Allow to self reference associated types in where clauses

compiler/rustc_middle/src/query/mod.rs

@@ -436,6 +436,16 @@ rustc_queries! {
             desc { |tcx| "computing the supertraits of `{}`", tcx.def_path_str(key) }
         }
 
+        /// Maps from the `DefId` of a trait to the list of
+        /// super-predicates. This is a subset of the full list of
+        /// predicates. We store these in a separate map because we must
+        /// evaluate them even during type conversion, often before the
+        /// full predicates are available (note that supertraits have
+        /// additional acyclicity requirements).
+        query super_predicates_that_define_assoc_type(key: (DefId, Option<rustc_span::symbol::Ident>)) -> ty::GenericPredicates<'tcx> {
+            desc { |tcx| "computing the supertraits of `{}`", tcx.def_path_str(key.0) }
+        }
+
         /// To avoid cycles within the predicates of a single item we compute
         /// per-type-parameter predicates for resolving `T::AssocTy`.
         query type_param_predicates(key: (DefId, LocalDefId, rustc_span::symbol::Ident)) -> ty::GenericPredicates<'tcx> {

compiler/rustc_middle/src/ty/query/keys.rs

@@ -149,6 +149,17 @@ impl Key for (LocalDefId, DefId) {
     }
 }
 
+impl Key for (DefId, Option<Ident>) {
+    type CacheSelector = DefaultCacheSelector;
+
+    fn query_crate(&self) -> CrateNum {
+        self.0.krate
+    }
+    fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
+        tcx.def_span(self.0)
+    }
+}
+
 impl Key for (DefId, LocalDefId, Ident) {
     type CacheSelector = DefaultCacheSelector;