Lambdas sometimes leak

[msullivan]

fn main() {
    let x = @0;
    lambda() { log_err *x; };
}

leaks memory.

Interestingly,

fn main() {
    let x = @0;
    let f = lambda() { log_err *x; };
}

does not.

[msullivan]

fn force(f: fn()) { f() }
fn main() {
    let x = @0;
    force(lambda() { log_err *x; });    
}

also doesn't leak.
But

fn force(f: &fn()) { f() }
fn main() {
    let x = @0;
    force(lambda() { log_err *x; });    
}

does.