A Stack with Box

pub struct List<T> {
    head: Option<Box<Node<T>>>,
}

struct Node<T> {
    elem: T,
    next: Option<Box<Node<T>>>,
}

Box<T> is a pointer to the heap that owns its contents. It’s what makes a recursive type finite: Node is one T plus one pointer, regardless of how long the list is.

Option provides the “no next node” case at zero extra cost — the compiler represents Option<Box<T>> as a nullable pointer, so the enum tag is free.

Push and pop both need Option::take:

pub fn push(&mut self, elem: T) {
    let node = Box::new(Node { elem, next: self.head.take() });
    self.head = Some(node);
}

take() swaps the value out and leaves None behind. You need it because you can’t move self.head out of a &mut self — that would leave the struct in a half-initialized state, which Rust forbids.