Is RefCell::as_ptr guaranteed to stay the same for a given RefCell?

I am looking for a way to impl Hash for MyStruct where MyStruct contains `Rc<RefCell> (yes, it's a linked list of a sort).

This answer suggests std::ptr::hash(self.next.as_ref()), but I have doubts whether there are any guarantees reg the RefCell address.

In Ruby, I'd use object_id method inplace of as_ptr as a unique identifier to hash. If as_ptr may change during execution, is there any similar identifier for a given RefCell ?

I'm trying to solve https://leetcode.com/problems/copy-list-with-random-pointer/

• tried #[Derive(Debug)] - didn't work, as RefCell doesn't impl Hash by default, what makes sense
• came across suggestions to RefCell::borrow and avoid panic-s somehow - didn't sound reliable

No, RefCell::as_ptr is not guaranteed to be stable in the general case. An example:

let a = RefCell::new(42);
println!("address: {:?}", a.as_ptr());
let b = Some(a);
println!("address: {:?}", b.unwrap().as_ptr());

Prints something like this:

address: 0x7ffe3737d228
address: 0x7ffe3737d300

The reason is that the RefCell object was moved. See it in action on Playground.

As for an actual solution: Rust doesn't make it easy, but there's a way. In your case, the object is an Rc<RefCell<_>>. The internal pointer of an immutable Rc cannot change since every Rc instance points to the same heap address. Thus, Rc::as_ref() is stable.

I wrote some code that demonstrates using Rcs as HashMap keys by heap address, but it's not pretty: Playground.