#![allow(dead_code)]

use std::ops::Drop;
use std::rc::Rc;

enum Value {
    Number(i64),
    Cons(Cons),
    Null,
}

impl Value {
    fn take(&mut self) -> Value {
        std::mem::replace(self, Value::Null)
    }
}

struct Cons(Rc<ConsInternals>);

struct ConsInternals {
    left: Value,
    right: Value,
}

// Putting `Drop` on the internal representation so we don't
// have to reach through an `Rc`.
impl Drop for ConsInternals {
    fn drop(&mut self) {
        let mut list = self.right.take();
        while let Value::Cons(head) = list {
            if let Ok(mut head) = Rc::try_unwrap(head.0) {
                list = head.right.take();
            } else {
                break
            }
        }
    }
}

fn main() {}