Implement `ToTrace` for `ty::Const`

src/librustc/infer/at.rs

@@ -27,6 +27,7 @@
 
 use super::*;
 
+use crate::ty::Const;
 use crate::ty::relate::{Relate, TypeRelation};
 
 pub struct At<'a, 'gcx: 'tcx, 'tcx: 'a> {
@@ -308,6 +309,20 @@ impl<'tcx> ToTrace<'tcx> for ty::Region<'tcx> {
     }
 }
 
+impl<'tcx> ToTrace<'tcx> for &'tcx Const<'tcx> {
+    fn to_trace(cause: &ObligationCause<'tcx>,
+                a_is_expected: bool,
+                a: Self,
+                b: Self)
+                -> TypeTrace<'tcx>
+    {
+        TypeTrace {
+            cause: cause.clone(),
+            values: Consts(ExpectedFound::new(a_is_expected, a, b))
+        }
+    }
+}
+
 impl<'tcx> ToTrace<'tcx> for ty::TraitRef<'tcx> {
     fn to_trace(cause: &ObligationCause<'tcx>,
                 a_is_expected: bool,

src/librustc/infer/error_reporting/mod.rs

@@ -1260,6 +1260,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         match *values {
             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
+            infer::Consts(ref exp_found) => self.expected_found_str(exp_found),
             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
         }

src/librustc/infer/mod.rs

@@ -232,6 +232,7 @@ pub type PlaceholderMap<'tcx> = BTreeMap<ty::BoundRegion, ty::Region<'tcx>>;
 pub enum ValuePairs<'tcx> {
     Types(ExpectedFound<Ty<'tcx>>),
     Regions(ExpectedFound<ty::Region<'tcx>>),
+    Consts(ExpectedFound<&'tcx ty::Const<'tcx>>),
     TraitRefs(ExpectedFound<ty::TraitRef<'tcx>>),
     PolyTraitRefs(ExpectedFound<ty::PolyTraitRef<'tcx>>),
 }
@@ -1730,6 +1731,7 @@ EnumTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for ValuePairs<'tcx> {
         (ValuePairs::Types)(a),
         (ValuePairs::Regions)(a),
+        (ValuePairs::Consts)(a),
         (ValuePairs::TraitRefs)(a),
         (ValuePairs::PolyTraitRefs)(a),
     }