The nodes in LinkedList look like this:

struct Node<T> {
    next: Option<Box<Node<T>>>,
    prev: Option<Shared<Node<T>>>,
    value: T,
}

Box uses Unique, which requires that it "the referent of the pointer should not be modified without a unique path to the Unique reference". This effectively means that Unique doesn't allow aliases, but this rule violated by the prev pointer which point to the same node as a Unique.

While Rust currently doesn't give noalias semantics to Unique, this is planned in the future and will silently break LinkedList.