Illegal mutable borrow incorrectly allowed

[samyatchmenoff]

Example:

fn main() {
    let mut x = Some(~1);
    let mut p: proc(&mut Option<~int>) = proc(_) {};
    match x {
        Some(ref y) => {
            p = proc(z: &mut Option<~int>) {
                *z = None;
                println!("{}", y);
            }
        }
        None => {}
    }
    p(&mut x);
}

The last statement p(&mut x) is mutably borrowing x but x is already borrowed (or rather should be) by the proc.

The result is a program that compiles fine but segfaults when run.

[alexcrichton]

cc @nikomatsakis

Nominating, closure shouldn't be able to capture borrowed variables.

[nikomatsakis]

I'm pretty sure the proc "code" needs a thorough review, giving this issue along with all the other similar proc bugs.

[flaper87]

cc @flaper87

[brson]

P-backcompat-lang, 1.0.