Rust, how to copy the inner value out from Rc> and return it?

Question

Brief: I'm new to Rust so I decided to practice by implementing a double linked list. For debugging purpose, I implemented the get() method, but I failed to copy the value out from a Rc<RefCell<_>>. (Sorry for asking dumb question)

Problem: I'm trying to return a Result<T, &'static str> in .get() where the T is the type of the data stored in node and &str is the error message string. The borrow checker tells me that I cannot return a reference to an in-method variable, so I tried to copy the inner value out and return it, but failed to do so.

Source code:

use std::{rc::Rc, cell::RefCell};

struct Node<T> {
    data: Option<T>,
    prev: Option<Rc<RefCell<Node<T>>>>,
    next: Option<Rc<RefCell<Node<T>>>>,
}

impl<T> Node<T> {
    /// Instantiate a new dummy node.
    /// This node is used to mark the start and end of the list.
    /// It is not counted in the size of the list.
    fn new() -> Self {
        Node {
            data: None,
            prev: None,
            next: None,
        }
    }
    /// Instantiate a new content node.
    /// This node is used to store data.
    /// It is counted in the size of the list.
    fn from(data: T) -> Self {
        Node {
            data: Some(data),
            prev: None,
            next: None,
        }
    }
}

struct List<T> {
    head: Rc<RefCell<Node<T>>>,
    tail: Rc<RefCell<Node<T>>>,
    size: usize,
}

impl<T> List<T> {
    pub fn new() -> Self {
        let head = Rc::new(RefCell::new(Node::new()));
        let tail = Rc::new(RefCell::new(Node::new()));
        head.borrow_mut().next = Some(Rc::clone(&tail));
        tail.borrow_mut().prev = Some(Rc::clone(&head));
        List { head, tail, size: 0 }
    }
    pub fn prepend(&self, data: T) {
        let node = Rc::new(RefCell::new(Node::from(data)));
        let mut head = self.head.borrow_mut();
        
        node.borrow_mut().next = Some(head.next.take().unwrap());
        node.borrow_mut().prev = Some(Rc::clone(&self.head));
        head.next = Some(Rc::clone(&node));
        if let Some(next) = node.borrow().next.as_ref() {
            next.borrow_mut().prev = Some(Rc::clone(&node));
        };
    }
    pub fn append(&self, data: T) {
        let node = Rc::new(RefCell::new(Node::from(data)));
        let mut tail = self.tail.borrow_mut();
        
        node.borrow_mut().prev = Some(Rc::clone(&tail.prev.take().unwrap()));
        node.borrow_mut().next = Some(Rc::clone(&self.tail));
        tail.prev = Some(Rc::clone(&node));
        if let Some(prev) = node.borrow().prev.as_ref() {
            prev.borrow_mut().next = Some(Rc::clone(&node));
        };
    }
    pub fn get(&self, index: isize) -> Result<T, &'static str> {
        let mut current: Rc<RefCell<Node<T>>> = Rc::clone(self.head.borrow().next.as_ref().unwrap());
        for _ in 0..index {
            let tmp = Rc::clone(current.borrow().next.as_ref().ok_or("Index out of range")?);
            current = tmp;
        }
        let result = current.borrow().data.as_ref().ok_or("Index out of range")?;  // error[E0716]
        Ok(*result)  // error[E0507]
    }
}
/*
error[E0716]: temporary value dropped while borrowed
  --> src\linked.rs:74:22
   |
74 |         let result = current.borrow().data.as_ref().ok_or("Index out of range")?;
   |                      ^^^^^^^^^^^^^^^^                                           - temporary value is freed at the end of this statement
   |                      |
   |                      creates a temporary value which is freed while still in use
75 |         Ok(*result)
   |            ------- borrow later used here
   |
help: consider using a `let` binding to create a longer lived value
   |
74 ~         let binding = current.borrow();
75 ~         let result = binding.data.as_ref().ok_or("Index out of range")?;
   |

error[E0507]: cannot move out of `*result` which is behind a shared reference
  --> src\linked.rs:75:12
   |
75 |         Ok(*result)
   |            ^^^^^^^ move occurs because `*result` has type `T`, which does not implement the `Copy` trait
*/

I've Tried:

• This post about making a reference into value, but that's what I'm trying to do and failed.
• This post about modifying a RefCell, but this did not help. I was trying to return it, not mutate it.
• This post about how to borrow a RefCell, but I can't return the borrowed value because the borrowed Rc is short lived (but the inner value is not).
• This post about how to return something inside a RefCell and this post about how to return it with .map(), but I the compiler says "trait bound not satisfied" when I tried to use .into() and borrow checker complains "cannot move out" if I remove the .into().
• This post about using Rc::try_unwarp(), but it won't work since the inner data has more than one owner.

Also: I may did these wrong, please forgive me if one of the posts solved my problem but I did not implement it in the right way, and please teach me how to do it properly. Many thanks.

Answer 1

You don't.

If the value is Copy, you can copy it. If it is Clone, you can clone it. But if it is not, or it is too expensive to clone?

You're out of luck.

Interior mutability (like RefCell) is bad for APIs. It tend to propagate across API boundaries, and sometimes even stops you altogether from implementing them. It is good (sometimes) inside the implementation, but not as it leaks.

Of course, there is a proper way to implement a doubly linked list (since std has one), but this proper way requires unsafe code. You may want to return to it when you've already mastered Rust, as an exercise. And make sure to read Learn Rust With Entirely Too Many Linked Lists.

Answer 2

This worked for me:

    pub fn get(&self, index: isize) -> Result<T, &'static str>
        where T: Clone
    {
        let mut current: Rc<RefCell<Node<T>>> = Rc::clone(self.head.borrow().next.as_ref().unwrap());
        for _ in 0..index {
            let tmp = Rc::clone(current.borrow().next.as_ref().ok_or("Index out of range")?);
            current = tmp;
        }
        let guard = current.borrow();
        guard.data.clone().ok_or("Index out of range")
    }

(playground link)

You need the line where T: Clone to enable the .clone() method.

---

Some say it's a mistake to implement a doubly-linked list in order to learn Rust... I'm sorry to report that they are right. I did the exact same thing when I started. The other thing I tried was writing a ray-tracer; that went a lot better.

Core data structures are implemented using raw pointers. That means writing unsafe code and putting a safe API around it: an advanced Rust skill.