Use real opaque type instead of just saying impl Trait

compiler/rustc_middle/src/infer/mod.rs

@@ -2,9 +2,8 @@ pub mod canonical;
 pub mod unify_key;
 
 use crate::ty::Region;
-use crate::ty::Ty;
+use crate::ty::{OpaqueTypeKey, Ty};
 use rustc_data_structures::sync::Lrc;
-use rustc_hir::def_id::LocalDefId;
 use rustc_span::Span;
 
 /// Requires that `region` must be equal to one of the regions in `choice_regions`.
@@ -15,8 +14,9 @@ use rustc_span::Span;
 /// ```
 #[derive(Debug, Clone, HashStable, TypeFoldable, TypeVisitable, Lift)]
 pub struct MemberConstraint<'tcx> {
-    /// The `DefId` of the opaque type causing this constraint: used for error reporting.
-    pub opaque_type_def_id: LocalDefId,
+    /// The `DefId` and substs of the opaque type causing this constraint.
+    /// Used for error reporting.
+    pub key: OpaqueTypeKey<'tcx>,
 
     /// The span where the hidden type was instantiated.
     pub definition_span: Span,