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

Question

I'd like to have struct members that know their parent. This is approximately what I'm trying to do:

struct Parent<'me> {     children: Vec<Child<'me>>, }  struct Child<'me> {     parent: &'me Parent<'me>,     i: i32, }  fn main() {     let mut p = Parent { children: vec![] };     let c1 = Child { parent: &p, i: 1 };     p.children.push(c1); } 

I tried to appease the compiler with lifetimes without completely understanding what I was doing.

Here's the error message I'm stuck on:

error[E0502]: cannot borrow `p.children` as mutable because `p` is also borrowed as immutable   --> src/main.rs:13:5    | 12 |     let c1 = Child { parent: &p, i: 1 };    |                               - immutable borrow occurs here 13 |     p.children.push(c1);    |     ^^^^^^^^^^ mutable borrow occurs here 14 | }    | - immutable borrow ends here 

That makes some sense, but I'm not at all sure where to go from here.

Answer 1

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).