Why can't we use RefCell for recursive data structures?

Question

Why can't we use RefCell for recursive data structures in Rust?

Invalid:

enum List {
    Cons(i32, RefCell<List>),
    Nil,
}

Valid:

enum List {
    Cons(i32, Rc<List>), // or Box<List>
    Nil,
}

Is this similar enough to [Why are recursive struct types illegal in Rust?](https://stackoverflow.com/questions/25296195/why-are-recursive-struct-types-illegal-in-rust) that it can be closed as a duplicate? — trent, Jan 03 '19 at 17:12
This question is different. I understand why recursive types are invalid and that we can solve that using indirection. But I assume all smart pointers provide indirection since, after all, they are pointers. So my question is is why doesn't RefCell provide indirection? — sidoshi, Jan 04 '19 at 04:27

score 4 · Accepted Answer · edited Jan 06 '19 at 02:38

RefCell contains the object inside, wrapping it, it's not a heap allocated value. That's why the compiler says "recursive without indirection": RefCell is not an indirection.

Box and Rc, on the other hand, contain a reference to an object allocated somewhere else, and therefore are indirections.

Without an indirection, the List enum ends up being infinite since every Cons contains a full List enum inside its RefCell.

You can not do recursive data structures that contain themselves as values, only as pointers or references.

Why can't we use RefCell for recursive data structures?

1 Answers1