Ownership Guide: Borrowing example doesn't work with previously introduced main function

[lgrahl]

In the ownership guide is an example for borrowing that doesn't work with the previously introduced main function. I guess this is because cross-borrowing is not allowed anymore?

fn main() {
    let x = box 5i;
    add_one(x);
    println!("{}", x);
}

fn add_one(num: &mut int) {
    *num += 1;
}

See: https://github.com/rust-lang/rust/blob/master/src/doc/guide-ownership.md#borrowing

[steveklabnik]

Yes. Do you have an idea of how to fix this? I'm not sure what makes the most sense, exactly.

[lgrahl]

Well, yeah... I've found a way to borrow the box but it's extremely ugly. Prepare yourself...

fn main() {
    let mut x = box 5i;
    add_one(&mut x);
    println!("{}", x);
}

fn add_one(num: &mut Box<int>) {
    **num += 1;
}

No, honestly, this can't be it. I'd rather hit myself with an axe.

There are a few RFCs out there which seem to address this problem, but I'm not sure whether they address mutable references as well. Maybe you could have a look. I'm not even sure if these RFCs are related to this problem.

[tomjakubowski]

Try this:

fn main() {
    let mut x = box 5i;
    add_one(&mut *x);
    println!("{}", x);
}

fn add_one(num: &mut int) {
    *num += 1;
}

(edit: I first submitted this post by mistake with a paste from a stale clipboard. Updated to reflect the necessary changes to the snippet)

[lgrahl]

Yeah, that works. Thanks!