io::writeln_number!

macro_rules! writeln_number {
    ($number: expr) => ({
        writeln_number!($number, 1)
    });
    ($number: expr, $out: expr) => ({
        write_number!($number, $out) && write!("\n".as_ptr(), 1)
    });
}

The writeln_number macro writes to output the number with a breakline and returns the Some 0i32 or None according to success.