Only register hidden types for opaque types from the current crate, nothing else would work anyway.

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -101,7 +101,7 @@ pub fn handle_opaque_type(
         }
         let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
         let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
-            ty::Opaque(def_id, substs) => {
+            ty::Opaque(def_id, substs) if def_id.is_local() => {
                 let origin = if self.defining_use_anchor.is_some() {
                     // Check that this is `impl Trait` type is
                     // declared by `parent_def_id` -- i.e., one whose

compiler/rustc_middle/src/ty/mod.rs

@@ -1105,6 +1105,7 @@ pub fn is_empty(&self) -> bool {
     Lift
 )]
 pub struct OpaqueTypeKey<'tcx> {
+    // FIXME(oli-obk): make this a LocalDefId
     pub def_id: DefId,
     pub substs: SubstsRef<'tcx>,
 }