Is a Drop for LinkedListLink feasible?

[cehteh]

Ben Kimock pointed me to a bug in my code https://github.com/cehteh/cachedb/pull/3

This clearly was caused by my misunderstanding. Still I am wondering whats the reason that LinkedListLink has no Drop implementation that checks if an Link is linked and if so removes it from the list? Do I miss some ill side effect or is it only for performance?

[Amanieu]

Due to the design of the linked list, it's not possible to do this automatically in Drop. Consider the case where this is the last element in the list: the link has no pointer back to the LinkedList object itself since the list is null-terminated (not circular) so it can't reset the head/tail pointers to null.

The reason why the linked list isn't circular is because Rust doesn't have move constructors like C++: the LinkedList must remain valid even when it is moved and its address changes. In C++ this would be handled in the move constructor which can fixup the linked in the head and tail nodes.

[cehteh]

Bummer. That explains it, thanks for the explanation.

Wondering if it would be possible to design this with List being circular and just being a

struct List{
    links: Link; // head and tail
    ...
}

Ensuring that Links are non movable (they shouldn't anyway [how?]). Then add a Link::swap(&mut self,other: &mut Self) to retain moveability. Then one can construct a moveable List (and Links with some efforts).

• Would be a breaking change.
• Needs some care that the List::links is not mistaken for a data node
• And I may've overseen something again.

[Amanieu]

Rust doesn't have a way to prevent objects from being moved. Except maybe for Pin, but that would make the API of this crate much uglier, so I'm generally against it.

[cehteh]

Yes I figured that out meanwhile. I may give it a shot and implement a tiny/ugly cyclic list for my use case.