mut qualifier cannot be applied to closure arguments

[nikomatsakis]

The following example should parse and work correctly:

fn swap(f: &fn(~[int]) -> ~[int]) -> ~[int] {
    let x = ~[1, 2, 3];
    f(x)
}

fn main() {
    let v = swap(|mut x| { x.push(4); x });
    let w = do swap |mut x| { x.push(4); x };
    assert_eq!(v, w);
}

It does not parse now (mut in identifier position)

[nikomatsakis]

Nominating for a milestone, though I'm not really clear on which one. Well-defined? Feature complete? It's a backwards compatible change to the grammar, anyhow.

[graydon]

accepted for feature complete

[catamorphism]

This works now. Adding a test.