io::write_character!

macro_rules! write_character {
    ($character: expr) => ({
        write_character!($character, 1)
    });
    ($character: expr, $out: expr) => ({
        write!([$character].as_ptr(), 1, $out)
    });
}

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