fn rot13 (string: String) -> String {
    let mut bytes: Vec<u8> = string.into();
    for byte in &mut bytes {
        match *byte {
            b'a'...b'm' | b'A'...b'M' => *byte += 13,
            b'n'...b'z' | b'N'...b'Z' => *byte -= 13,
            _ => (), // do nothing
        }
    }
    String::from_utf8(bytes).unwrap()
}

fn main () {
    let a =  rot13("abc".to_owned());
    assert_eq!(a, "nop");
}