Auto merge of #121462 - compiler-errors:eq-and-sub, r=lcnr

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/mod.rs

@@ -1066,7 +1066,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             &cause,
                             param_env,
                             hidden_ty.ty,
-                            true,
                             &mut obligations,
                         )?;
 

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -120,7 +120,6 @@ impl<'me, 'bccx, 'tcx> NllTypeRelating<'me, 'bccx, 'tcx> {
     fn relate_opaques(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let infcx = self.type_checker.infcx;
         debug_assert!(!infcx.next_trait_solver());
-        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
         // `handle_opaque_type` cannot handle subtyping, so to support subtyping
         // we instead eagerly generalize here. This is a bit of a mess but will go
         // away once we're using the new solver.
@@ -161,8 +160,7 @@ impl<'me, 'bccx, 'tcx> NllTypeRelating<'me, 'bccx, 'tcx> {
             ),
         };
         let cause = ObligationCause::dummy_with_span(self.span());
-        let obligations =
-            infcx.handle_opaque_type(a, b, true, &cause, self.param_env())?.obligations;
+        let obligations = infcx.handle_opaque_type(a, b, &cause, self.param_env())?.obligations;
         self.register_obligations(obligations);
         Ok(())
     }
@@ -331,10 +329,6 @@ impl<'bccx, 'tcx> TypeRelation<'tcx> for NllTypeRelating<'_, 'bccx, 'tcx> {
         "nll::subtype"
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     #[instrument(skip(self, info), level = "trace", ret)]
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
@@ -349,12 +343,15 @@ 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")]
@@ -579,10 +576,6 @@ 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_hir_typeck/src/coercion.rs

@@ -1493,6 +1493,21 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             return;
         }
 
+        let (expected, found) = if label_expression_as_expected {
+            // In the case where this is a "forced unit", like
+            // `break`, we want to call the `()` "expected"
+            // since it is implied by the syntax.
+            // (Note: not all force-units work this way.)"
+            (expression_ty, self.merged_ty())
+        } else {
+            // Otherwise, the "expected" type for error
+            // reporting is the current unification type,
+            // which is basically the LUB of the expressions
+            // we've seen so far (combined with the expected
+            // type)
+            (self.merged_ty(), expression_ty)
+        };
+
         // Handle the actual type unification etc.
         let result = if let Some(expression) = expression {
             if self.pushed == 0 {
@@ -1540,12 +1555,11 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             // Another example is `break` with no argument expression.
             assert!(expression_ty.is_unit(), "if let hack without unit type");
             fcx.at(cause, fcx.param_env)
-                // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
-                .eq_exp(
+                .eq(
+                    // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
                     DefineOpaqueTypes::Yes,
-                    label_expression_as_expected,
-                    expression_ty,
-                    self.merged_ty(),
+                    expected,
+                    found,
                 )
                 .map(|infer_ok| {
                     fcx.register_infer_ok_obligations(infer_ok);
@@ -1579,20 +1593,6 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 fcx.set_tainted_by_errors(
                     fcx.dcx().span_delayed_bug(cause.span, "coercion error but no error emitted"),
                 );
-                let (expected, found) = if label_expression_as_expected {
-                    // In the case where this is a "forced unit", like
-                    // `break`, we want to call the `()` "expected"
-                    // since it is implied by the syntax.
-                    // (Note: not all force-units work this way.)"
-                    (expression_ty, self.merged_ty())
-                } else {
-                    // Otherwise, the "expected" type for error
-                    // reporting is the current unification type,
-                    // which is basically the LUB of the expressions
-                    // we've seen so far (combined with the expected
-                    // type)
-                    (self.merged_ty(), expression_ty)
-                };
                 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
 
                 let mut err;

compiler/rustc_infer/src/infer/at.rs

@@ -49,7 +49,6 @@ pub struct At<'a, 'tcx> {
 
 pub struct Trace<'a, 'tcx> {
     at: At<'a, 'tcx>,
-    a_is_expected: bool,
     trace: TypeTrace<'tcx>,
 }
 
@@ -105,23 +104,6 @@ pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {
 }
 
 impl<'a, 'tcx> At<'a, 'tcx> {
-    /// Makes `a <: b`, where `a` may or may not be expected.
-    ///
-    /// See [`At::trace_exp`] and [`Trace::sub`] for a version of
-    /// this method that only requires `T: Relate<'tcx>`
-    pub fn sub_exp<T>(
-        self,
-        define_opaque_types: DefineOpaqueTypes,
-        a_is_expected: bool,
-        a: T,
-        b: T,
-    ) -> InferResult<'tcx, ()>
-    where
-        T: ToTrace<'tcx>,
-    {
-        self.trace_exp(a_is_expected, a, b).sub(define_opaque_types, a, b)
-    }
-
     /// Makes `actual <: expected`. For example, if type-checking a
     /// call like `foo(x)`, where `foo: fn(i32)`, you might have
     /// `sup(i32, x)`, since the "expected" type is the type that
@@ -138,7 +120,7 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.sub_exp(define_opaque_types, false, actual, expected)
+        self.trace(expected, actual).sup(define_opaque_types, expected, actual)
     }
 
     /// Makes `expected <: actual`.
@@ -154,24 +136,7 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.sub_exp(define_opaque_types, true, expected, actual)
-    }
-
-    /// Makes `expected <: actual`.
-    ///
-    /// See [`At::trace_exp`] and [`Trace::eq`] for a version of
-    /// this method that only requires `T: Relate<'tcx>`
-    pub fn eq_exp<T>(
-        self,
-        define_opaque_types: DefineOpaqueTypes,
-        a_is_expected: bool,
-        a: T,
-        b: T,
-    ) -> InferResult<'tcx, ()>
-    where
-        T: ToTrace<'tcx>,
-    {
-        self.trace_exp(a_is_expected, a, b).eq(define_opaque_types, a, b)
+        self.trace(expected, actual).sub(define_opaque_types, expected, actual)
     }
 
     /// Makes `expected <: actual`.
@@ -260,48 +225,50 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        self.trace_exp(true, expected, actual)
+        let trace = ToTrace::to_trace(self.cause, true, expected, actual);
+        Trace { at: self, trace }
     }
+}
 
-    /// Like `trace`, but the expected value is determined by the
-    /// boolean argument (if true, then the first argument `a` is the
-    /// "expected" value).
-    pub fn trace_exp<T>(self, a_is_expected: bool, a: T, b: T) -> Trace<'a, 'tcx>
+impl<'a, 'tcx> Trace<'a, 'tcx> {
+    /// Makes `a <: b`.
+    #[instrument(skip(self), level = "debug")]
+    pub fn sub<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
-        T: ToTrace<'tcx>,
+        T: Relate<'tcx>,
     {
-        let trace = ToTrace::to_trace(self.cause, a_is_expected, a, b);
-        Trace { at: self, trace, a_is_expected }
+        let Trace { at, trace } = self;
+        let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
+        fields
+            .sub()
+            .relate(a, b)
+            .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
-}
 
-impl<'a, 'tcx> Trace<'a, 'tcx> {
-    /// Makes `a <: b` where `a` may or may not be expected (if
-    /// `a_is_expected` is true, then `a` is expected).
+    /// Makes `a :> b`.
     #[instrument(skip(self), level = "debug")]
-    pub fn sub<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
+    pub fn sup<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .sub(a_is_expected)
+            .sup()
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
 
-    /// Makes `a == b`; the expectation is set by the call to
-    /// `trace()`.
+    /// Makes `a == b`.
     #[instrument(skip(self), level = "debug")]
     pub fn eq<T>(self, define_opaque_types: DefineOpaqueTypes, a: T, b: T) -> InferResult<'tcx, ()>
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .equate(StructurallyRelateAliases::No, a_is_expected)
+            .equate(StructurallyRelateAliases::No)
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
@@ -313,11 +280,11 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         debug_assert!(at.infcx.next_trait_solver());
         let mut fields = at.infcx.combine_fields(trace, at.param_env, DefineOpaqueTypes::No);
         fields
-            .equate(StructurallyRelateAliases::Yes, a_is_expected)
+            .equate(StructurallyRelateAliases::Yes)
             .relate(a, b)
             .map(move |_| InferOk { value: (), obligations: fields.obligations })
     }
@@ -327,10 +294,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .lub(a_is_expected)
+            .lub()
             .relate(a, b)
             .map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
@@ -340,10 +307,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        let Trace { at, trace, a_is_expected } = self;
+        let Trace { at, trace } = self;
         let mut fields = at.infcx.combine_fields(trace, at.param_env, define_opaque_types);
         fields
-            .glb(a_is_expected)
+            .glb()
             .relate(a, b)
             .map(move |t| InferOk { value: t, obligations: fields.obligations })
     }

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

@@ -2654,10 +2654,6 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         "SameTypeModuloInfer"
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     fn relate_with_variance<T: relate::Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,

compiler/rustc_infer/src/infer/mod.rs

@@ -836,7 +836,6 @@ impl<'tcx> InferCtxt<'tcx> {
         CombineFields {
             infcx: self,
             trace,
-            cause: None,
             param_env,
             obligations: PredicateObligations::new(),
             define_opaque_types,
@@ -1033,7 +1032,11 @@ impl<'tcx> InferCtxt<'tcx> {
         }
 
         self.enter_forall(predicate, |ty::SubtypePredicate { a_is_expected, a, b }| {
-            Ok(self.at(cause, param_env).sub_exp(DefineOpaqueTypes::No, a_is_expected, a, b))
+            if a_is_expected {
+                Ok(self.at(cause, param_env).sub(DefineOpaqueTypes::No, a, b))
+            } else {
+                Ok(self.at(cause, param_env).sup(DefineOpaqueTypes::No, b, a))
+            }
         })
     }