Cycles can be created in the exchange heap

[msullivan]

The following code typechecks and runs; it causes a memory leak (and thus the runtime to trip an assert).

enum cycle {
    node({mut a: ~cycle}),
    empty
}
fn main() {
    let x = ~node({mut a: ~empty});
    // Create a cycle!
    alt check *x { node(y) { y.a <- x } };
}

It works by moving a unique pointer into the object it points to. (This code also works if the <- is a regular assignment, thanks to the last-use analysis.) Trying to log the value produces a segfault.

[nikomatsakis]

borrowck detected this error. now that it's merged, issue closed.