allow mutation of moved upvars

[nikomatsakis]

Something like this should work:

fn foo() {
    let mut x = [mut ...];
    task::spawn(fn~[move x]() {
        x += [3];
    }):
}

[brson]

Also when the move is done implicitly by last use? This seems right, but I can't recall ever running up against it.

[nikomatsakis]

I guess?

[graydon]

blocked on resolution of #6308

[nikomatsakis]

The syntax and particulars of this example do not relate,
but it would still be useful to be able to have mutable upvars.
Though if we remove copying closures it will not be relevant.

[emberian]

Visiting for triage; nothing to add

[catamorphism]

Obsolete given current plans for closures