6 files changed, 33 insertions, 3 deletions
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index 4503af03ca3..c82438f05e1 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -842,7 +842,7 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
         let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
 
         self.register_predicates([ty::Binder::dummy(if self.tcx().trait_solver_next() {
-            ty::PredicateKind::AliasEq(a.into(), b.into())
+            ty::PredicateKind::AliasEq(a.into(), b.into(), ty::AliasRelationDirection::Equate)
         } else {
             ty::PredicateKind::ConstEquate(a, b)
         })]);
@@ -852,13 +852,15 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     ///
     /// If they aren't equal then the relation doesn't hold.
     fn register_type_equate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
-        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
-
         self.register_predicates([ty::Binder::dummy(ty::PredicateKind::AliasEq(
             a.into(),
             b.into(),
+            self.alias_relate_direction(),
         ))]);
     }
+
+    /// Relation direction emitted for `AliasEq` predicates
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection;
 }
 
 fn int_unification_error<'tcx>(
diff --git a/compiler/rustc_infer/src/infer/equate.rs b/compiler/rustc_infer/src/infer/equate.rs
index c92a74b6241..38002357cde 100644
--- a/compiler/rustc_infer/src/infer/equate.rs
+++ b/compiler/rustc_infer/src/infer/equate.rs
@@ -210,4 +210,8 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Equate<'_, '_, 'tcx> {
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.fields.register_obligations(obligations);
     }
+
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        ty::AliasRelationDirection::Equate
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/glb.rs b/compiler/rustc_infer/src/infer/glb.rs
index 5c12351226a..6395c4d4b20 100644
--- a/compiler/rustc_infer/src/infer/glb.rs
+++ b/compiler/rustc_infer/src/infer/glb.rs
@@ -155,4 +155,9 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, 'tcx> {
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.fields.register_obligations(obligations);
     }
+
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        // FIXME(deferred_projection_equality): This isn't right, I think?
+        ty::AliasRelationDirection::Equate
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/lub.rs b/compiler/rustc_infer/src/infer/lub.rs
index dbef42db8f1..98cbd4c561c 100644
--- a/compiler/rustc_infer/src/infer/lub.rs
+++ b/compiler/rustc_infer/src/infer/lub.rs
@@ -155,4 +155,9 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Lub<'_, '_, 'tcx> {
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.fields.register_obligations(obligations)
     }
+
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        // FIXME(deferred_projection_equality): This isn't right, I think?
+        ty::AliasRelationDirection::Equate
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index 573cd91a2a2..216d2965da7 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -777,6 +777,16 @@ where
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.delegate.register_obligations(obligations);
     }
+
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        match self.ambient_variance {
+            ty::Variance::Covariant => ty::AliasRelationDirection::Subtype,
+            ty::Variance::Contravariant => ty::AliasRelationDirection::Supertype,
+            ty::Variance::Invariant => ty::AliasRelationDirection::Equate,
+            // FIXME(deferred_projection_equality): Implement this when we trigger it
+            ty::Variance::Bivariant => unreachable!(),
+        }
+    }
 }
 
 /// When we encounter a binder like `for<..> fn(..)`, we actually have
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index 230cadb1184..fc73ca7606d 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -236,4 +236,8 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Sub<'_, '_, 'tcx> {
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.fields.register_obligations(obligations);
     }
+
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        ty::AliasRelationDirection::Subtype
+    }
 }