Weak

To use a Weak<T> value you must call borrow() to check if it's still in scope. This avoids circular references, below is an example where it's useful when we want two way references so you can get to a parent from the child:

use std::borrow::{Borrow, BorrowMut};
use std::cell::RefCell;
use std::rc::{Rc, Weak};

#[derive(Debug)]
struct Node {
    value: i32,
    parent: RefCell<Weak<Node>>,
    children: RefCell<Vec<Rc<Node>>>,
}
fn main() {
    let leaf = Rc::new(Node {
        value: 3,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![]),
    });

    let branch = Rc::new(Node {
        value: 5,
        parent: RefCell::new(Weak::new()),
        children: RefCell::new(vec![Rc::clone(&leaf)]),
    });

    *leaf.parent.borrow_mut() = Rc::downgrade(&branch);
    let b = leaf.parent.borrow().upgrade();

    if let Some(node) = b {
        println!("banch value: {}", node.value);
    }

	println!(
        "branch refs - strong: {} weak: {}",
        Rc::strong_count(&branch),
        Rc::weak_count(&branch)
    )
}

This gets the value of the branch from the child after checking it's still there by using upgrade()

The branch still goes out of scope after the main function ends because its strong reference count remains at 1