Auto merge of #115039 - jackh726:impl_compare_add_alias_obligations, r=aliemjay

Add projection obligations when comparing impl too

Fixes #115033

In the test, when we ask for WF obligations of `DatasetIter<'a, ArrayBase<D>>`, we get back two important obligations: `[<D as Data>::Elem -> ?1, ?1: 'a]`. If we don't add the projection obligation, `?1` remains unconstrained.

An alternative solution would be to use unnormalized obligations, where we only have one relevant obligation: `<D as Data>::Elem: 'a`. This would leave no inference vars unconstrained.

1 files changed, 8 insertions, 1 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index ad02ca252c4..bd0ab6463f0 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -342,9 +342,16 @@ fn compare_method_predicate_entailment<'tcx>(
                 continue;
             };
             for obligation in obligations {
+                debug!(?obligation);
                 match obligation.predicate.kind().skip_binder() {
+                    // We need to register Projection oblgiations too, because we may end up with
+                    // an implied `X::Item: 'a`, which gets desugared into `X::Item = ?0`, `?0: 'a`.
+                    // If we only register the region outlives obligation, this leads to an unconstrained var.
+                    // See `implied_bounds_entailment_alias_var` test.
                     ty::PredicateKind::Clause(
-                        ty::ClauseKind::RegionOutlives(..) | ty::ClauseKind::TypeOutlives(..),
+                        ty::ClauseKind::RegionOutlives(..)
+                        | ty::ClauseKind::TypeOutlives(..)
+                        | ty::ClauseKind::Projection(..),
                     ) => ocx.register_obligation(obligation),
                     ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
                         if wf_args_seen.insert(arg) {