> One can already do this with `&T` (for the backpointer) and `Box<T>` or `&mut T` (for the forward pointer). The hard part is letting you safely mutate somethi

> One can already do this with `&T` (for the backpointer) and `Box<T>` or `&mut T` (for the forward pointer). The hard part is letting you safely mutate something once the back reference has been added, without any runtime checks and with arbitrary data in the pointer. Can one? A &mut T and &T both pointing to the same memory and existing at the same time causes undefined behavior, and such a pairing could only be constructed via a compiler bug or (wrong) unsafe code.