`op2` allows `#[smi]` annotation for return type if we return "()"

[bartlomieju]

This code should not compile:

#[op2(fast)]
#[smi]
fn op_get_exit_code(state: &mut OpState) {
  state.borrow_mut::<ExitCode>().get();
}

Notice that we annotate with #[smi] for return type, but we are returning ().