Make the MaybeUninit array consist of bytes

Author: 1c3t3a

compiler/rustc_mir_transform/src/check_enums.rs

@@ -263,7 +263,7 @@ fn insert_discr_cast_to_u128<'tcx>(
     } else if discriminant_size.bytes() < op_size.bytes() {
         // The discriminant is less wide than the operand, cast the operand into
         // [MaybeUninit; N] and then index into it.
-        let mu = Ty::new_maybe_uninit(tcx, discriminant_ty);
+        let mu = Ty::new_maybe_uninit(tcx, tcx.types.u8);
         let array_len = op_size.bytes();
         let mu_array_ty = Ty::new_array(tcx, mu, array_len);
         let mu_array =