Rust: How to implement linked list?

It looks like you're trying to walk your own list to find the final element, but you don't actually have a loop. Assuming you fix that, your issue with mutability can be fixed by using ref mut instead of ref.

To try it myself I used a recursive implementation of add() and this works:

fn add(&mut self, item: T) {
    match *self {
        Node(_, ref mut next) => next.add(item),
        Nil => *self = Node(item, ~Nil)
    }
}

Offhand I'm not sure how to implement this using an iterative approach, due to issues with mutable borrowing.