only instantiate opaques with rigid types

Author: lcnr

compiler/rustc_trait_selection/src/solve/project_goals/opaques.rs

@@ -53,6 +53,20 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                         return self.flounder(&matches);
                     }
                 }
+
+                let expected = match self.try_normalize_ty(goal.param_env, expected) {
+                    Some(ty) => if ty.is_ty_var() {
+                        return self.evaluate_added_goals_and_make_canonical_response(
+                            Certainty::AMBIGUOUS,
+                        )
+                    } else {
+                        ty
+                    },
+                    None => return self.evaluate_added_goals_and_make_canonical_response(
+                        Certainty::OVERFLOW,
+                    ),
+                };
+
                 // Otherwise, define a new opaque type
                 self.insert_hidden_type(opaque_type_key, goal.param_env, expected)?;
                 self.add_item_bounds_for_hidden_type(