Remove opaque type obligation and just register opaque types as they are encountered.

This also registers obligations for the hidden type immediately.

compiler/rustc_middle/src/ty/mod.rs

@@ -632,11 +632,6 @@ 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
@@ -1004,7 +999,6 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::TypeOutlives(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
-            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -1023,7 +1017,6 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::ClosureKind(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
-            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }