From 2b2c4afc295aca527a3edce266d8d8d50232fbc9 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Fri, 27 Oct 2023 13:33:59 +0200
Subject: [PATCH] only instantiate opaques with rigid types

---
 compiler/rustc_borrowck/src/type_check/mod.rs      | 12 ++++++------
 .../src/solve/project_goals/opaques.rs             | 14 ++++++++++++++
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 9eb02be2f155b..0e02ababdcbbf 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -188,11 +188,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
         &mut borrowck_context,
     );
 
-    // FIXME(-Ztrait-solver=next): A bit dubious that we're only registering
-    // predefined opaques in the typeck root.
-    if infcx.next_trait_solver() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
-        checker.register_predefined_opaques_in_new_solver();
-    }
+    checker.check_user_type_annotations();
 
     let mut verifier = TypeVerifier::new(&mut checker, promoted);
     verifier.visit_body(&body);
@@ -1021,7 +1017,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             borrowck_context,
             reported_errors: Default::default(),
         };
-        checker.check_user_type_annotations();
+        // FIXME(-Ztrait-solver=next): A bit dubious that we're only registering
+        // predefined opaques in the typeck root.
+        if infcx.next_trait_solver() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
+            checker.register_predefined_opaques_in_new_solver();
+        }
         checker
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/project_goals/opaques.rs b/compiler/rustc_trait_selection/src/solve/project_goals/opaques.rs
index ebd129f32b919..26dd63b70ff78 100644
--- a/compiler/rustc_trait_selection/src/solve/project_goals/opaques.rs
+++ b/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(