bind allows data to be moved multiple times

[nikomatsakis]

This code crashes today

// 'x' can have any mode but & or &&
fn foo(-x: ~int) { }
fn main() {
     let b = bind foo(~3);
     b();
     b();
}

The error is that bind needs to "re-copy" the ~3 each time b() is invoked, but it doesn't. In other words, that bind code is equivalent to let x = ~3 in fn () -> foo(x) (in some weird O'Caml/Rust hybrid) but it would need to be fn() -> let x = ~3 in foo(x) to be safe.

Not sure the best fix here. One thought is to ensure that bind let b = bind f(E1, ..., En) is syntactic sugar for something like:

let b = {
    let v1 = E1;
    ...
    let vN = En;
    lambda() { f(v1, ..., vN) }
}

Similarly, upvars in lambdas should have reference mode (referencing the environment, not creator stack frame) which would prevent moves.

[marijnh]

I think disallowing binding of by-move or by-copy arguments is the sanest solution to this.

[marijnh]

@nikomatsakis Feel free to reopen if you feel this solution is unsatisfactory.

[nikomatsakis]

Yes, that's fine, as long as lambdas also prevent moving out of upvars (and I just tested and found that they do). That is what I meant, not sure why I phrased it in such a complicated way. I guess I wanted to know if there were other corner cases I was not thinking of.