Are there any differences between these two drop implementations for a singly-linked list

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I'm working through Learning Rust with Entirely Too Many Linked Lists and had a question about the drop implementation for the initial linked list from that book. The linked list is composed of a stack allocated link, which is either empty or points to a heap allocated Node with some associated data.

struct List {
    head: Link,
}

enum Link {
    Empty,
    More(Box<Node>),
}

struct Node {
    data: i32,
    next: Link,
}

The implementation of the drop trait given in the book uses std::mem::replace to take ownership of the nodes in the list.

// Copied from the book, omitting comments
impl Drop for List {
    fn drop(&mut self) {
        let mut cur_link = mem::replace(&mut self.head, Link::Empty);
        while let Link::More(mut boxed_node) = cur_link {
            cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
        }
    }
}

I'm confused why the book is using mem::replace to take ownership of the next pointer of the node. It seems like we already have owndership of boxed_node from the while let line and we can just set cur_link to node.next instead of using mem::replace.

I wrote this alternative implementation which compiles just fine. I'm wondering if there is any difference between the two drop methods.

impl Drop for List {
    fn drop(&mut self) {
        let mut link = mem::replace(&mut self.head, Link::Empty);
        while let Link::More(node) = link {
            link = node.next;
        }
    }
}
rust
1 Answers

Yes, you are absolutely right. The whole point of implementing Drop in that tutorial is to avoid a recursive call, and your implementation achieves the same thing using less code (and with less assembly at the end).

Having said that, the implementation in the tutorial is a lot more explicit, and it could be argued that it is easier to see what is going on. The mem::replace ensures that the Link is replaced with an Empty, making it clear that all of the boxes are dropped too. This might have been a conscious decision by the tutorial authors, or it could have been an oversight.

The bonus section might also give a clue as to what the author was thinking. Their idea of simplifying the Drop implementation was to add a method which is useful for simplifying other methods too, and therefore would still need to maintain the integrity of the structure. The fact that this isn't necessary in a Drop implementation may have passed them by while looking at the bigger picture, or else they may have specifically wanted that implementation so that the bonus section makes sense.

Your version is probably more efficient, but might take a second glance to understand what it is really doing. I'd still go with yours though, since it is shorter and is doing less work.

user_4937290
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