From d2bacb18d2bf9eadca86fe7c1483e38ce774750e Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Tue, 28 Apr 2020 13:04:48 -0700
Subject: [PATCH] Ensure that `new_self_ty` has no escaping bound vars

Otherwise inserting it to the `Binder` used by `trait_ref` would cause
problems. This is just to be extra carefult: we aren't going to
start recommending that the user start using HKTs anytime soon.
---
 src/librustc_trait_selection/traits/error_reporting/mod.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/librustc_trait_selection/traits/error_reporting/mod.rs b/src/librustc_trait_selection/traits/error_reporting/mod.rs
index 20de0b786ed70..c59abd3602116 100644
--- a/src/librustc_trait_selection/traits/error_reporting/mod.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/mod.rs
@@ -1385,6 +1385,8 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
         trait_ref: &ty::PolyTraitRef<'tcx>,
         new_self_ty: Ty<'tcx>,
     ) -> PredicateObligation<'tcx> {
+        assert!(!new_self_ty.has_escaping_bound_vars());
+
         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
             ..*tr