Add ConstError

Co-Authored-By: Gabriel Smith <yodaldevoid@users.noreply.github.com>

src/librustc/ty/error.rs

@@ -44,6 +44,14 @@ pub enum TypeError<'tcx> {
     ProjectionMismatched(ExpectedFound<DefId>),
     ProjectionBoundsLength(ExpectedFound<usize>),
     ExistentialMismatch(ExpectedFound<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>),
+
+    ConstError(ConstError<'tcx>),
+}
+
+// Data structure used in const unification
+#[derive(Clone, Debug)]
+pub enum ConstError<'tcx> {
+    Mismatch(ExpectedFound<&'tcx ty::LazyConst<'tcx>>),
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Debug, Copy)]
@@ -163,6 +171,21 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
                 report_maybe_different(f, &format!("trait `{}`", values.expected),
                                        &format!("trait `{}`", values.found))
             }
+            ConstError(ref err) => {
+                write!(f, "{}", err)
+            }
+        }
+    }
+}
+
+impl<'tcx> fmt::Display for ConstError<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use self::ConstError::*;
+
+        match *self {
+            Mismatch(ref values) => {
+                write!(f, "expected `{:?}`, found `{:?}`", values.expected, values.found)
+            }
         }
     }
 }

src/librustc/ty/structural_impls.rs

@@ -737,11 +737,23 @@ impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
             ProjectionMismatched(x) => ProjectionMismatched(x),
             ProjectionBoundsLength(x) => ProjectionBoundsLength(x),
             Sorts(ref x) => return tcx.lift(x).map(Sorts),
-            ExistentialMismatch(ref x) => return tcx.lift(x).map(ExistentialMismatch)
+            ExistentialMismatch(ref x) => return tcx.lift(x).map(ExistentialMismatch),
+            ConstError(ref x) => return tcx.lift(x).map(ConstError),
         })
     }
 }
 
+impl<'a, 'tcx> Lift<'tcx> for ty::error::ConstError<'a> {
+    type Lifted = ty::error::ConstError<'tcx>;
+    fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
+        use ty::error::ConstError::*;
+
+        match *self {
+            Mismatch(ref x) => return tcx.lift(x).map(Mismatch),
+        }
+    }
+}
+
 impl<'a, 'tcx> Lift<'tcx> for ty::InstanceDef<'a> {
     type Lifted = ty::InstanceDef<'tcx>;
     fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
@@ -1320,6 +1332,13 @@ EnumTypeFoldableImpl! {
         (ty::error::TypeError::ProjectionBoundsLength)(x),
         (ty::error::TypeError::Sorts)(x),
         (ty::error::TypeError::ExistentialMismatch)(x),
+        (ty::error::TypeError::ConstError)(x),
+    }
+}
+
+EnumTypeFoldableImpl! {
+    impl<'tcx> TypeFoldable<'tcx> for ty::error::ConstError<'tcx> {
+        (ty::error::ConstError::Mismatch)(x),
     }
 }