Fallout from removing a_is_expected

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -160,8 +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(())
     }
@@ -330,10 +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,

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/opaque_types.rs

@@ -78,9 +78,7 @@ impl<'tcx> InferCtxt<'tcx> {
                         span,
                     });
                     obligations.extend(
-                        self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
-                            .unwrap()
-                            .obligations,
+                        self.handle_opaque_type(ty, ty_var, &cause, param_env).unwrap().obligations,
                     );
                     ty_var
                 }
@@ -94,14 +92,12 @@ impl<'tcx> InferCtxt<'tcx> {
         &self,
         a: Ty<'tcx>,
         b: Ty<'tcx>,
-        a_is_expected: bool,
         cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> InferResult<'tcx, ()> {
         if a.references_error() || b.references_error() {
             return Ok(InferOk { value: (), obligations: vec![] });
         }
-        let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
         let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) if def_id.is_local() => {
                 let def_id = def_id.expect_local();

compiler/rustc_infer/src/infer/outlives/test_type_match.rs

@@ -144,10 +144,6 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstHigherRankedOutlives<'tcx> {
         self.tcx
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    } // irrelevant
-
     #[instrument(level = "trace", skip(self))]
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,

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

@@ -17,12 +17,6 @@
 //!
 //! On success, the  LUB/GLB operations return the appropriate bound. The
 //! return value of `Equate` or `Sub` shouldn't really be used.
-//!
-//! ## Contravariance
-//!
-//! We explicitly track which argument is expected using
-//! [TypeRelation::a_is_expected], so when dealing with contravariance
-//! this should be correctly updated.
 
 use super::glb::Glb;
 use super::lub::Lub;
@@ -57,7 +51,6 @@ impl<'tcx> InferCtxt<'tcx> {
     where
         R: ObligationEmittingRelation<'tcx>,
     {
-        let a_is_expected = relation.a_is_expected();
         debug_assert!(!a.has_escaping_bound_vars());
         debug_assert!(!b.has_escaping_bound_vars());
 
@@ -68,20 +61,20 @@ impl<'tcx> InferCtxt<'tcx> {
                     .borrow_mut()
                     .int_unification_table()
                     .unify_var_var(a_id, b_id)
-                    .map_err(|e| int_unification_error(a_is_expected, e))?;
+                    .map_err(|e| int_unification_error(true, e))?;
                 Ok(a)
             }
             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
-                self.unify_integral_variable(a_is_expected, v_id, IntType(v))
+                self.unify_integral_variable(true, v_id, IntType(v))
             }
             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
-                self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
+                self.unify_integral_variable(false, v_id, IntType(v))
             }
             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
-                self.unify_integral_variable(a_is_expected, v_id, UintType(v))
+                self.unify_integral_variable(true, v_id, UintType(v))
             }
             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
-                self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
+                self.unify_integral_variable(false, v_id, UintType(v))
             }
 
             // Relate floating-point variables to other types
@@ -90,14 +83,14 @@ impl<'tcx> InferCtxt<'tcx> {
                     .borrow_mut()
                     .float_unification_table()
                     .unify_var_var(a_id, b_id)
-                    .map_err(|e| float_unification_error(a_is_expected, e))?;
+                    .map_err(|e| float_unification_error(true, e))?;
                 Ok(a)
             }
             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
-                self.unify_float_variable(a_is_expected, v_id, v)
+                self.unify_float_variable(true, v_id, v)
             }
             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
-                self.unify_float_variable(!a_is_expected, v_id, v)
+                self.unify_float_variable(false, v_id, v)
             }
 
             // We don't expect `TyVar` or `Fresh*` vars at this point with lazy norm.
@@ -130,7 +123,7 @@ impl<'tcx> InferCtxt<'tcx> {
 
             // All other cases of inference are errors
             (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
-                Err(TypeError::Sorts(ty::relate::expected_found(relation, a, b)))
+                Err(TypeError::Sorts(ty::relate::expected_found(a, b)))
             }
 
             // During coherence, opaque types should be treated as *possibly*
@@ -228,12 +221,12 @@ impl<'tcx> InferCtxt<'tcx> {
             }
 
             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
-                self.instantiate_const_var(relation, relation.a_is_expected(), vid, b)?;
+                self.instantiate_const_var(relation, true, vid, b)?;
                 Ok(b)
             }
 
             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
-                self.instantiate_const_var(relation, !relation.a_is_expected(), vid, a)?;
+                self.instantiate_const_var(relation, false, vid, a)?;
                 Ok(a)
             }
 
@@ -250,8 +243,6 @@ impl<'tcx> InferCtxt<'tcx> {
             {
                 match relation.structurally_relate_aliases() {
                     StructurallyRelateAliases::No => {
-                        let (a, b) = if relation.a_is_expected() { (a, b) } else { (b, a) };
-
                         relation.register_predicates([if self.next_trait_solver() {
                             ty::PredicateKind::AliasRelate(
                                 a.into(),

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

@@ -130,7 +130,7 @@ impl<'tcx> InferCtxt<'tcx> {
             // instantiate_ty_var(?b, A) # expected and variance flipped
             // A rel A'
             // ```
-            if target_is_expected == relation.a_is_expected() {
+            if target_is_expected {
                 relation.relate(generalized_ty, source_ty)?;
             } else {
                 debug!("flip relation");
@@ -204,9 +204,9 @@ impl<'tcx> InferCtxt<'tcx> {
             .const_unification_table()
             .union_value(target_vid, ConstVariableValue::Known { value: generalized_ct });
 
-        // HACK: make sure that we `a_is_expected` continues to be
-        // correct when relating the generalized type with the source.
-        if target_is_expected == relation.a_is_expected() {
+        // Make sure that the order is correct when relating the
+        // generalized const and the source.
+        if target_is_expected {
             relation.relate_with_variance(
                 ty::Variance::Invariant,
                 ty::VarianceDiagInfo::default(),
@@ -398,10 +398,6 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         "Generalizer"
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     fn relate_item_args(
         &mut self,
         item_def_id: DefId,

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

@@ -30,10 +30,6 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

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

@@ -116,9 +116,7 @@ where
                 && !this.infcx().next_trait_solver() =>
         {
             this.register_obligations(
-                infcx
-                    .handle_opaque_type(a, b, this.a_is_expected(), this.cause(), this.param_env())?
-                    .obligations,
+                infcx.handle_opaque_type(a, b, this.cause(), this.param_env())?.obligations,
             );
             Ok(a)
         }

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

@@ -30,10 +30,6 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

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

@@ -35,10 +35,6 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
         self.fields.infcx.tcx
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,
@@ -139,7 +135,7 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
             {
                 self.fields.obligations.extend(
                     infcx
-                        .handle_opaque_type(a, b, true, &self.fields.trace.cause, self.param_env())?
+                        .handle_opaque_type(a, b, &self.fields.trace.cause, self.param_env())?
                         .obligations,
                 );
             }
@@ -158,10 +154,6 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
         b: ty::Region<'tcx>,
     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
         debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
-
-        // FIXME -- we have more fine-grained information available
-        // from the "cause" field, we could perhaps give more tailored
-        // error messages.
         let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
 
         match self.ambient_variance {
@@ -184,7 +176,6 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
                     .make_subregion(origin, a, b);
             }
             ty::Invariant => {
-                // The order of `make_eqregion` apparently matters.
                 self.fields
                     .infcx
                     .inner

compiler/rustc_middle/src/ty/_match.rs

@@ -37,10 +37,6 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstFreshVars<'tcx> {
         self.tcx
     }
 
-    fn a_is_expected(&self) -> bool {
-        true
-    } // irrelevant
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,
@@ -75,7 +71,7 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstFreshVars<'tcx> {
             ) => Ok(a),
 
             (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
-                Err(TypeError::Sorts(relate::expected_found(self, a, b)))
+                Err(TypeError::Sorts(relate::expected_found(a, b)))
             }
 
             (&ty::Error(guar), _) | (_, &ty::Error(guar)) => Ok(Ty::new_error(self.tcx(), guar)),
@@ -100,7 +96,7 @@ impl<'tcx> TypeRelation<'tcx> for MatchAgainstFreshVars<'tcx> {
             }
 
             (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
-                return Err(TypeError::ConstMismatch(relate::expected_found(self, a, b)));
+                return Err(TypeError::ConstMismatch(relate::expected_found(a, b)));
             }
 
             _ => {}