From d9bc86032d0aa20db522e04a550deb1034005784 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Wed, 2 Nov 2016 11:50:16 -0400
Subject: [PATCH] normalize trait-ref in context of impl

The `specializes()` function was trying to normalize the impl trait in
an empty environment. This could lead to inexplicable failures.
---
 src/librustc/traits/specialize/mod.rs | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/librustc/traits/specialize/mod.rs b/src/librustc/traits/specialize/mod.rs
index e37425901c8c..6a27558b0b77 100644
--- a/src/librustc/traits/specialize/mod.rs
+++ b/src/librustc/traits/specialize/mod.rs
@@ -148,6 +148,8 @@ pub fn find_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                              impl1_def_id: DefId,
                              impl2_def_id: DefId) -> bool {
+    debug!("specializes({:?}, {:?})", impl1_def_id, impl2_def_id);
+
     if let Some(r) = tcx.specializes_cache.borrow().check(impl1_def_id, impl2_def_id) {
         return r;
     }
@@ -177,21 +179,22 @@ pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 
     // create a parameter environment corresponding to a (skolemized) instantiation of impl1
-    let mut penv = tcx.construct_parameter_environment(DUMMY_SP,
-                                                       impl1_def_id,
-                                                       region::DUMMY_CODE_EXTENT);
+    let penv = tcx.construct_parameter_environment(DUMMY_SP,
+                                                   impl1_def_id,
+                                                   region::DUMMY_CODE_EXTENT);
     let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id)
                              .unwrap()
                              .subst(tcx, &penv.free_substs);
 
-    let result = tcx.infer_ctxt(None, None, Reveal::ExactMatch).enter(|mut infcx| {
+    // Create a infcx, taking the predicates of impl1 as assumptions:
+    let result = tcx.infer_ctxt(None, Some(penv), Reveal::ExactMatch).enter(|mut infcx| {
         // Normalize the trait reference, adding any obligations
         // that arise into the impl1 assumptions.
         let Normalized { value: impl1_trait_ref, obligations: normalization_obligations } = {
             let selcx = &mut SelectionContext::new(&infcx);
             traits::normalize(selcx, ObligationCause::dummy(), &impl1_trait_ref)
         };
-        penv.caller_bounds.extend(normalization_obligations.into_iter().map(|o| {
+        infcx.parameter_environment.caller_bounds.extend(normalization_obligations.into_iter().map(|o| {
             match tcx.lift_to_global(&o.predicate) {
                 Some(predicate) => predicate,
                 None => {
@@ -200,9 +203,6 @@ pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             }
         }));
 
-        // Install the parameter environment, taking the predicates of impl1 as assumptions:
-        infcx.parameter_environment = penv;
-
         // Attempt to prove that impl2 applies, given all of the above.
         fulfill_implication(&infcx, impl1_trait_ref, impl2_def_id).is_ok()
     });