Implement TypeFoldable for InferConst

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

src/librustc/ty/structural_impls.rs

@@ -6,7 +6,7 @@
 use crate::hir::def::Namespace;
 use crate::mir::ProjectionKind;
 use crate::mir::interpret::ConstValue;
-use crate::ty::{self, Lift, Ty, TyCtxt, ConstVid};
+use crate::ty::{self, Lift, Ty, TyCtxt, ConstVid, InferConst};
 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::print::{FmtPrinter, Printer};
 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
@@ -1352,8 +1352,7 @@ impl<'tcx> TypeFoldable<'tcx> for ConstValue<'tcx> {
     fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
         match *self {
             ConstValue::ByRef(ptr, alloc) => ConstValue::ByRef(ptr, alloc),
-            // FIXME(const_generics): implement TypeFoldable for InferConst
-            ConstValue::Infer(ic) => ConstValue::Infer(ic),
+            ConstValue::Infer(ic) => ConstValue::Infer(ic.fold_with(folder)),
             ConstValue::Param(p) => ConstValue::Param(p.fold_with(folder)),
             ConstValue::Placeholder(p) => ConstValue::Placeholder(p),
             ConstValue::Scalar(a) => ConstValue::Scalar(a),
@@ -1366,8 +1365,7 @@ impl<'tcx> TypeFoldable<'tcx> for ConstValue<'tcx> {
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
         match *self {
             ConstValue::ByRef(..) => false,
-            // FIXME(const_generics): implement TypeFoldable for InferConst
-            ConstValue::Infer(_) => false,
+            ConstValue::Infer(ic) => ic.visit_with(visitor),
             ConstValue::Param(p) => p.visit_with(visitor),
             ConstValue::Placeholder(_) => false,
             ConstValue::Scalar(_) => false,
@@ -1376,3 +1374,13 @@ impl<'tcx> TypeFoldable<'tcx> for ConstValue<'tcx> {
         }
     }
 }
+
+impl<'tcx> TypeFoldable<'tcx> for InferConst<'tcx> {
+    fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, _folder: &mut F) -> Self {
+        *self
+    }
+
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, _visitor: &mut V) -> bool {
+        false
+    }
+}