It should not be possible with normal borrows (and safe Rust) since they're statically checked to be acyclic.

Of course, leaking memory is as easy as using `Box::leak`, although that's probably never going to happen accidentally.

Or you could use the recently-stabilized `new_cyclic` method: https://doc.rust-lang.org/std/rc/struct.Rc.html#method.new_c...