Borrowck doesn't consider calls to fn~ to be borrows

[nikomatsakis]

This program should fail:

fn call(x: @{mut f: fn~()}) {
    x.f(); //~ ERROR foo
    //~^ NOTE bar
}

fn main() {}

the reason is that it would be possible for the closure to cause itself to be freed. well, of course it's not actually possible in this case but it'd require alias analysis to know that.