Remove predicates on associated types from traits

These need to only be bounds to avoid cycle errors in trait checking.

compiler/rustc_middle/src/query/mod.rs

@@ -391,6 +391,24 @@ rustc_queries! {
             desc { |tcx| "computing predicates of `{}`", tcx.def_path_str(key) }
         }
 
+        /// Returns everything that looks like a predicate written explicitly
+        /// by the user on a trait item.
+        ///
+        /// Traits are unusual, because predicates on associated types are
+        /// converted into bounds on that type for backwards compatibility:
+        ///
+        /// trait X where Self::U: Copy { type U; }
+        ///
+        /// becomes
+        ///
+        /// trait X { type U: Copy; }
+        ///
+        /// `explicit_predicates_of` and `explicit_item_bounds` will then take
+        /// the appropriate subsets of the predicates here.
+        query trait_explicit_predicates_and_bounds(key: LocalDefId) -> ty::GenericPredicates<'tcx> {
+            desc { |tcx| "computing explicit predicates of trait `{}`", tcx.def_path_str(key.to_def_id()) }
+        }
+
         /// Returns the predicates written explicitly by the user.
         query explicit_predicates_of(key: DefId) -> ty::GenericPredicates<'tcx> {
             desc { |tcx| "computing explicit predicates of `{}`", tcx.def_path_str(key) }