
///
/// This behavior is a somewhat of a hack to prevent overconstraining inference variables
/// in cases like #91762.
pub(super) fn match_projection_projections(
&mut self,
obligation: &ProjectionTyObligation<'tcx>,
env_predicate: PolyProjectionPredicate<'tcx>,
potentially_unnormalized_candidates: bool,
) -> bool {
) -> ProjectionMatchesProjection {
let mut nested_obligations = Vec::new();
let (infer_predicate, _) = self.infcx.replace_bound_vars_with_fresh_vars(
obligation.cause.span,
@@ -1521,7 +1527,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
infer_predicate.projection_ty
};
self.infcx
let is_match = self
.infcx
.at(&obligation.cause, obligation.param_env)
.sup(obligation.predicate, infer_projection)
.map_or(false, |InferOk { obligations, value: () }| {
@@ -1530,7 +1537,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
nested_obligations.into_iter().chain(obligations),
)
.map_or(false, |res| res.may_apply())
})
});
if is_match {
let generics = self.tcx().generics_of(obligation.predicate.item_def_id);
// FIXME(generic-associated-types): Addresses aggressive inference in #92917.
// If this type is a GAT, and of the GAT substs resolve to something new,
// that means that we must have newly inferred something about the GAT.
// We should give up in that case.
if !generics.params.is_empty()
&& obligation.predicate.substs[generics.parent_count..]
.iter()
.any(|&p| p.has_infer_types_or_consts() && self.infcx.shallow_resolve(p) != p)
{
ProjectionMatchesProjection::Ambiguous
} else {
ProjectionMatchesProjection::Yes
}
} else {
ProjectionMatchesProjection::No
}
}
///////////////////////////////////////////////////////////////////////////
@@ -2709,3 +2735,9 @@ impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
write!(f, "TraitObligationStack({:?})", self.obligation)
}
}
pub enum ProjectionMatchesProjection {
Yes,
Ambiguous,
No,
}