io::write_err!

macro_rules! write_err {
    ($text: expr) => ({
        let mut result: bool = false;

        for len in 0i32..std::i32::MAX {
            if unsafe {
                *$text.offset(len as isize)
            } == 0u8 {
                result = write_err!($text, len);
                break ;
            }
        }
        result
    });
    ($text: expr, $len: expr) => ({
        write!($text, $len, 2)
    });
}

The write_error macro writes to output the error and returns the Some 0i32 or None according to success.