io::write_number!

macro_rules! write_number {
    ($number: expr) => ({
        write_number!($number, 1)
    });
    ($number: expr, $out: expr) => ({
        let mut decimal = $number;
        let mut buf: [u8; 64] = [0; 64];
        let mut result: bool = false;

        for target in (0..64).rev() {
            buf[target] = {decimal % 10 + 48} as u8;
            decimal /= 10;
            if decimal == 0 {
                result = write! (
                    buf.as_ptr().offset (
                        target as isize
                    ),
                    64 - target,
                    $out
                );
                break ;
            }
        }
        result
    });
}

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