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

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
}