rust/src/test/run-pass/cycle-collection4.rs

type foo = { mut z : fn@() };

fn nop() { }
fn nop_foo(_y: ~[int], _x : @foo) { }

fn main() {
    let w = @{ mut z: {||nop()} };
    let x = {||nop_foo(~[], w)};
    w.z = x;
}