Introduce Rvalue::ShallowInitBox

compiler/rustc_const_eval/src/interpret/step.rs

@@ -289,6 +289,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.write_scalar(Scalar::from_machine_usize(val, self), &dest)?;
             }
 
+            ShallowInitBox(ref operand, _) => {
+                let src = self.eval_operand(operand, None)?;
+                let v = self.read_immediate(&src)?;
+                self.write_immediate(*v, &dest)?;
+            }
+
             Cast(cast_kind, ref operand, cast_ty) => {
                 let src = self.eval_operand(operand, None)?;
                 let cast_ty = self.subst_from_current_frame_and_normalize_erasing_regions(cast_ty);