Lazily resolve type-alias-impl-trait defining uses

by using an opaque type obligation to bubble up comparisons between opaque types and other types

Also uses proper obligation causes so that the body id works, because out of some reason nll uses body ids for logic instead of just diagnostics.

compiler/rustc_middle/src/ty/mod.rs

@@ -628,6 +628,11 @@ pub enum PredicateKind<'tcx> {
     ///
     /// Only used for Chalk.
     TypeWellFormedFromEnv(Ty<'tcx>),
+
+    /// Represents a hidden type assignment for an opaque type.
+    /// Such obligations get processed by checking whether the item currently being
+    /// type-checked may acually define it.
+    OpaqueType(Ty<'tcx>, Ty<'tcx>),
 }
 
 /// The crate outlives map is computed during typeck and contains the
@@ -987,6 +992,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::TypeOutlives(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -1005,6 +1011,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::ClosureKind(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -1045,7 +1052,18 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, TypeFoldable)]
+#[derive(
+    Copy,
+    Clone,
+    Debug,
+    PartialEq,
+    Eq,
+    HashStable,
+    TyEncodable,
+    TyDecodable,
+    TypeFoldable,
+    Lift
+)]
 pub struct OpaqueTypeKey<'tcx> {
     pub def_id: DefId,
     pub substs: SubstsRef<'tcx>,