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//! `CactusRef` can be used to implement collections that own strong references
//! to themselves.
//!
//! # Doubly-linked List
//!
//! The following implements a doubly-linked list that is fully deallocated once
//! the `list` binding is dropped.
//!
//! ```rust
//! use std::cell::RefCell;
//! use std::iter;
//!
//! use cactusref::{Adopt, Rc};
//!
//! struct Node<T> {
//! pub prev: Option<Rc<RefCell<Self>>>,
//! pub next: Option<Rc<RefCell<Self>>>,
//! pub data: T,
//! }
//!
//! struct List<T> {
//! pub head: Option<Rc<RefCell<Node<T>>>>,
//! }
//!
//! impl<T> List<T> {
//! fn pop(&mut self) -> Option<Rc<RefCell<Node<T>>>> {
//! let head = self.head.take()?;
//! let tail = head.borrow_mut().prev.take();
//! let next = head.borrow_mut().next.take();
//! if let Some(ref tail) = tail {
//! Rc::unadopt(&head, tail);
//! Rc::unadopt(tail, &head);
//!
//! tail.borrow_mut().next.clone_from(&next);
//! if let Some(ref next) = next {
//! unsafe {
//! Rc::adopt_unchecked(tail, next);
//! }
//! }
//! }
//! if let Some(ref next) = next {
//! Rc::unadopt(&head, next);
//! Rc::unadopt(next, &head);
//!
//! next.borrow_mut().prev.clone_from(&tail);
//! if let Some(ref tail) = tail {
//! unsafe {
//! Rc::adopt_unchecked(next, tail);
//! }
//! }
//! }
//! self.head = next;
//! Some(head)
//! }
//! }
//!
//! impl<T> From<Vec<T>> for List<T> {
//! fn from(list: Vec<T>) -> Self {
//! let nodes = list
//! .into_iter()
//! .map(|data| {
//! Rc::new(RefCell::new(Node {
//! prev: None,
//! next: None,
//! data,
//! }))
//! })
//! .collect::<Vec<_>>();
//! for i in 0..nodes.len() - 1 {
//! let curr = &nodes[i];
//! let next = &nodes[i + 1];
//! curr.borrow_mut().next = Some(Rc::clone(next));
//! next.borrow_mut().prev = Some(Rc::clone(curr));
//! unsafe {
//! Rc::adopt_unchecked(curr, next);
//! Rc::adopt_unchecked(next, curr);
//! }
//! }
//! let tail = &nodes[nodes.len() - 1];
//! let head = &nodes[0];
//! tail.borrow_mut().next = Some(Rc::clone(head));
//! head.borrow_mut().prev = Some(Rc::clone(tail));
//! unsafe {
//! Rc::adopt_unchecked(tail, head);
//! Rc::adopt_unchecked(head, tail);
//! }
//!
//! let head = Rc::clone(head);
//! Self { head: Some(head) }
//! }
//! }
//!
//! let list = iter::repeat(())
//! .map(|_| "a".repeat(1024 * 1024))
//! .take(10)
//! .collect::<Vec<_>>();
//! let mut list = List::from(list);
//!
//! let head = list.pop().unwrap();
//! assert_eq!(Rc::strong_count(&head), 1);
//! assert!(head.borrow().data.starts_with('a'));
//!
//! // The new head of the list is owned three times:
//! //
//! // - itself.
//! // - the `prev` pointer to it from it's next element.
//! // - the `next` pointer from the list's tail.
//! assert_eq!(list.head.as_ref().map(Rc::strong_count), Some(3));
//!
//! // The popped head is no longer part of the graph and can be safely dropped
//! // and deallocated.
//! let weak = Rc::downgrade(&head);
//! drop(head);
//! assert!(weak.upgrade().is_none());
//!
//! drop(list);
//! // all memory consumed by the list nodes is reclaimed.
//! ```