reference - Cannot move out of borrowed content when trying to transfer ownership

Question

I'm writing a linked list to wrap my head around Rust lifetimes, ownership and references. I have the following code:

pub struct LinkedList {
    head: Option<Box<LinkedListNode>>,
}

pub struct LinkedListNode {
    next: Option<Box<LinkedListNode>>,
}

impl LinkedList {
    pub fn new() -> LinkedList {
        LinkedList { head: None }
    }

    pub fn prepend_value(&mut self) {
        let mut new_node = LinkedListNode { next: None };

        match self.head {
            Some(ref head) => new_node.next = Some(*head),
            None => new_node.next = None,
        };

        self.head = Some(Box::new(new_node));
    }
}

fn main() {}

But I am getting the following compilation error:

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:18:52
   |
18 |             Some(ref head) => new_node.next = Some(*head),
   |                                                    ^^^^^ cannot move out of borrowed content

Newer versions of Rust have a slightly different error:

error[E0507]: cannot move out of `*head` which is behind a shared reference
  --> src/main.rs:18:52
   |
18 |             Some(ref head) => new_node.next = Some(*head),
   |                                                    ^^^^^ move occurs because `*head` has type `std::boxed::Box<LinkedListNode>`, which does not implement the `Copy` trait

I'm thinking that the head node must currently be owned by self, which is the linked list. When I assign it to new_node.next, there is probably a change of ownership that will happen.

I would rather not clone the value if possible as that seems wasteful. I don't want to just "borrow" it for the duration of the function. I really want to transfer its ownership.

How do I do that?

I have already looked at cannot move out of borrowed content when unwrapping a member variable in a &mut self method and Cannot move out of borrowed content / cannot move out of behind a shared reference.

I tried removing the match arm as suggested in the accepted answer in one of those questions and defining next in the creation of the new LinkedListNode, but I get the same error message.

I have successfully added an append method which takes a LinkedListNode to add to the end of the list.

Answer 1

Cannot move out of borrowed content when trying to transfer ownership

At a high-level, this is against-the-grain for Rust. You cannot transfer ownership of something borrowed because you don't own it. You shouldn't borrow my car (&Car) and then give it to the first person you see on the street! This is still true even if I lend you my car and allow you to make changes to it (&mut Car).

You cannot move head out of a &self at all because you cannot mutate the value.

You cannot move head out of a &mut self because this would leave the LinkedList struct in an inconsistent state - one of the fields would have an undefined value. This is a core measure of Rust's safety guarantees.

In general, you will need to follow something from How can I swap in a new value for a field in a mutable reference to a structure? to replace the existing value.

In this case, you can use Option::take. This will leave the variable where it is, changing it in-place to a None and returning the previous value. You can then use that value to build the new head of the list:

pub fn prepend_value(&mut self) {
    let head = self.head.take();
    self.head = Some(Box::new(LinkedListNode { next: head }));
}

A more generic solution is to take ownership of the struct instead of borrowing it. This allows you to do whatever you want to it. Note that we take self by-value, not by-reference:

pub fn prepend_value(mut self) -> LinkedList {
    self.head = Some(Box::new(LinkedListNode { next: self.head }));
    self
}