Fallout from removing a_is_expected

12 files changed, 46 insertions, 127 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/relate_tys.rs b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
index 5c5274d7d86..78609a482ed 100644
--- a/compiler/rustc_borrowck/src/type_check/relate_tys.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 911b2f16c8b..1cf990fef04 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index 07245643ef5..7a789a1b41b 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/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();
diff --git a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs b/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
index d547f51f381..29c11d4247d 100644
--- a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/relate/combine.rs b/compiler/rustc_infer/src/infer/relate/combine.rs
index 099b7ff7c04..28b7db275a3 100644
--- a/compiler/rustc_infer/src/infer/relate/combine.rs
+++ b/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(),
diff --git a/compiler/rustc_infer/src/infer/relate/generalize.rs b/compiler/rustc_infer/src/infer/relate/generalize.rs
index b18c8a8b844..5fb9d9341e0 100644
--- a/compiler/rustc_infer/src/infer/relate/generalize.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/relate/glb.rs b/compiler/rustc_infer/src/infer/relate/glb.rs
index f6796861b12..b86d1b2671d 100644
--- a/compiler/rustc_infer/src/infer/relate/glb.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/relate/lattice.rs b/compiler/rustc_infer/src/infer/relate/lattice.rs
index 744e2dfa380..747158585db 100644
--- a/compiler/rustc_infer/src/infer/relate/lattice.rs
+++ b/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)
         }
diff --git a/compiler/rustc_infer/src/infer/relate/lub.rs b/compiler/rustc_infer/src/infer/relate/lub.rs
index 3d9cfe7bf05..20f5f65c984 100644
--- a/compiler/rustc_infer/src/infer/relate/lub.rs
+++ b/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,
diff --git a/compiler/rustc_infer/src/infer/relate/type_relating.rs b/compiler/rustc_infer/src/infer/relate/type_relating.rs
index 7464b525724..c053adc5e07 100644
--- a/compiler/rustc_infer/src/infer/relate/type_relating.rs
+++ b/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
diff --git a/compiler/rustc_middle/src/ty/_match.rs b/compiler/rustc_middle/src/ty/_match.rs
index 425a2dbd890..e28e4d66faf 100644
--- a/compiler/rustc_middle/src/ty/_match.rs
+++ b/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)));
             }
 
             _ => {}
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index abd39914cfd..990e78aff8a 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -15,21 +15,12 @@ use std::iter;
 
 pub type RelateResult<'tcx, T> = Result<T, TypeError<'tcx>>;
 
-#[derive(Clone, Debug)]
-pub enum Cause {
-    ExistentialRegionBound, // relating an existential region bound
-}
-
 pub trait TypeRelation<'tcx>: Sized {
     fn tcx(&self) -> TyCtxt<'tcx>;
 
     /// Returns a static string we can use for printouts.
     fn tag(&self) -> &'static str;
 
-    /// Returns `true` if the value `a` is the "expected" type in the
-    /// relation. Just affects error messages.
-    fn a_is_expected(&self) -> bool;
-
     /// Generic relation routine suitable for most anything.
     fn relate<T: Relate<'tcx>>(&mut self, a: T, b: T) -> RelateResult<'tcx, T> {
         Relate::relate(self, a, b)
@@ -171,11 +162,7 @@ impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
         let tcx = relation.tcx();
 
         if a.c_variadic != b.c_variadic {
-            return Err(TypeError::VariadicMismatch(expected_found(
-                relation,
-                a.c_variadic,
-                b.c_variadic,
-            )));
+            return Err(TypeError::VariadicMismatch(expected_found(a.c_variadic, b.c_variadic)));
         }
         let unsafety = relation.relate(a.unsafety, b.unsafety)?;
         let abi = relation.relate(a.abi, b.abi)?;
@@ -220,39 +207,31 @@ impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
 
 impl<'tcx> Relate<'tcx> for ty::BoundConstness {
     fn relate<R: TypeRelation<'tcx>>(
-        relation: &mut R,
+        _relation: &mut R,
         a: ty::BoundConstness,
         b: ty::BoundConstness,
     ) -> RelateResult<'tcx, ty::BoundConstness> {
-        if a != b {
-            Err(TypeError::ConstnessMismatch(expected_found(relation, a, b)))
-        } else {
-            Ok(a)
-        }
+        if a != b { Err(TypeError::ConstnessMismatch(expected_found(a, b))) } else { Ok(a) }
     }
 }
 
 impl<'tcx> Relate<'tcx> for hir::Unsafety {
     fn relate<R: TypeRelation<'tcx>>(
-        relation: &mut R,
+        _relation: &mut R,
         a: hir::Unsafety,
         b: hir::Unsafety,
     ) -> RelateResult<'tcx, hir::Unsafety> {
-        if a != b {
-            Err(TypeError::UnsafetyMismatch(expected_found(relation, a, b)))
-        } else {
-            Ok(a)
-        }
+        if a != b { Err(TypeError::UnsafetyMismatch(expected_found(a, b))) } else { Ok(a) }
     }
 }
 
 impl<'tcx> Relate<'tcx> for abi::Abi {
     fn relate<R: TypeRelation<'tcx>>(
-        relation: &mut R,
+        _relation: &mut R,
         a: abi::Abi,
         b: abi::Abi,
     ) -> RelateResult<'tcx, abi::Abi> {
-        if a == b { Ok(a) } else { Err(TypeError::AbiMismatch(expected_found(relation, a, b))) }
+        if a == b { Ok(a) } else { Err(TypeError::AbiMismatch(expected_found(a, b))) }
     }
 }
 
@@ -263,7 +242,7 @@ impl<'tcx> Relate<'tcx> for ty::AliasTy<'tcx> {
         b: ty::AliasTy<'tcx>,
     ) -> RelateResult<'tcx, ty::AliasTy<'tcx>> {
         if a.def_id != b.def_id {
-            Err(TypeError::ProjectionMismatched(expected_found(relation, a.def_id, b.def_id)))
+            Err(TypeError::ProjectionMismatched(expected_found(a.def_id, b.def_id)))
         } else {
             let args = match relation.tcx().def_kind(a.def_id) {
                 DefKind::OpaqueTy => relate_args_with_variances(
@@ -291,7 +270,7 @@ impl<'tcx> Relate<'tcx> for ty::ExistentialProjection<'tcx> {
         b: ty::ExistentialProjection<'tcx>,
     ) -> RelateResult<'tcx, ty::ExistentialProjection<'tcx>> {
         if a.def_id != b.def_id {
-            Err(TypeError::ProjectionMismatched(expected_found(relation, a.def_id, b.def_id)))
+            Err(TypeError::ProjectionMismatched(expected_found(a.def_id, b.def_id)))
         } else {
             let term = relation.relate_with_variance(
                 ty::Invariant,
@@ -318,7 +297,7 @@ impl<'tcx> Relate<'tcx> for ty::TraitRef<'tcx> {
     ) -> RelateResult<'tcx, ty::TraitRef<'tcx>> {
         // Different traits cannot be related.
         if a.def_id != b.def_id {
-            Err(TypeError::Traits(expected_found(relation, a.def_id, b.def_id)))
+            Err(TypeError::Traits(expected_found(a.def_id, b.def_id)))
         } else {
             let args = relate_args_invariantly(relation, a.args, b.args)?;
             Ok(ty::TraitRef::new(relation.tcx(), a.def_id, args))
@@ -334,7 +313,7 @@ impl<'tcx> Relate<'tcx> for ty::ExistentialTraitRef<'tcx> {
     ) -> RelateResult<'tcx, ty::ExistentialTraitRef<'tcx>> {
         // Different traits cannot be related.
         if a.def_id != b.def_id {
-            Err(TypeError::Traits(expected_found(relation, a.def_id, b.def_id)))
+            Err(TypeError::Traits(expected_found(a.def_id, b.def_id)))
         } else {
             let args = relate_args_invariantly(relation, a.args, b.args)?;
             Ok(ty::ExistentialTraitRef { def_id: a.def_id, args })
@@ -510,9 +489,9 @@ pub fn structurally_relate_tys<'tcx, R: TypeRelation<'tcx>>(
                     let sz_b = sz_b.try_to_target_usize(tcx);
 
                     match (sz_a, sz_b) {
-                        (Some(sz_a_val), Some(sz_b_val)) if sz_a_val != sz_b_val => Err(
-                            TypeError::FixedArraySize(expected_found(relation, sz_a_val, sz_b_val)),
-                        ),
+                        (Some(sz_a_val), Some(sz_b_val)) if sz_a_val != sz_b_val => {
+                            Err(TypeError::FixedArraySize(expected_found(sz_a_val, sz_b_val)))
+                        }
                         _ => Err(err),
                     }
                 }
@@ -531,9 +510,9 @@ pub fn structurally_relate_tys<'tcx, R: TypeRelation<'tcx>>(
                     iter::zip(as_, bs).map(|(a, b)| relation.relate(a, b)),
                 )?)
             } else if !(as_.is_empty() || bs.is_empty()) {
-                Err(TypeError::TupleSize(expected_found(relation, as_.len(), bs.len())))
+                Err(TypeError::TupleSize(expected_found(as_.len(), bs.len())))
             } else {
-                Err(TypeError::Sorts(expected_found(relation, a, b)))
+                Err(TypeError::Sorts(expected_found(a, b)))
             }
         }
 
@@ -554,7 +533,7 @@ pub fn structurally_relate_tys<'tcx, R: TypeRelation<'tcx>>(
             Ok(Ty::new_alias(tcx, a_kind, alias_ty))
         }
 
-        _ => Err(TypeError::Sorts(expected_found(relation, a, b))),
+        _ => Err(TypeError::Sorts(expected_found(a, b))),
     }
 }
 
@@ -652,13 +631,13 @@ pub fn structurally_relate_consts<'tcx, R: TypeRelation<'tcx>>(
                     let related_args = tcx.mk_const_list(&related_args);
                     Expr::FunctionCall(func, related_args)
                 }
-                _ => return Err(TypeError::ConstMismatch(expected_found(r, a, b))),
+                _ => return Err(TypeError::ConstMismatch(expected_found(a, b))),
             };
             return Ok(ty::Const::new_expr(tcx, expr, a.ty()));
         }
         _ => false,
     };
-    if is_match { Ok(a) } else { Err(TypeError::ConstMismatch(expected_found(relation, a, b))) }
+    if is_match { Ok(a) } else { Err(TypeError::ConstMismatch(expected_found(a, b))) }
 }
 
 impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> {
@@ -680,7 +659,7 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> {
         b_v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
         b_v.dedup();
         if a_v.len() != b_v.len() {
-            return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
+            return Err(TypeError::ExistentialMismatch(expected_found(a, b)));
         }
 
         let v = iter::zip(a_v, b_v).map(|(ep_a, ep_b)| {
@@ -692,7 +671,7 @@ impl<'tcx> Relate<'tcx> for &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> {
                     relation.relate(ep_a.rebind(a), ep_b.rebind(b))?.skip_binder(),
                 ))),
                 (AutoTrait(a), AutoTrait(b)) if a == b => Ok(ep_a.rebind(AutoTrait(a))),
-                _ => Err(TypeError::ExistentialMismatch(expected_found(relation, a, b))),
+                _ => Err(TypeError::ExistentialMismatch(expected_found(a, b))),
             }
         });
         tcx.mk_poly_existential_predicates_from_iter(v)
@@ -792,15 +771,11 @@ impl<'tcx> Relate<'tcx> for GenericArg<'tcx> {
 
 impl<'tcx> Relate<'tcx> for ty::ImplPolarity {
     fn relate<R: TypeRelation<'tcx>>(
-        relation: &mut R,
+        _relation: &mut R,
         a: ty::ImplPolarity,
         b: ty::ImplPolarity,
     ) -> RelateResult<'tcx, ty::ImplPolarity> {
-        if a != b {
-            Err(TypeError::PolarityMismatch(expected_found(relation, a, b)))
-        } else {
-            Ok(a)
-        }
+        if a != b { Err(TypeError::PolarityMismatch(expected_found(a, b))) } else { Ok(a) }
     }
 }
 
@@ -834,9 +809,6 @@ impl<'tcx> Relate<'tcx> for Term<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 // Error handling
 
-pub fn expected_found<'tcx, R, T>(relation: &mut R, a: T, b: T) -> ExpectedFound<T>
-where
-    R: TypeRelation<'tcx>,
-{
-    ExpectedFound::new(relation.a_is_expected(), a, b)
+pub fn expected_found<T>(a: T, b: T) -> ExpectedFound<T> {
+    ExpectedFound::new(true, a, b)
 }