Rustで双方向リンクリストをどのように実装しますか？

Question

この質問は、Rust before Rust 1.0のバージョンを参照していることに注意してください。構文は変更されていますが、概念は引き続き有効です。

次のような所有ポインターを使用して、転送のみのリンクリストを簡単に実装できます。

struct Node<T> {
  next: Option<~Node<T>>,
  data: T
}

ただし、次の4つの基本操作をサポートするキューを効率的に実装する場合を想像してみてください。

• Push：リストの最後に追加
• pop：リストの最後から削除して戻る
• unshift：リストの先頭に追加
• shift：リストの最後から削除して戻る

通常のポインタを使用する言語では、双方向のリンクリストと、リストの最初と最後の要素へのfirstおよびlastポインタを格納するルートオブジェクトを使用してこれを実装できます。

Rustでこれをどのように実装するかわかりません。

私はあなたがたくさんの参照を使用するだろうと漠然と推測することができます、そしておそらく次のようなものです：

struct Node<T> {
  next: Option<&Node<T>>,
  prev: Option<&Node<T>>,
  data: T
}

...しかし、これらの変数の有効期間スコープをどのように管理するかはわかりません。

誰かが私をこれの方向に向けることができますか、またはオブジェクト間の参照を持つ複雑なライフタイムを含む同様の例ですか？

（このスタイルのコードのもう1つの典型的な例は、多くのオブジェクトがイベントの更新を単一の場所に公開する必要があるオブザーバーパターンです。例：UINode <> ---- EventObserver <>- --EventCore <> ---- UINodes;ポインターを共有する複雑な階層内の複数のオブジェクト。イベントはリーフノードからコアに伝播し、別のリーフノードにプッシュされます）

Answer 1

Lars Bergstrom によって書かれた 錆のパターン をご覧になることをお勧めします。

二重リンクリストを実装するためのコードは次のとおりです。Rust 1.12 from @Yurume、（完全にはテストされていません）

use std::mem;
use std::ptr;

pub struct List<T> {
    list_head: Option<Box<Node<T>>>,
    list_tail: Rawlink<Node<T>>,
}

struct Rawlink<T> { p: *mut T }

impl<T> Copy for Rawlink<T> {}

impl<T> Clone for Rawlink<T> {
    fn clone(&self) -> Self { Rawlink { p: self.p } }
}

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

impl<T> List<T> {
    pub fn is_empty(&self) -> bool {
        self.list_head.is_none()
    }

    pub fn len(&self) -> usize {
        let mut node = &self.list_head;
        let mut i = 0;
        loop {
            match *node {
                Some(ref n) => {
                    i+=1;
                    node=&n.next;
                }
                None => {
                    return i;
                }
            }
        }
    }

    /// Create an empty DList
    pub fn new() -> List<T> {
        List{list_head: None, list_tail: Rawlink::none()}
    }

    pub fn Push_front(&mut self, elt: T) {
        self.Push_front_node(Box::new(Node::new(elt)))
    }

    pub fn Push_front_node(&mut self, mut new_head: Box<Node<T>>) {
        match self.list_head {
            None => {
                self.list_tail = Rawlink::some(&mut new_head);
                new_head.prev = Rawlink::none();
                self.list_head = Some(new_head);
            }
            Some(ref mut head) => {
                new_head.prev = Rawlink::none();
                head.prev = Rawlink::some(&mut new_head);
                mem::swap(head, &mut new_head);
                head.next = Some(new_head);
            }
        }
    }

    /// Provide a forward iterator
    #[inline]
    pub fn iter<'a>(&'a self) -> ListIterator<'a, T> {
        ListIterator{nelem: self.len(), head: &self.list_head, tail: self.list_tail}
    }
}

impl<T> Node<T> {
    fn new(v: T) -> Node<T> {
        Node{value: v, next: None, prev: Rawlink::none()}
    }
}

/// Rawlink is a type like Option<T> but for holding a raw pointer
impl<T> Rawlink<T> {
    /// Like Option::None for Rawlink
    fn none() -> Rawlink<T> {
        Rawlink{p: ptr::null_mut()}
    }

    /// Like Option::Some for Rawlink
    fn some(n: &mut T) -> Rawlink<T> {
        Rawlink{p: n as *mut T}
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve_immut<'a>(&self) -> Option<&'a T> {
        unsafe { self.p.as_ref() }
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve<'a>(&mut self) -> Option<&'a mut T> {
        unsafe { self.p.as_mut() }
    }

    /// Return the `Rawlink` and replace with `Rawlink::none()`
    fn take(&mut self) -> Rawlink<T> {
        mem::replace(self, Rawlink::none())
    }
}

pub struct ListIterator<'a, T: 'a> {
    head: &'a Option<Box<Node<T>>>,
    tail: Rawlink<Node<T>>,
    nelem: usize,
}

impl<'a, A> Iterator for ListIterator<'a, A> {
    type Item = &'a A;

    #[inline]
    fn next(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.as_ref().map(|head| {
            self.nelem -= 1;
            self.head = &head.next;
            &head.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.nelem, Some(self.nelem))
    }
}

impl<'a, A> DoubleEndedIterator for ListIterator<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        let tmp = self.tail.resolve_immut();
        tmp.as_ref().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &prev.value
        })
    }
}

fn main() {
}

遊び場

Answer 2

Rust標準ライブラリ、collections :: dlistに、二重にリンクされたリストの実装があります。これには、徹底的なテストスイートが付属しており、読みやすく、使いやすいツールです。

ここにソースへのリンクがあります。

コメントで示唆されているように、ここにソース自体があります（簡潔にするためにテストは省略されているので、可能な場合はリンクを読んでください）：

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://Rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-Apache or
// http://www.Apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! A doubly-linked list with owned nodes.
//!
//! The `DList` allows pushing and popping elements at either end and is thus
//! efficiently usable as a double-ended queue.

// DList is constructed like a singly-linked list over the field `next`.
// including the last link being None; each Node owns its `next` field.
//
// Backlinks over DList::prev are raw pointers that form a full chain in
// the reverse direction.

#![stable(feature = "Rust1", since = "1.0.0")]

use core::prelude::*;

use alloc::boxed::Box;
use core::cmp::Ordering;
use core::default::Default;
use core::fmt;
use core::hash::{Writer, Hasher, Hash};
use core::iter::{self, FromIterator};
use core::mem;
use core::ptr;

/// A doubly-linked list.
#[stable(feature = "Rust1", since = "1.0.0")]
pub struct DList<T> {
    length: uint,
    list_head: Link<T>,
    list_tail: Rawlink<Node<T>>,
}

type Link<T> = Option<Box<Node<T>>>;

struct Rawlink<T> {
    p: *mut T,
}

impl<T> Copy for Rawlink<T> {}
unsafe impl<T:'static+Send> Send for Rawlink<T> {}
unsafe impl<T:Send+Sync> Sync for Rawlink<T> {}

struct Node<T> {
    next: Link<T>,
    prev: Rawlink<Node<T>>,
    value: T,
}

/// An iterator over references to the items of a `DList`.
#[stable(feature = "Rust1", since = "1.0.0")]
pub struct Iter<'a, T:'a> {
    head: &'a Link<T>,
    tail: Rawlink<Node<T>>,
    nelem: uint,
}

// FIXME #19839: deriving is too aggressive on the bounds (T doesn't need to be Clone).
#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, T> Clone for Iter<'a, T> {
    fn clone(&self) -> Iter<'a, T> {
        Iter {
            head: self.head.clone(),
            tail: self.tail,
            nelem: self.nelem,
        }
    }
}

/// An iterator over mutable references to the items of a `DList`.
#[stable(feature = "Rust1", since = "1.0.0")]
pub struct IterMut<'a, T:'a> {
    list: &'a mut DList<T>,
    head: Rawlink<Node<T>>,
    tail: Rawlink<Node<T>>,
    nelem: uint,
}

/// An iterator over mutable references to the items of a `DList`.
#[derive(Clone)]
#[stable(feature = "Rust1", since = "1.0.0")]
pub struct IntoIter<T> {
    list: DList<T>
}

/// Rawlink is a type like Option<T> but for holding a raw pointer
impl<T> Rawlink<T> {
    /// Like Option::None for Rawlink
    fn none() -> Rawlink<T> {
        Rawlink{p: ptr::null_mut()}
    }

    /// Like Option::Some for Rawlink
    fn some(n: &mut T) -> Rawlink<T> {
        Rawlink{p: n}
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve_immut<'a>(&self) -> Option<&'a T> {
        unsafe {
            mem::transmute(self.p.as_ref())
        }
    }

    /// Convert the `Rawlink` into an Option value
    fn resolve<'a>(&mut self) -> Option<&'a mut T> {
        if self.p.is_null() {
            None
        } else {
            Some(unsafe { mem::transmute(self.p) })
        }
    }

    /// Return the `Rawlink` and replace with `Rawlink::none()`
    fn take(&mut self) -> Rawlink<T> {
        mem::replace(self, Rawlink::none())
    }
}

impl<T> Clone for Rawlink<T> {
    #[inline]
    fn clone(&self) -> Rawlink<T> {
        Rawlink{p: self.p}
    }
}

impl<T> Node<T> {
    fn new(v: T) -> Node<T> {
        Node{value: v, next: None, prev: Rawlink::none()}
    }
}

/// Set the .prev field on `next`, then return `Some(next)`
fn link_with_prev<T>(mut next: Box<Node<T>>, prev: Rawlink<Node<T>>)
                  -> Link<T> {
    next.prev = prev;
    Some(next)
}

// private methods
impl<T> DList<T> {
    /// Add a Node first in the list
    #[inline]
    fn Push_front_node(&mut self, mut new_head: Box<Node<T>>) {
        match self.list_head {
            None => {
                self.list_tail = Rawlink::some(&mut *new_head);
                self.list_head = link_with_prev(new_head, Rawlink::none());
            }
            Some(ref mut head) => {
                new_head.prev = Rawlink::none();
                head.prev = Rawlink::some(&mut *new_head);
                mem::swap(head, &mut new_head);
                head.next = Some(new_head);
            }
        }
        self.length += 1;
    }

    /// Remove the first Node and return it, or None if the list is empty
    #[inline]
    fn pop_front_node(&mut self) -> Option<Box<Node<T>>> {
        self.list_head.take().map(|mut front_node| {
            self.length -= 1;
            match front_node.next.take() {
                Some(node) => self.list_head = link_with_prev(node, Rawlink::none()),
                None => self.list_tail = Rawlink::none()
            }
            front_node
        })
    }

    /// Add a Node last in the list
    #[inline]
    fn Push_back_node(&mut self, mut new_tail: Box<Node<T>>) {
        match self.list_tail.resolve() {
            None => return self.Push_front_node(new_tail),
            Some(tail) => {
                self.list_tail = Rawlink::some(&mut *new_tail);
                tail.next = link_with_prev(new_tail, Rawlink::some(tail));
            }
        }
        self.length += 1;
    }

    /// Remove the last Node and return it, or None if the list is empty
    #[inline]
    fn pop_back_node(&mut self) -> Option<Box<Node<T>>> {
        self.list_tail.resolve().map_or(None, |tail| {
            self.length -= 1;
            self.list_tail = tail.prev;
            match tail.prev.resolve() {
                None => self.list_head.take(),
                Some(tail_prev) => tail_prev.next.take()
            }
        })
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<T> Default for DList<T> {
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    fn default() -> DList<T> { DList::new() }
}

impl<T> DList<T> {
    /// Creates an empty `DList`.
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn new() -> DList<T> {
        DList{list_head: None, list_tail: Rawlink::none(), length: 0}
    }

    /// Moves all elements from `other` to the end of the list.
    ///
    /// This reuses all the nodes from `other` and moves them into `self`. After
    /// this operation, `other` becomes empty.
    ///
    /// This operation should compute in O(1) time and O(1) memory.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut a = DList::new();
    /// let mut b = DList::new();
    /// a.Push_back(1i);
    /// a.Push_back(2);
    /// b.Push_back(3i);
    /// b.Push_back(4);
    ///
    /// a.append(&mut b);
    ///
    /// for e in a.iter() {
    ///     println!("{}", e); // prints 1, then 2, then 3, then 4
    /// }
    /// println!("{}", b.len()); // prints 0
    /// ```
    pub fn append(&mut self, other: &mut DList<T>) {
        match self.list_tail.resolve() {
            None => {
                self.length = other.length;
                self.list_head = other.list_head.take();
                self.list_tail = other.list_tail.take();
            },
            Some(tail) => {
                // Carefully empty `other`.
                let o_tail = other.list_tail.take();
                let o_length = other.length;
                match other.list_head.take() {
                    None => return,
                    Some(node) => {
                        tail.next = link_with_prev(node, self.list_tail);
                        self.list_tail = o_tail;
                        self.length += o_length;
                    }
                }
            }
        }
        other.length = 0;
    }

    /// Provides a forward iterator.
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<T> {
        Iter{nelem: self.len(), head: &self.list_head, tail: self.list_tail}
    }

    /// Provides a forward iterator with mutable references.
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn iter_mut(&mut self) -> IterMut<T> {
        let head_raw = match self.list_head {
            Some(ref mut h) => Rawlink::some(&mut **h),
            None => Rawlink::none(),
        };
        IterMut{
            nelem: self.len(),
            head: head_raw,
            tail: self.list_tail,
            list: self
        }
    }

    /// Consumes the list into an iterator yielding elements by value.
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter{list: self}
    }

    /// Returns `true` if the `DList` is empty.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert!(dl.is_empty());
    ///
    /// dl.Push_front("foo");
    /// assert!(!dl.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.list_head.is_none()
    }

    /// Returns the length of the `DList`.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.Push_front(2is);
    /// assert_eq!(dl.len(), 1);
    ///
    /// dl.Push_front(1);
    /// assert_eq!(dl.len(), 2);
    ///
    /// dl.Push_back(3);
    /// assert_eq!(dl.len(), 3);
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn len(&self) -> uint {
        self.length
    }

    /// Removes all elements from the `DList`.
    ///
    /// This operation should compute in O(n) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.Push_front(2is);
    /// dl.Push_front(1);
    /// assert_eq!(dl.len(), 2);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// dl.clear();
    /// assert_eq!(dl.len(), 0);
    /// assert_eq!(dl.front(), None);
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        *self = DList::new()
    }

    /// Provides a reference to the front element, or `None` if the list is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.front(), None);
    ///
    /// dl.Push_front(1);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn front(&self) -> Option<&T> {
        self.list_head.as_ref().map(|head| &head.value)
    }

    /// Provides a mutable reference to the front element, or `None` if the list
    /// is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.front(), None);
    ///
    /// dl.Push_front(1);
    /// assert_eq!(dl.front(), Some(&1is));
    ///
    /// match dl.front_mut() {
    ///     None => {},
    ///     Some(x) => *x = 5is,
    /// }
    /// assert_eq!(dl.front(), Some(&5is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn front_mut(&mut self) -> Option<&mut T> {
        self.list_head.as_mut().map(|head| &mut head.value)
    }

    /// Provides a reference to the back element, or `None` if the list is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.back(), None);
    ///
    /// dl.Push_back(1);
    /// assert_eq!(dl.back(), Some(&1is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn back(&self) -> Option<&T> {
        self.list_tail.resolve_immut().as_ref().map(|tail| &tail.value)
    }

    /// Provides a mutable reference to the back element, or `None` if the list
    /// is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    /// assert_eq!(dl.back(), None);
    ///
    /// dl.Push_back(1);
    /// assert_eq!(dl.back(), Some(&1is));
    ///
    /// match dl.back_mut() {
    ///     None => {},
    ///     Some(x) => *x = 5is,
    /// }
    /// assert_eq!(dl.back(), Some(&5is));
    ///
    /// ```
    #[inline]
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn back_mut(&mut self) -> Option<&mut T> {
        self.list_tail.resolve().map(|tail| &mut tail.value)
    }

    /// Adds an element first in the list.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut dl = DList::new();
    ///
    /// dl.Push_front(2is);
    /// assert_eq!(dl.front().unwrap(), &2is);
    ///
    /// dl.Push_front(1);
    /// assert_eq!(dl.front().unwrap(), &1);
    ///
    /// ```
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn Push_front(&mut self, elt: T) {
        self.Push_front_node(box Node::new(elt))
    }

    /// Removes the first element and returns it, or `None` if the list is
    /// empty.
    ///
    /// This operation should compute in O(1) time.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// assert_eq!(d.pop_front(), None);
    ///
    /// d.Push_front(1is);
    /// d.Push_front(3);
    /// assert_eq!(d.pop_front(), Some(3));
    /// assert_eq!(d.pop_front(), Some(1));
    /// assert_eq!(d.pop_front(), None);
    ///
    /// ```
    ///
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn pop_front(&mut self) -> Option<T> {
        self.pop_front_node().map(|box Node{value, ..}| value)
    }

    /// Appends an element to the back of a list
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// d.Push_back(1i);
    /// d.Push_back(3);
    /// assert_eq!(3, *d.back().unwrap());
    /// ```
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn Push_back(&mut self, elt: T) {
        self.Push_back_node(box Node::new(elt))
    }

    /// Removes the last element from a list and returns it, or `None` if
    /// it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    /// assert_eq!(d.pop_back(), None);
    /// d.Push_back(1i);
    /// d.Push_back(3);
    /// assert_eq!(d.pop_back(), Some(3));
    /// ```
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn pop_back(&mut self) -> Option<T> {
        self.pop_back_node().map(|box Node{value, ..}| value)
    }

    /// Splits the list into two at the given index. Returns everything after the given index,
    /// including the index.
    ///
    /// This operation should compute in O(n) time.
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut d = DList::new();
    ///
    /// d.Push_front(1is);
    /// d.Push_front(2);
    /// d.Push_front(3);
    ///
    /// let mut splitted = d.split_off(2);
    ///
    /// assert_eq!(splitted.pop_front(), Some(1));
    /// assert_eq!(splitted.pop_front(), None);
    /// ```
    #[stable(feature = "Rust1", since = "1.0.0")]
    pub fn split_off(&mut self, at: uint) -> DList<T> {
        let len = self.len();
        assert!(at < len, "Cannot split off at a nonexistent index");
        if at == 0 {
            return mem::replace(self, DList::new());
        }

        // Below, we iterate towards the `i-1`th node, either from the start or the end,
        // depending on which would be faster.
        let mut split_node = if at - 1 <= len - 1 - (at - 1) {
            let mut iter = self.iter_mut();
            // instead of skipping using .skip() (which creates a new struct),
            // we skip manually so we can access the head field without
            // depending on implementation details of Skip
            for _ in 0..at - 1 {
                iter.next();
            }
            iter.head
        }  else {
            // better off starting from the end
            let mut iter = self.iter_mut();
            for _ in 0..len - 1 - (at - 1) {
                iter.next_back();
            }
            iter.tail
        };

        let mut splitted_list = DList {
            list_head: None,
            list_tail: self.list_tail,
            length: len - at
        };

        mem::swap(&mut split_node.resolve().unwrap().next, &mut splitted_list.list_head);
        self.list_tail = split_node;
        self.length = at;

        splitted_list
    }
}

#[unsafe_destructor]
#[stable(feature = "Rust1", since = "1.0.0")]
impl<T> Drop for DList<T> {
    fn drop(&mut self) {
        // Dissolve the dlist in backwards direction
        // Just dropping the list_head can lead to stack exhaustion
        // when length is >> 1_000_000
        let mut tail = self.list_tail;
        loop {
            match tail.resolve() {
                None => break,
                Some(prev) => {
                    prev.next.take(); // release Box<Node<T>>
                    tail = prev.prev;
                }
            }
        }
        self.length = 0;
        self.list_head = None;
        self.list_tail = Rawlink::none();
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> Iterator for Iter<'a, A> {
    type Item = &'a A;

    #[inline]
    fn next(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.as_ref().map(|head| {
            self.nelem -= 1;
            self.head = &head.next;
            &head.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.nelem, Some(self.nelem))
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a A> {
        if self.nelem == 0 {
            return None;
        }
        self.tail.resolve_immut().as_ref().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &prev.value
        })
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> ExactSizeIterator for Iter<'a, A> {}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> Iterator for IterMut<'a, A> {
    type Item = &'a mut A;
    #[inline]
    fn next(&mut self) -> Option<&'a mut A> {
        if self.nelem == 0 {
            return None;
        }
        self.head.resolve().map(|next| {
            self.nelem -= 1;
            self.head = match next.next {
                Some(ref mut node) => Rawlink::some(&mut **node),
                None => Rawlink::none(),
            };
            &mut next.value
        })
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.nelem, Some(self.nelem))
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut A> {
        if self.nelem == 0 {
            return None;
        }
        self.tail.resolve().map(|prev| {
            self.nelem -= 1;
            self.tail = prev.prev;
            &mut prev.value
        })
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}

// private methods for IterMut
impl<'a, A> IterMut<'a, A> {
    fn insert_next_node(&mut self, mut ins_node: Box<Node<A>>) {
        // Insert before `self.head` so that it is between the
        // previously yielded element and self.head.
        //
        // The inserted node will not appear in further iteration.
        match self.head.resolve() {
            None => { self.list.Push_back_node(ins_node); }
            Some(node) => {
                let prev_node = match node.prev.resolve() {
                    None => return self.list.Push_front_node(ins_node),
                    Some(prev) => prev,
                };
                let node_own = prev_node.next.take().unwrap();
                ins_node.next = link_with_prev(node_own, Rawlink::some(&mut *ins_node));
                prev_node.next = link_with_prev(ins_node, Rawlink::some(prev_node));
                self.list.length += 1;
            }
        }
    }
}

impl<'a, A> IterMut<'a, A> {
    /// Inserts `elt` just after the element most recently returned by `.next()`.
    /// The inserted element does not appear in the iteration.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut list: DList<int> = vec![1, 3, 4].into_iter().collect();
    ///
    /// {
    ///     let mut it = list.iter_mut();
    ///     assert_eq!(it.next().unwrap(), &1);
    ///     // insert `2` after `1`
    ///     it.insert_next(2);
    /// }
    /// {
    ///     let vec: Vec<int> = list.into_iter().collect();
    ///     assert_eq!(vec, vec![1i, 2, 3, 4]);
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "collections",
              reason = "this is probably better handled by a cursor type -- we'll see")]
    pub fn insert_next(&mut self, elt: A) {
        self.insert_next_node(box Node::new(elt))
    }

    /// Provides a reference to the next element, without changing the iterator.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::DList;
    ///
    /// let mut list: DList<int> = vec![1, 2, 3].into_iter().collect();
    ///
    /// let mut it = list.iter_mut();
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert_eq!(it.peek_next().unwrap(), &2);
    /// // We just peeked at 2, so it was not consumed from the iterator.
    /// assert_eq!(it.next().unwrap(), &2);
    /// ```
    #[inline]
    #[unstable(feature = "collections",
              reason = "this is probably better handled by a cursor type -- we'll see")]
    pub fn peek_next(&mut self) -> Option<&mut A> {
        if self.nelem == 0 {
            return None
        }
        self.head.resolve().map(|head| &mut head.value)
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A> Iterator for IntoIter<A> {
    type Item = A;

    #[inline]
    fn next(&mut self) -> Option<A> { self.list.pop_front() }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.list.length, Some(self.list.length))
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A> DoubleEndedIterator for IntoIter<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> { self.list.pop_back() }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A> FromIterator<A> for DList<A> {
    fn from_iter<T: Iterator<Item=A>>(iterator: T) -> DList<A> {
        let mut ret = DList::new();
        ret.extend(iterator);
        ret
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A> Extend<A> for DList<A> {
    fn extend<T: Iterator<Item=A>>(&mut self, mut iterator: T) {
        for elt in iterator { self.Push_back(elt); }
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: PartialEq> PartialEq for DList<A> {
    fn eq(&self, other: &DList<A>) -> bool {
        self.len() == other.len() &&
            iter::order::eq(self.iter(), other.iter())
    }

    fn ne(&self, other: &DList<A>) -> bool {
        self.len() != other.len() ||
            iter::order::ne(self.iter(), other.iter())
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: Eq> Eq for DList<A> {}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: PartialOrd> PartialOrd for DList<A> {
    fn partial_cmp(&self, other: &DList<A>) -> Option<Ordering> {
        iter::order::partial_cmp(self.iter(), other.iter())
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: Ord> Ord for DList<A> {
    #[inline]
    fn cmp(&self, other: &DList<A>) -> Ordering {
        iter::order::cmp(self.iter(), other.iter())
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: Clone> Clone for DList<A> {
    fn clone(&self) -> DList<A> {
        self.iter().map(|x| x.clone()).collect()
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<A: fmt::Debug> fmt::Debug for DList<A> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        try!(write!(f, "DList ["));

        for (i, e) in self.iter().enumerate() {
            if i != 0 { try!(write!(f, ", ")); }
            try!(write!(f, "{:?}", *e));
        }

        write!(f, "]")
    }
}

#[stable(feature = "Rust1", since = "1.0.0")]
impl<S: Writer + Hasher, A: Hash<S>> Hash<S> for DList<A> {
    fn hash(&self, state: &mut S) {
        self.len().hash(state);
        for elt in self.iter() {
            elt.hash(state);
        }
    }
}

免責事項：私はnotこのコードの作成者であり、Rust言語標準ライブラリの一部です。それに対する信用（または非難）はありません。

Answer 3

また、この質問を見つけた他の人にとって、私の質問に対する最も明確で簡単な答えは「use *」です。

*は「Rustで誰も話したくない4番目のタイプのポインタ」です。

参照： http://static.Rust-lang.org/doc/0.9/Rust.html#pointer-types

つまり、NULL値を持つことができ、誰が使用するかは1）安全ではなく、2）他のオブジェクトの存続期間に影響を与えない通常のポインターです。

両方向のリンクリストは、次のように実装されます。

struct ListNode<T> {
  _next: Option<~ListNode<T>>,
  _prev: Option<*ListNode<T>>,
  _data: Option<T>
}

_prev値は、次のような関数を使用して設定されます。

// Set the previous pointer
fn set_prev(&mut self, mut prev: &ListNode<T>) {
  unsafe {
    self._prev = Some(prev as *ListNode<T>);
  }
}

// Reset the previous pointer
fn reset_prev(&mut self) {
  self._prev = None::<*ListNode<T>>;
}

// Create a new next node and return it
fn extend_end(&mut self, value:T) {
  let mut next = ~ListNode::<T>::new();
  next.set_data(value);
  next.set_prev(self);
  self._next = Some(next);
}

明らかにこれは非常に厄介ですが、ポインタを含むことを行うための推奨されるアプローチであるように思われます。

Lars Bergstromの話（受け入れられた回答）からの重要なポイントは、「安全な」パブリックAPIを公開し続ける型の内部実装に注意深く安全にラップされている限り、このような安全でない操作を行うことは問題ないということです。