sauron_the_borrow_checker.rs

extern crate core;
use core::cell::Cell;

#[deriving(Copy, Show)]
struct Pos(u32, u32);

#[deriving(Show)]
struct Monster {
    name: String,
    pos: Cell<Pos>
}

impl Monster {
    pub fn goto(&self, pos: Pos) {
        self.pos.set(pos);
    }
}

struct Game {
    monsters: Vec<Monster>
}

impl Game {
    pub fn new() -> Game {
        Game { monsters: vec![] }
    }

    pub fn add_monster(&mut self, monster: Monster) {
        self.monsters.push(monster)
    }

    #[allow(unused_variables)]
    fn can_goto(&self, pos: Pos) -> bool {
        true
    }

    pub fn update(&self) {
        for monster in self.monsters.iter() {
            let Pos(x, y) = monster.pos.get();
            let dst = Pos(x + 1, y + 1);
            if self.can_goto(dst) {
                monster.goto(dst)
            }
        }
    }

    // pub fn update(&mut self) {
    //     let game : &Game = unsafe { ::std::mem::transmute_copy(&self) };
    //     for monster in self.monsters.iter_mut() {
    //         let (x, y) : (uint, uint) = (monster.x + 1, monster.y + 1);
    //         if game.can_goto(x, y) {
    //             monster.goto(x, y)
    //         }
    //     }
    // }
}

fn main() {
    let mut game = Game::new();
    game.add_monster(Monster { name: "Sauron".to_string(),  pos : Cell::new(Pos(0,1))});
    game.add_monster(Monster { name: "Morgoth".to_string(), pos: Cell::new(Pos(40, 40)) } );
    game.update();
}

/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:36:16: 36:20 error: cannot borrow *self as immutable because self.monsters is also borrowed as mutable
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:36 if self.can_goto(x, y) {
^~~~
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:34:24: 34:37 note: previous borrow of self.monsters occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of self.monsters until the borrow ends
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:34 for monster in self.monsters.iter_mut() {
^~~~~~~~~~~~~
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:40:6: 40:6 note: previous borrow ends here
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:34 for monster in self.monsters.iter_mut() {
...
/Users/pzol/Dropbox/src/rust/rogue/test/bc1.rs:40 }
^