you'd use a cell/refcell/box or just wouldn't get this excercise as everyone who got past it in rust know it's not easy and critically it's not supposed to be easy.

it's difficult in rust because recurrent data structures are super hard to reason about in general in terms of borrowing and ownership when AOT compiling and you unfortunately must know this to be efficient at rust; this realization is one of the stages of learning the language.