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

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

@@ -605,8 +605,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     {
         debug!(?obligation, "confirm_fn_pointer_candidate");
 
-        // Okay to skip binder; it is reintroduced below.
-        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
+        let self_ty = self
+            .infcx
+            .shallow_resolve(obligation.self_ty().no_bound_vars())
+            .expect("fn pointer should not capture bound vars from predicate");
         let sig = self_ty.fn_sig(self.tcx());
         let trait_ref = closure_trait_ref_and_return_type(
             self.tcx(),
@@ -621,15 +623,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
         let cause = obligation.derived_cause(BuiltinDerivedObligation);
-        // The binder on the Fn obligation is "less" important than the one on
-        // the signature, as evidenced by how we treat it during projection.
-        // The safe thing to do here is to liberate it, though, which should
-        // have no worse effect than skipping the binder here.
-        let liberated_fn_ty =
-            self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
-        let output_ty = self
-            .infcx
-            .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
+        let output_ty = self.infcx.replace_bound_vars_with_placeholders(sig.output());
         let output_ty = normalize_with_depth_to(
             self,
             obligation.param_env,
@@ -693,16 +687,19 @@ 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
-        // that the self-type is an generator type and hence is
+        // NOTE: The self-type is a generator type and hence is
         // in fact unparameterized (or at least does not reference any
-        // regions bound in the obligation). Still probably some
-        // refactoring could make this nicer.
+        // regions bound in the obligation).
+        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
-        // that the self-type is an unboxed closure type and hence is
+        // NOTE: 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
-        // refactoring could make this nicer.
+        // regions bound in the obligation).
+        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))
 }