message: Introduce `Rvalue::ShallowInitBox`
diff: @@ -701,6 +701,13 @@ fn is_fat_ptr<'tcx>(fx: &FunctionCx<'_, '_, 'tcx>, ty: Ty<'tcx>) -> bool {
  let len = codegen_array_len(fx, place);
  lval.write_cvalue(fx, CValue::by_val(len, usize_layout));
  }
  + Rvalue::ShallowInitBox(ref operand, content_ty) => {
  + let content_ty = fx.monomorphize(content_ty);
  + let box_layout = fx.layout_of(fx.tcx.mk_box(content_ty));
  + let operand = codegen_operand(fx, operand);
  + let operand = operand.load_scalar(fx);
  + lval.write_cvalue(fx, CValue::by_val(operand, box_layout));
  + }
  Rvalue::NullaryOp(NullOp::Box, content_ty) => {
  let usize_type = fx.clif_type(fx.tcx.types.usize).unwrap();
  let content_ty = fx.monomorphize(content_ty);
  