What lifetimes do I use to create Rust structs that reference each other cyclically?

It is not possible to create cyclic structures with borrowed pointers.

There is not any good way of achieving cyclic data structures at present; the only real solutions are:

1. Use reference counting with Rc<T> with a cyclic structure with Rc::new() and Rc:downgrade(). Read the rc module documentation and be careful to not create cyclic structures that use strong references, as these will cause memory leaks.
2. Use raw/unsafe pointers (*T).