Register member constraints on the final merged hidden type

Previously we did this per hidden type candiate, which didn't always have all the information available.

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -723,7 +723,7 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
         true
     }
 
-    fn constrain_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool) {
+    fn register_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool) {
         self.obligations.push(self.infcx.opaque_ty_obligation(
             a,
             b,

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

@@ -90,7 +90,7 @@ pub trait TypeRelatingDelegate<'tcx> {
         info: ty::VarianceDiagInfo<'tcx>,
     );
 
-    fn constrain_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool);
+    fn register_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool);
 
     fn const_equate(&mut self, a: &'tcx ty::Const<'tcx>, b: &'tcx ty::Const<'tcx>);
 
@@ -591,7 +591,7 @@ where
                     (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
                     _ => unreachable!(),
                 };
-                self.delegate.constrain_opaque_type(a, b, true);
+                self.delegate.register_opaque_type(a, b, true);
                 trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
                 Ok(a)
             }

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -1,5 +1,5 @@
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use crate::infer::{InferCtxt, InferOk, InferResult};
+use crate::infer::{InferCtxt, InferOk};
 use crate::traits::{self, PredicateObligation, PredicateObligations};
 use hir::def_id::{DefId, LocalDefId};
 use hir::OpaqueTyOrigin;
@@ -379,13 +379,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// - `free_region_relations` -- something that can be used to relate
     ///   the free regions (`'a`) that appear in the impl trait.
     #[instrument(level = "debug", skip(self))]
-    pub fn constrain_opaque_type(
+    pub fn register_member_constraints(
         &self,
         param_env: ty::ParamEnv<'tcx>,
         opaque_type_key: OpaqueTypeKey<'tcx>,
         concrete_ty: Ty<'tcx>,
         span: Span,
-    ) -> InferResult<'tcx, ()> {
+    ) {
         let def_id = opaque_type_key.def_id;
 
         let tcx = self.tcx;
@@ -445,7 +445,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 )
             },
         });
-        Ok(InferOk { value: (), obligations: vec![] })
     }
 
     pub fn opaque_ty_obligation(