Fix a bug in chalk lowering

Some where clauses were not subtituted for generic bound type vars.

Author: scalexm

src/librustc_traits/lowering/mod.rs

@@ -209,6 +209,10 @@ fn program_clauses_for_trait<'a, 'tcx>(
     let implemented_from_env = Clause::ForAll(ty::Binder::bind(implemented_from_env));
 
     let predicates = &tcx.predicates_defined_on(def_id).predicates;
+
+    // Warning: these where clauses are not substituted for bound vars yet,
+    // so that we don't need to adjust binders in the `FromEnv` rules below
+    // (see the FIXME).
     let where_clauses = &predicates
         .iter()
         .map(|(wc, _)| wc.lower())
@@ -258,6 +262,7 @@ fn program_clauses_for_trait<'a, 'tcx>(
     // `WellFormed(WC)`
     let wf_conditions = where_clauses
         .into_iter()
+        .map(|wc| wc.subst(tcx, bound_vars))
         .map(|wc| wc.map_bound(|goal| goal.into_well_formed_goal()));
 
     // `WellFormed(Self: Trait<P1..Pn>) :- Implemented(Self: Trait<P1..Pn>) && WellFormed(WC)`
@@ -341,7 +346,9 @@ pub fn program_clauses_for_type_def<'a, 'tcx>(
     // `Ty<...>`
     let ty = tcx.type_of(def_id).subst(tcx, bound_vars);
 
-    // `WC`
+    // Warning: these where clauses are not substituted for bound vars yet,
+    // so that we don't need to adjust binders in the `FromEnv` rules below
+    // (see the FIXME).
     let where_clauses = tcx.predicates_of(def_id).predicates
         .iter()
         .map(|(wc, _)| wc.lower())