lvalues can update themselves in Rust? (there is probably a nicer way to write this)

flags.rs

enum Flag {
    X0 = 1,
    X1 = 2,
    X2 = 4,
    X3 = 8,
    X4 = 16,
}

type Flagset = uint;

trait FlagsetMethods {
    fn add(&mut self, Flag);
    fn has(Flag) -> bool;
}
impl Flagset: FlagsetMethods {
    fn add(&mut self, f: Flag) {
        *self |= (f as uint);
    }

    fn has(f: Flag) -> bool {
        return self & (f as uint) != 0;
    }
}

fn main() {
    let mut flags: Flagset = 0;
    (&mut flags).add(X0);
    (&mut flags).add(X2);

    io::println(fmt!("%?", flags.has(X0)));
    io::println(fmt!("%?", flags.has(X1)));
    io::println(fmt!("%?", flags.has(X2)));
    io::println(fmt!("%?", flags.has(X3)));
    io::println(fmt!("%?", flags.has(X4)));
}