You can do that now. Its not quite the same but pretty close:

    fn f(n: Option<u8>) -> u8 {
       match n {
          Some(n0) => n0,
          None => 10
       }
    }
    fn main() {
       let n1 = f(Some(20));
       let n2 = f(None);
       println!("{} {}", n1, n2);
    }