Allow Downcast projections if const_if_match enabled

These are generated when matching on enum variants to extract the value
within. We should have no problem evaluating these, but care should be
taken that we aren't accidentally allowing some other operation.

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/ops.rs

@@ -52,7 +52,11 @@ pub trait NonConstOp: std::fmt::Debug {
 /// A `Downcast` projection.
 #[derive(Debug)]
 pub struct Downcast;
-impl NonConstOp for Downcast {}
+impl NonConstOp for Downcast {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_if_match)
+    }
+}
 
 /// A function call where the callee is a pointer.
 #[derive(Debug)]