Assert that we don't capture escaping bound vars in Fn trait selection

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -693,16 +693,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let gen_sig = substs.as_generator().poly_sig();
 
-        // (1) Feels icky to skip the binder here, but OTOH we know
+        // NOTE: The self-type is a generator type and hence is
-        // that the self-type is an generator type and hence is
         // in fact unparameterized (or at least does not reference any
-        // regions bound in the obligation). Still probably some
+        // regions bound in the obligation).
-        // refactoring could make this nicer.
+        let self_ty = obligation
+            .predicate
+            .self_ty()
+            .no_bound_vars()
+            .expect("unboxed closure type should not capture bound vars from the predicate");
 
         let trait_ref = super::util::generator_trait_ref_and_outputs(
             self.tcx(),
             obligation.predicate.def_id(),
-            obligation.predicate.skip_binder().self_ty(), // (1)
+            self_ty,
             gen_sig,
         )
         .map_bound(|(trait_ref, ..)| trait_ref);

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2271,15 +2271,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?closure_sig);
 
-        // (1) Feels icky to skip the binder here, but OTOH we know
+        // NOTE: The self-type is an unboxed closure type and hence is
-        // that the self-type is an unboxed closure type and hence is
         // in fact unparameterized (or at least does not reference any
-        // regions bound in the obligation). Still probably some
+        // regions bound in the obligation).
-        // refactoring could make this nicer.
+        let self_ty = obligation
+            .predicate
+            .self_ty()
+            .no_bound_vars()
+            .expect("unboxed closure type should not capture bound vars from the predicate");
+
         closure_trait_ref_and_return_type(
             self.tcx(),
             obligation.predicate.def_id(),
-            obligation.predicate.skip_binder().self_ty(), // (1)
+            self_ty,
             closure_sig,
             util::TupleArgumentsFlag::No,
         )

compiler/rustc_trait_selection/src/traits/util.rs

@@ -298,11 +298,11 @@ pub fn closure_trait_ref_and_return_type<'tcx>(
     sig: ty::PolyFnSig<'tcx>,
     tuple_arguments: TupleArgumentsFlag,
 ) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
+    assert!(!self_ty.has_escaping_bound_vars());
     let arguments_tuple = match tuple_arguments {
         TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
         TupleArgumentsFlag::Yes => tcx.intern_tup(sig.skip_binder().inputs()),
     };
-    debug_assert!(!self_ty.has_escaping_bound_vars());
     let trait_ref = tcx.mk_trait_ref(fn_trait_def_id, [self_ty, arguments_tuple]);
     sig.map_bound(|sig| (trait_ref, sig.output()))
 }
@@ -313,7 +313,7 @@ pub fn generator_trait_ref_and_outputs<'tcx>(
     self_ty: Ty<'tcx>,
     sig: ty::PolyGenSig<'tcx>,
 ) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)> {
-    debug_assert!(!self_ty.has_escaping_bound_vars());
+    assert!(!self_ty.has_escaping_bound_vars());
     let trait_ref = tcx.mk_trait_ref(fn_trait_def_id, [self_ty, sig.skip_binder().resume_ty]);
     sig.map_bound(|sig| (trait_ref, sig.yield_ty, sig.return_ty))
 }
@@ -324,7 +324,7 @@ pub fn future_trait_ref_and_outputs<'tcx>(
     self_ty: Ty<'tcx>,
     sig: ty::PolyGenSig<'tcx>,
 ) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
-    debug_assert!(!self_ty.has_escaping_bound_vars());
+    assert!(!self_ty.has_escaping_bound_vars());
     let trait_ref = tcx.mk_trait_ref(fn_trait_def_id, [self_ty]);
     sig.map_bound(|sig| (trait_ref, sig.return_ty))
 }