Learning Rust traits ... poorly

monster.rs

fn main() {
    let mut mon1: ~Sleestak = ~Monster::new();
    let mut mon2: ~Reaver   = ~Monster::new();
    let mut monsters = [mon1 as ~Monster, mon2 as ~Monster];

    println!("{:?}", monsters);
    println("------------------------------");

    // attempt 1: fail
    for m in monsters.iter() {
        m.attack(22u);  // ERROR: cannot borrow immutable dereference of ~ pointer as mutable
        println!("{:?}", m);
    }

    // attempt 2: fail
    for m in monsters.iter() {
        let mut mm = m;
        mm.attack(22u);  // ERROR: cannot borrow immutable dereference of ~ pointer as mutable
        println!("{:?}", mm);
    }

    // attempt 3: fail
    for m in monsters.iter() {
        let mut mmm = &mut m;  // error: cannot borrow immutable local variable as mutable
        mmm.attack(22u);
        println!("{:?}", mmm);
    }
    
}

trait Monster {
    fn new() -> Self;
    fn attack(&mut self, strength: uint);
}

struct Reaver {
    attack_strength: uint,
    health: uint
}

struct Sleestak {
    attack_strength: uint,
    health: uint
}


impl Monster for Reaver {
    fn new() -> Reaver {
        Reaver{attack_strength: 50, health: 100}
    }
    
    fn attack(&mut self, strength: uint) {
        if strength > self.health {
            self.health = 0;
        } else {
            self.health = self.health - strength;
        }
    }
}

impl Monster for Sleestak {
    fn new() -> Sleestak {
        Sleestak{attack_strength: 22, health: 100}
    }

    fn attack(&mut self, strength: uint) {
        if strength > self.health {
            self.health = 0;
        } else {
            self.health = self.health - strength;
        }        
    }
}