bad.rs

fn tupleize(num: ~int) -> (~int,) {
    (num,)
}

fn main() {
    let mut num = ~1;

    loop {
        (num,) = tupleize(num);
    }
}

good.rs

fn identity(num: ~int) -> ~int {
    num
}

fn main() {
    let mut num = ~1;

    loop {
        num = identity(num);
    }
}

output

$ rustc bad.rs

loop.rs:9:26: 9:29 error: use of moved value: `num`
loop.rs:9         (num,) = tupleize(num);
                                    ^~~
loop.rs:9:26: 9:29 note: `num` moved here because it has type `~int`, which is moved by default (use `copy` to override)
loop.rs:9         (num,) = tupleize(num);
                                    ^~~
loop.rs:9:9: 9:12 error: use of moved value: `num`
loop.rs:9         (num,) = tupleize(num);
                   ^~~
loop.rs:9:26: 9:29 note: `num` moved here because it has type `~int`, which is moved by default (use `copy` to override)
loop.rs:9         (num,) = tupleize(num);
                                    ^~~

$ rustc good.rs
(nothing)