Allow Downcast projections unconditionally

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/ops.rs

@@ -53,15 +53,6 @@ pub trait NonConstOp: std::fmt::Debug {
     }
 }
 
-/// A `Downcast` projection.
-#[derive(Debug)]
-pub struct Downcast;
-impl NonConstOp for Downcast {
-    fn feature_gate() -> Option<Symbol> {
-        Some(sym::const_if_match)
-    }
-}
-
 /// A function call where the callee is a pointer.
 #[derive(Debug)]
 pub struct FnCallIndirect;

src/librustc_mir/transform/check_consts/validation.rs

@@ -448,6 +448,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
             }
 
             ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(_) => {
@@ -460,10 +461,6 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                     _ => {}
                 }
             }
-
-            ProjectionElem::Downcast(..) => {
-                self.check_op(ops::Downcast);
-            }
         }
     }
 

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -286,11 +286,6 @@ fn check_place(
     while let &[ref proj_base @ .., elem] = cursor {
         cursor = proj_base;
         match elem {
-            ProjectionElem::Downcast(..) if !feature_allowed(tcx, def_id, sym::const_if_match) => {
-                return Err((span, "`match` or `if let` in `const fn` is unstable".into()));
-            }
-            ProjectionElem::Downcast(_symbol, _variant_index) => {}
-
             ProjectionElem::Field(..) => {
                 let base_ty = Place::ty_from(place.local, &proj_base, body, tcx).ty;
                 if let Some(def) = base_ty.ty_adt_def() {
@@ -303,6 +298,7 @@ fn check_place(
                 }
             }
             ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Deref
             | ProjectionElem::Index(_) => {}