Add a convenience function

compiler/rustc_hir_analysis/src/check/check.rs

@@ -397,14 +397,7 @@ fn check_opaque_meets_bounds<'tcx>(
 ) {
     let defining_use_anchor = match *origin {
         hir::OpaqueTyOrigin::FnReturn(did) | hir::OpaqueTyOrigin::AsyncFn(did) => did,
-        hir::OpaqueTyOrigin::TyAlias { .. } => {
+        hir::OpaqueTyOrigin::TyAlias { .. } => tcx.impl_trait_parent(def_id),
-            let mut def_id = def_id;
-            // Find the surrounding item (type alias or assoc type)
-            while let DefKind::OpaqueTy = tcx.def_kind(def_id) {
-                def_id = tcx.local_parent(def_id);
-            }
-            def_id
-        }
     };
     let param_env = tcx.param_env(defining_use_anchor);
 

compiler/rustc_middle/src/ty/mod.rs

@@ -2476,6 +2476,18 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Returns the `DefId` of the item within which the `impl Trait` is declared.
+    /// For type-alias-impl-trait this is the `type` alias.
+    /// For impl-trait-in-assoc-type this is the assoc type.
+    /// For return-position-impl-trait this is the function.
+    pub fn impl_trait_parent(self, mut def_id: LocalDefId) -> LocalDefId {
+        // Find the surrounding item (type alias or assoc type)
+        while let DefKind::OpaqueTy = self.def_kind(def_id) {
+            def_id = self.local_parent(def_id);
+        }
+        def_id
+    }
+
     pub fn impl_method_has_trait_impl_trait_tys(self, def_id: DefId) -> bool {
         if self.def_kind(def_id) != DefKind::AssocFn {
             return false;