Auto merge of #121462 - compiler-errors:eq-and-sub, r=<try>

Combine `Sub` and `Equate`

Combine `Sub` and `Equate` into a new relation called `TypeRelating` (that name sounds familiar...)

Tracks the difference between `Sub` and `Equate` via `ambient_variance: ty::Variance` much like the `NllTypeRelating` relation, but implemented slightly jankier because it's a more general purpose relation.

r? lcnr

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -348,12 +348,15 @@ impl<'bccx, 'tcx> TypeRelation<'tcx> for NllTypeRelating<'_, 'bccx, 'tcx> {
 
         debug!(?self.ambient_variance);
         // In a bivariant context this always succeeds.
-        let r =
-            if self.ambient_variance == ty::Variance::Bivariant { a } else { self.relate(a, b)? };
+        let r = if self.ambient_variance == ty::Variance::Bivariant {
+            Ok(a)
+        } else {
+            self.relate(a, b)
+        };
 
         self.ambient_variance = old_ambient_variance;
 
-        Ok(r)
+        r
     }
 
     #[instrument(skip(self), level = "debug")]
@@ -576,10 +579,6 @@ impl<'bccx, 'tcx> ObligationEmittingRelation<'tcx> for NllTypeRelating<'_, 'bccx
         );
     }
 
-    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
-        unreachable!("manually overridden to handle ty::Variance::Contravariant ambient variance")
-    }
-
     fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
         self.register_predicates([ty::Binder::dummy(match self.ambient_variance {
             ty::Variance::Covariant => ty::PredicateKind::AliasRelate(

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -1,4 +1,6 @@
-//! There are four type combiners: [Equate], [Sub], [Lub], and [Glb].
+//! There are four type combiners: [TypeRelating], [Lub], and [Glb],
+//! (and `NllTypeRelating` in rustc_borrowck, which is only used for NLL).
+//!
 //! Each implements the trait [TypeRelation] and contains methods for
 //! combining two instances of various things and yielding a new instance.
 //! These combiner methods always yield a `Result<T>`. To relate two
@@ -22,10 +24,9 @@
 //! [TypeRelation::a_is_expected], so when dealing with contravariance
 //! this should be correctly updated.
 
-use super::equate::Equate;
 use super::glb::Glb;
 use super::lub::Lub;
-use super::sub::Sub;
+use super::type_relating::TypeRelating;
 use crate::infer::{DefineOpaqueTypes, InferCtxt, TypeTrace};
 use crate::traits::{Obligation, PredicateObligations};
 use rustc_middle::infer::canonical::OriginalQueryValues;
@@ -303,12 +304,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         self.infcx.tcx
     }
 
-    pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
-        Equate::new(self, a_is_expected)
+    pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> TypeRelating<'a, 'infcx, 'tcx> {
+        TypeRelating::new(self, a_is_expected, ty::Invariant)
     }
 
-    pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
-        Sub::new(self, a_is_expected)
+    pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> TypeRelating<'a, 'infcx, 'tcx> {
+        TypeRelating::new(self, a_is_expected, ty::Covariant)
     }
 
     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
@@ -343,19 +344,8 @@ pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     /// be used if control over the obligation causes is required.
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ToPredicate<'tcx>>);
 
-    /// Register an obligation that both types must be related to each other according to
-    /// the [`ty::AliasRelationDirection`] given by [`ObligationEmittingRelation::alias_relate_direction`]
-    fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
-        self.register_predicates([ty::Binder::dummy(ty::PredicateKind::AliasRelate(
-            a.into(),
-            b.into(),
-            self.alias_relate_direction(),
-        ))]);
-    }
-
-    /// Relation direction emitted for `AliasRelate` predicates, corresponding to the direction
-    /// of the relation.
-    fn alias_relate_direction(&self) -> ty::AliasRelationDirection;
+    /// Register `AliasRelate` obligation(s) that both types must be related to each other.
+    fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>);
 }
 
 fn int_unification_error<'tcx>(

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -404,9 +404,9 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         debug!(?self.ambient_variance, "new ambient variance");
         // Recursive calls to `relate` can overflow the stack. For example a deeper version of
         // `ui/associated-consts/issue-93775.rs`.
-        let r = ensure_sufficient_stack(|| self.relate(a, b))?;
+        let r = ensure_sufficient_stack(|| self.relate(a, b));
         self.ambient_variance = old_ambient_variance;
-        Ok(r)
+        r
     }
 
     #[instrument(level = "debug", skip(self, t2), ret)]

compiler/rustc_infer/src/infer/relate/glb.rs

@@ -151,8 +151,12 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, '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
+    fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
+        self.register_predicates([ty::Binder::dummy(ty::PredicateKind::AliasRelate(
+            a.into(),
+            b.into(),
+            // FIXME(deferred_projection_equality): This isn't right, I think?
+            ty::AliasRelationDirection::Equate,
+        ))]);
     }
 }

compiler/rustc_infer/src/infer/relate/lub.rs

@@ -151,8 +151,12 @@ impl<'tcx> ObligationEmittingRelation<'tcx> for Lub<'_, '_, '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
+    fn register_type_relate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
+        self.register_predicates([ty::Binder::dummy(ty::PredicateKind::AliasRelate(
+            a.into(),
+            b.into(),
+            // FIXME(deferred_projection_equality): This isn't right, I think?
+            ty::AliasRelationDirection::Equate,
+        ))]);
     }
 }

compiler/rustc_infer/src/infer/relate/mod.rs

@@ -2,10 +2,9 @@
 //! (except for some relations used for diagnostics and heuristics in the compiler).
 
 pub(super) mod combine;
-mod equate;
 mod generalize;
 mod glb;
 mod higher_ranked;
 mod lattice;
 mod lub;
-mod sub;
+mod type_relating;