Drop

Like the mutable lists, we have a recursive destructor problem. Admittedly, this isn't as bad of a problem for the immutable list: if we ever hit another node that's the head of another list somewhere, we won't recursively drop it. However it's still a thing we should care about, and how to deal with isn't as clear. Here's how we solved it before:

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut cur_link = self.head.take();
        while let Some(mut boxed_node) = cur_link {
            cur_link = boxed_node.next.take();
        }
    }
}

The problem is the body of the loop:

cur_link = boxed_node.next.take();

This is mutating the Node inside the Box, but we can't do that with Rc; it only gives us shared access, because any number of other Rc's could be pointing at it.

But if we know that we're the last list that knows about this node, it would actually be fine to move the Node out of the Rc. Then we could also know when to stop: whenever we can't hoist out the Node.

And look at that, Rc has a method that does exactly this: try_unwrap:

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut head = self.head.take();
        while let Some(node) = head {
            if let Ok(mut node) = Rc::try_unwrap(node) {
                head = node.next.take();
            } else {
                break;
            }
        }
    }
}

cargo test
   Compiling lists v0.1.0 (/Users/ADesires/dev/too-many-lists/lists)
    Finished dev [unoptimized + debuginfo] target(s) in 1.10s
     Running /Users/ADesires/dev/too-many-lists/lists/target/debug/deps/lists-86544f1d97438f1f

running 8 tests
test first::test::basics ... ok
test second::test::basics ... ok
test second::test::into_iter ... ok
test second::test::iter ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::basics ... ok
test third::test::iter ... ok

test result: ok. 8 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Great! Nice.