io::writeln!

macro_rules! writeln {
    ($text: expr) => ({
        write!($text) && write!("\n".as_ptr(), 1)
    });
    ($text: expr, $len: expr) => ({
        write!($text, $len) && write!("\n".as_ptr(), 1)
    });
    ($text: expr, $len: expr, $out: expr) => ({
        write!($text, $len, $out) && write!("\n".as_ptr(), 1)
    });
}

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