traits - adding to an existing type

gistfile1.rs

trait ConvertToInt {
    fn to_int(&self) -> Option<i64>;
}

impl ConvertToInt for String {
    fn to_int(&self) -> Option<i64> {
        let buf = self.as_slice();
        let result: Option<i64> = from_str(buf);
        return result
    }
}

#[test]
fn test_convert_to_int() {
    let a = "10".to_string().to_int().unwrap();
    assert_eq!(10i64, a);

    let a = "11".to_string().to_int().unwrap();
    assert_eq!(11i64, a);
}