libcollections

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diff --git a/libcollections/linked_list.rs b/libcollections/linked_list.rs
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+// 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 `LinkedList` allows pushing and popping elements at either end and is thus
+//! efficiently usable as a double-ended queue.
+
+// LinkedList 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 LinkedList::prev are raw pointers that form a full chain in
+// the reverse direction.
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use alloc::boxed::{Box, IntermediateBox};
+use core::cmp::Ordering;
+use core::fmt;
+use core::hash::{Hasher, Hash};
+use core::iter::FromIterator;
+use core::mem;
+use core::ops::{BoxPlace, InPlace, Place, Placer};
+use core::ptr::{self, Shared};
+
+/// A doubly-linked list.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct LinkedList<T> {
+    length: usize,
+    list_head: Link<T>,
+    list_tail: Rawlink<Node<T>>,
+}
+
+type Link<T> = Option<Box<Node<T>>>;
+
+struct Rawlink<T> {
+    p: Option<Shared<T>>,
+}
+
+impl<T> Copy for Rawlink<T> {}
+unsafe impl<T: Send> Send for Rawlink<T> {}
+unsafe impl<T: 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 `LinkedList`.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Iter<'a, T: 'a> {
+    head: &'a Link<T>,
+    tail: Rawlink<Node<T>>,
+    nelem: usize,
+}
+
+// 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 `LinkedList`.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct IterMut<'a, T: 'a> {
+    list: &'a mut LinkedList<T>,
+    head: Rawlink<Node<T>>,
+    tail: Rawlink<Node<T>>,
+    nelem: usize,
+}
+
+/// An iterator over the items of a `LinkedList`.
+#[derive(Clone)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct IntoIter<T> {
+    list: LinkedList<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: None }
+    }
+
+    /// Like Option::Some for Rawlink
+    fn some(n: &mut T) -> Rawlink<T> {
+        unsafe { Rawlink { p: Some(Shared::new(n)) } }
+    }
+
+    /// Convert the `Rawlink` into an Option value
+    ///
+    /// **unsafe** because:
+    ///
+    /// - Dereference of raw pointer.
+    /// - Returns reference of arbitrary lifetime.
+    unsafe fn resolve<'a>(&self) -> Option<&'a T> {
+        self.p.map(|p| &**p)
+    }
+
+    /// Convert the `Rawlink` into an Option value
+    ///
+    /// **unsafe** because:
+    ///
+    /// - Dereference of raw pointer.
+    /// - Returns reference of arbitrary lifetime.
+    unsafe fn resolve_mut<'a>(&mut self) -> Option<&'a mut T> {
+        self.p.map(|p| &mut **p)
+    }
+
+    /// Return the `Rawlink` and replace with `Rawlink::none()`
+    fn take(&mut self) -> Rawlink<T> {
+        mem::replace(self, Rawlink::none())
+    }
+}
+
+impl<'a, T> From<&'a mut Link<T>> for Rawlink<Node<T>> {
+    fn from(node: &'a mut Link<T>) -> Self {
+        match node.as_mut() {
+            None => Rawlink::none(),
+            Some(ptr) => Rawlink::some(ptr),
+        }
+    }
+}
+
+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(),
+        }
+    }
+
+    /// Update the `prev` link on `next`, then set self's next pointer.
+    ///
+    /// `self.next` should be `None` when you call this
+    /// (otherwise a Node is probably being dropped by mistake).
+    fn set_next(&mut self, mut next: Box<Node<T>>) {
+        debug_assert!(self.next.is_none());
+        next.prev = Rawlink::some(self);
+        self.next = Some(next);
+    }
+}
+
+/// Clear the .prev field on `next`, then return `Some(next)`
+fn link_no_prev<T>(mut next: Box<Node<T>>) -> Link<T> {
+    next.prev = Rawlink::none();
+    Some(next)
+}
+
+// private methods
+impl<T> LinkedList<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_head = link_no_prev(new_head);
+                self.list_tail = Rawlink::from(&mut self.list_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);
+            }
+        }
+        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_no_prev(node),
+                None => self.list_tail = Rawlink::none(),
+            }
+            front_node
+        })
+    }
+
+    /// Add a Node last in the list
+    #[inline]
+    fn push_back_node(&mut self, new_tail: Box<Node<T>>) {
+        match unsafe { self.list_tail.resolve_mut() } {
+            None => return self.push_front_node(new_tail),
+            Some(tail) => {
+                tail.set_next(new_tail);
+                self.list_tail = Rawlink::from(&mut tail.next);
+            }
+        }
+        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>>> {
+        unsafe {
+            self.list_tail.resolve_mut().and_then(|tail| {
+                self.length -= 1;
+                self.list_tail = tail.prev;
+                match tail.prev.resolve_mut() {
+                    None => self.list_head.take(),
+                    Some(tail_prev) => tail_prev.next.take(),
+                }
+            })
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Default for LinkedList<T> {
+    #[inline]
+    fn default() -> LinkedList<T> {
+        LinkedList::new()
+    }
+}
+
+impl<T> LinkedList<T> {
+    /// Creates an empty `LinkedList`.
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new() -> LinkedList<T> {
+        LinkedList {
+            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::LinkedList;
+    ///
+    /// let mut a = LinkedList::new();
+    /// let mut b = LinkedList::new();
+    /// a.push_back(1);
+    /// a.push_back(2);
+    /// b.push_back(3);
+    /// b.push_back(4);
+    ///
+    /// a.append(&mut b);
+    ///
+    /// for e in &a {
+    ///     println!("{}", e); // prints 1, then 2, then 3, then 4
+    /// }
+    /// println!("{}", b.len()); // prints 0
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn append(&mut self, other: &mut LinkedList<T>) {
+        match unsafe { self.list_tail.resolve_mut() } {
+            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.set_next(node);
+                        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> {
+        IterMut {
+            nelem: self.len(),
+            head: Rawlink::from(&mut self.list_head),
+            tail: self.list_tail,
+            list: self,
+        }
+    }
+
+    /// Returns `true` if the `LinkedList` is empty.
+    ///
+    /// This operation should compute in O(1) time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::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 `LinkedList`.
+    ///
+    /// This operation should compute in O(1) time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    ///
+    /// dl.push_front(2);
+    /// 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) -> usize {
+        self.length
+    }
+
+    /// Removes all elements from the `LinkedList`.
+    ///
+    /// This operation should compute in O(n) time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    ///
+    /// dl.push_front(2);
+    /// dl.push_front(1);
+    /// assert_eq!(dl.len(), 2);
+    /// assert_eq!(dl.front(), Some(&1));
+    ///
+    /// 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 = LinkedList::new()
+    }
+
+    /// Provides a reference to the front element, or `None` if the list is
+    /// empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    /// assert_eq!(dl.front(), None);
+    ///
+    /// dl.push_front(1);
+    /// assert_eq!(dl.front(), Some(&1));
+    ///
+    /// ```
+    #[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::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    /// assert_eq!(dl.front(), None);
+    ///
+    /// dl.push_front(1);
+    /// assert_eq!(dl.front(), Some(&1));
+    ///
+    /// match dl.front_mut() {
+    ///     None => {},
+    ///     Some(x) => *x = 5,
+    /// }
+    /// assert_eq!(dl.front(), Some(&5));
+    ///
+    /// ```
+    #[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::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    /// assert_eq!(dl.back(), None);
+    ///
+    /// dl.push_back(1);
+    /// assert_eq!(dl.back(), Some(&1));
+    ///
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn back(&self) -> Option<&T> {
+        unsafe { self.list_tail.resolve().map(|tail| &tail.value) }
+    }
+
+    /// Provides a mutable reference to the back element, or `None` if the list
+    /// is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    /// assert_eq!(dl.back(), None);
+    ///
+    /// dl.push_back(1);
+    /// assert_eq!(dl.back(), Some(&1));
+    ///
+    /// match dl.back_mut() {
+    ///     None => {},
+    ///     Some(x) => *x = 5,
+    /// }
+    /// assert_eq!(dl.back(), Some(&5));
+    ///
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn back_mut(&mut self) -> Option<&mut T> {
+        unsafe { self.list_tail.resolve_mut().map(|tail| &mut tail.value) }
+    }
+
+    /// Adds an element first in the list.
+    ///
+    /// This operation should compute in O(1) time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut dl = LinkedList::new();
+    ///
+    /// dl.push_front(2);
+    /// assert_eq!(dl.front().unwrap(), &2);
+    ///
+    /// 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::LinkedList;
+    ///
+    /// let mut d = LinkedList::new();
+    /// assert_eq!(d.pop_front(), None);
+    ///
+    /// d.push_front(1);
+    /// 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::LinkedList;
+    ///
+    /// let mut d = LinkedList::new();
+    /// d.push_back(1);
+    /// 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::LinkedList;
+    ///
+    /// let mut d = LinkedList::new();
+    /// assert_eq!(d.pop_back(), None);
+    /// d.push_back(1);
+    /// 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.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at > len`.
+    ///
+    /// This operation should compute in O(n) time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut d = LinkedList::new();
+    ///
+    /// d.push_front(1);
+    /// 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: usize) -> LinkedList<T> {
+        let len = self.len();
+        assert!(at <= len, "Cannot split off at a nonexistent index");
+        if at == 0 {
+            return mem::replace(self, LinkedList::new());
+        } else if at == len {
+            return LinkedList::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
+        };
+
+        // The split node is the new tail node of the first part and owns
+        // the head of the second part.
+        let mut second_part_head;
+
+        unsafe {
+            second_part_head = split_node.resolve_mut().unwrap().next.take();
+            match second_part_head {
+                None => {}
+                Some(ref mut head) => head.prev = Rawlink::none(),
+            }
+        }
+
+        let second_part = LinkedList {
+            list_head: second_part_head,
+            list_tail: self.list_tail,
+            length: len - at,
+        };
+
+        // Fix the tail ptr of the first part
+        self.list_tail = split_node;
+        self.length = at;
+
+        second_part
+    }
+
+    /// Returns a place for insertion at the front of the list.
+    ///
+    /// Using this method with placement syntax is equivalent to [`push_front`]
+    /// (#method.push_front), but may be more efficient.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(collection_placement)]
+    /// #![feature(placement_in_syntax)]
+    ///
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut list = LinkedList::new();
+    /// list.front_place() <- 2;
+    /// list.front_place() <- 4;
+    /// assert!(list.iter().eq(&[4, 2]));
+    /// ```
+    #[unstable(feature = "collection_placement",
+               reason = "method name and placement protocol are subject to change",
+               issue = "30172")]
+    pub fn front_place(&mut self) -> FrontPlace<T> {
+        FrontPlace { list: self, node: IntermediateBox::make_place() }
+    }
+
+    /// Returns a place for insertion at the back of the list.
+    ///
+    /// Using this method with placement syntax is equivalent to [`push_back`](#method.push_back),
+    /// but may be more efficient.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(collection_placement)]
+    /// #![feature(placement_in_syntax)]
+    ///
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut list = LinkedList::new();
+    /// list.back_place() <- 2;
+    /// list.back_place() <- 4;
+    /// assert!(list.iter().eq(&[2, 4]));
+    /// ```
+    #[unstable(feature = "collection_placement",
+               reason = "method name and placement protocol are subject to change",
+               issue = "30172")]
+    pub fn back_place(&mut self) -> BackPlace<T> {
+        BackPlace { list: self, node: IntermediateBox::make_place() }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Drop for LinkedList<T> {
+    #[unsafe_destructor_blind_to_params]
+    fn drop(&mut self) {
+        // Dissolve the linked_list in a loop.
+        // Just dropping the list_head can lead to stack exhaustion
+        // when length is >> 1_000_000
+        while let Some(mut head_) = self.list_head.take() {
+            self.list_head = head_.next.take();
+        }
+        self.length = 0;
+        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) -> (usize, Option<usize>) {
+        (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;
+        }
+        unsafe {
+            self.tail.resolve().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;
+        }
+        unsafe {
+            self.head.resolve_mut().map(|next| {
+                self.nelem -= 1;
+                self.head = Rawlink::from(&mut next.next);
+                &mut next.value
+            })
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (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;
+        }
+        unsafe {
+            self.tail.resolve_mut().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 unsafe { self.head.resolve_mut() } {
+            None => {
+                self.list.push_back_node(ins_node);
+            }
+            Some(node) => {
+                let prev_node = match unsafe { node.prev.resolve_mut() } {
+                    None => return self.list.push_front_node(ins_node),
+                    Some(prev) => prev,
+                };
+                let node_own = prev_node.next.take().unwrap();
+                ins_node.set_next(node_own);
+                prev_node.set_next(ins_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
+    ///
+    /// ```
+    /// #![feature(linked_list_extras)]
+    ///
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut list: LinkedList<_> = 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<_> = list.into_iter().collect();
+    ///     assert_eq!(vec, [1, 2, 3, 4]);
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "linked_list_extras",
+               reason = "this is probably better handled by a cursor type -- we'll see",
+               issue = "27794")]
+    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
+    ///
+    /// ```
+    /// #![feature(linked_list_extras)]
+    ///
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut list: LinkedList<_> = 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 = "linked_list_extras",
+               reason = "this is probably better handled by a cursor type -- we'll see",
+               issue = "27794")]
+    pub fn peek_next(&mut self) -> Option<&mut A> {
+        if self.nelem == 0 {
+            return None;
+        }
+        unsafe { self.head.resolve_mut().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) -> (usize, Option<usize>) {
+        (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> ExactSizeIterator for IntoIter<A> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> FromIterator<A> for LinkedList<A> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> LinkedList<A> {
+        let mut ret = LinkedList::new();
+        ret.extend(iter);
+        ret
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> IntoIterator for LinkedList<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+
+    /// Consumes the list into an iterator yielding elements by value.
+    #[inline]
+    fn into_iter(self) -> IntoIter<T> {
+        IntoIter { list: self }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> IntoIterator for &'a LinkedList<T> {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T> IntoIterator for &'a mut LinkedList<T> {
+    type Item = &'a mut T;
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(mut self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> Extend<A> for LinkedList<A> {
+    fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) {
+        for elt in iter {
+            self.push_back(elt);
+        }
+    }
+}
+
+#[stable(feature = "extend_ref", since = "1.2.0")]
+impl<'a, T: 'a + Copy> Extend<&'a T> for LinkedList<T> {
+    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
+        self.extend(iter.into_iter().cloned());
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: PartialEq> PartialEq for LinkedList<A> {
+    fn eq(&self, other: &LinkedList<A>) -> bool {
+        self.len() == other.len() && self.iter().eq(other.iter())
+    }
+
+    fn ne(&self, other: &LinkedList<A>) -> bool {
+        self.len() != other.len() || self.iter().ne(other.iter())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Eq> Eq for LinkedList<A> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: PartialOrd> PartialOrd for LinkedList<A> {
+    fn partial_cmp(&self, other: &LinkedList<A>) -> Option<Ordering> {
+        self.iter().partial_cmp(other.iter())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Ord> Ord for LinkedList<A> {
+    #[inline]
+    fn cmp(&self, other: &LinkedList<A>) -> Ordering {
+        self.iter().cmp(other.iter())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Clone> Clone for LinkedList<A> {
+    fn clone(&self) -> LinkedList<A> {
+        self.iter().cloned().collect()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: fmt::Debug> fmt::Debug for LinkedList<A> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_list().entries(self.iter()).finish()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A: Hash> Hash for LinkedList<A> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.len().hash(state);
+        for elt in self {
+            elt.hash(state);
+        }
+    }
+}
+
+unsafe fn finalize<T>(node: IntermediateBox<Node<T>>) -> Box<Node<T>> {
+    let mut node = node.finalize();
+    ptr::write(&mut node.next, None);
+    ptr::write(&mut node.prev, Rawlink::none());
+    node
+}
+
+/// A place for insertion at the front of a `LinkedList`.
+///
+/// See [`LinkedList::front_place`](struct.LinkedList.html#method.front_place) for details.
+#[must_use = "places do nothing unless written to with `<-` syntax"]
+#[unstable(feature = "collection_placement",
+           reason = "struct name and placement protocol are subject to change",
+           issue = "30172")]
+pub struct FrontPlace<'a, T: 'a> {
+    list: &'a mut LinkedList<T>,
+    node: IntermediateBox<Node<T>>,
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> Placer<T> for FrontPlace<'a, T> {
+    type Place = Self;
+
+    fn make_place(self) -> Self {
+        self
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> Place<T> for FrontPlace<'a, T> {
+    fn pointer(&mut self) -> *mut T {
+        unsafe { &mut (*self.node.pointer()).value }
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> InPlace<T> for FrontPlace<'a, T> {
+    type Owner = ();
+
+    unsafe fn finalize(self) {
+        let FrontPlace { list, node } = self;
+        list.push_front_node(finalize(node));
+    }
+}
+
+/// A place for insertion at the back of a `LinkedList`.
+///
+/// See [`LinkedList::back_place`](struct.LinkedList.html#method.back_place) for details.
+#[must_use = "places do nothing unless written to with `<-` syntax"]
+#[unstable(feature = "collection_placement",
+           reason = "struct name and placement protocol are subject to change",
+           issue = "30172")]
+pub struct BackPlace<'a, T: 'a> {
+    list: &'a mut LinkedList<T>,
+    node: IntermediateBox<Node<T>>,
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> Placer<T> for BackPlace<'a, T> {
+    type Place = Self;
+
+    fn make_place(self) -> Self {
+        self
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> Place<T> for BackPlace<'a, T> {
+    fn pointer(&mut self) -> *mut T {
+        unsafe { &mut (*self.node.pointer()).value }
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> InPlace<T> for BackPlace<'a, T> {
+    type Owner = ();
+
+    unsafe fn finalize(self) {
+        let BackPlace { list, node } = self;
+        list.push_back_node(finalize(node));
+    }
+}
+
+// Ensure that `LinkedList` and its read-only iterators are covariant in their type parameters.
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn a<'a>(x: LinkedList<&'static str>) -> LinkedList<&'a str> { x }
+    fn b<'i, 'a>(x: Iter<'i, &'static str>) -> Iter<'i, &'a str> { x }
+    fn c<'a>(x: IntoIter<&'static str>) -> IntoIter<&'a str> { x }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::clone::Clone;
+    use std::iter::{Iterator, IntoIterator, Extend};
+    use std::option::Option::{self, Some, None};
+    use std::__rand::{thread_rng, Rng};
+    use std::thread;
+    use std::vec::Vec;
+
+    use super::{LinkedList, Node};
+
+    #[cfg(test)]
+    fn list_from<T: Clone>(v: &[T]) -> LinkedList<T> {
+        v.iter().cloned().collect()
+    }
+
+    pub fn check_links<T>(list: &LinkedList<T>) {
+        let mut len = 0;
+        let mut last_ptr: Option<&Node<T>> = None;
+        let mut node_ptr: &Node<T>;
+        match list.list_head {
+            None => {
+                assert_eq!(0, list.length);
+                return;
+            }
+            Some(ref node) => node_ptr = &**node,
+        }
+        loop {
+            match unsafe { (last_ptr, node_ptr.prev.resolve()) } {
+                (None, None) => {}
+                (None, _) => panic!("prev link for list_head"),
+                (Some(p), Some(pptr)) => {
+                    assert_eq!(p as *const Node<T>, pptr as *const Node<T>);
+                }
+                _ => panic!("prev link is none, not good"),
+            }
+            match node_ptr.next {
+                Some(ref next) => {
+                    last_ptr = Some(node_ptr);
+                    node_ptr = &**next;
+                    len += 1;
+                }
+                None => {
+                    len += 1;
+                    break;
+                }
+            }
+        }
+        assert_eq!(len, list.length);
+    }
+
+    #[test]
+    fn test_append() {
+        // Empty to empty
+        {
+            let mut m = LinkedList::<i32>::new();
+            let mut n = LinkedList::new();
+            m.append(&mut n);
+            check_links(&m);
+            assert_eq!(m.len(), 0);
+            assert_eq!(n.len(), 0);
+        }
+        // Non-empty to empty
+        {
+            let mut m = LinkedList::new();
+            let mut n = LinkedList::new();
+            n.push_back(2);
+            m.append(&mut n);
+            check_links(&m);
+            assert_eq!(m.len(), 1);
+            assert_eq!(m.pop_back(), Some(2));
+            assert_eq!(n.len(), 0);
+            check_links(&m);
+        }
+        // Empty to non-empty
+        {
+            let mut m = LinkedList::new();
+            let mut n = LinkedList::new();
+            m.push_back(2);
+            m.append(&mut n);
+            check_links(&m);
+            assert_eq!(m.len(), 1);
+            assert_eq!(m.pop_back(), Some(2));
+            check_links(&m);
+        }
+
+        // Non-empty to non-empty
+        let v = vec![1, 2, 3, 4, 5];
+        let u = vec![9, 8, 1, 2, 3, 4, 5];
+        let mut m = list_from(&v);
+        let mut n = list_from(&u);
+        m.append(&mut n);
+        check_links(&m);
+        let mut sum = v;
+        sum.extend_from_slice(&u);
+        assert_eq!(sum.len(), m.len());
+        for elt in sum {
+            assert_eq!(m.pop_front(), Some(elt))
+        }
+        assert_eq!(n.len(), 0);
+        // let's make sure it's working properly, since we
+        // did some direct changes to private members
+        n.push_back(3);
+        assert_eq!(n.len(), 1);
+        assert_eq!(n.pop_front(), Some(3));
+        check_links(&n);
+    }
+
+    #[test]
+    fn test_insert_prev() {
+        let mut m = list_from(&[0, 2, 4, 6, 8]);
+        let len = m.len();
+        {
+            let mut it = m.iter_mut();
+            it.insert_next(-2);
+            loop {
+                match it.next() {
+                    None => break,
+                    Some(elt) => {
+                        it.insert_next(*elt + 1);
+                        match it.peek_next() {
+                            Some(x) => assert_eq!(*x, *elt + 2),
+                            None => assert_eq!(8, *elt),
+                        }
+                    }
+                }
+            }
+            it.insert_next(0);
+            it.insert_next(1);
+        }
+        check_links(&m);
+        assert_eq!(m.len(), 3 + len * 2);
+        assert_eq!(m.into_iter().collect::<Vec<_>>(),
+                   [-2, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1]);
+    }
+
+    #[test]
+    fn test_send() {
+        let n = list_from(&[1, 2, 3]);
+        thread::spawn(move || {
+            check_links(&n);
+            let a: &[_] = &[&1, &2, &3];
+            assert_eq!(a, &n.iter().collect::<Vec<_>>()[..]);
+        })
+            .join()
+            .ok()
+            .unwrap();
+    }
+
+    #[test]
+    fn test_fuzz() {
+        for _ in 0..25 {
+            fuzz_test(3);
+            fuzz_test(16);
+            fuzz_test(189);
+        }
+    }
+
+    #[test]
+    fn test_26021() {
+        use std::iter::ExactSizeIterator;
+        // There was a bug in split_off that failed to null out the RHS's head's prev ptr.
+        // This caused the RHS's dtor to walk up into the LHS at drop and delete all of
+        // its nodes.
+        //
+        // https://github.com/rust-lang/rust/issues/26021
+        let mut v1 = LinkedList::new();
+        v1.push_front(1);
+        v1.push_front(1);
+        v1.push_front(1);
+        v1.push_front(1);
+        let _ = v1.split_off(3); // Dropping this now should not cause laundry consumption
+        assert_eq!(v1.len(), 3);
+
+        assert_eq!(v1.iter().len(), 3);
+        assert_eq!(v1.iter().collect::<Vec<_>>().len(), 3);
+    }
+
+    #[test]
+    fn test_split_off() {
+        let mut v1 = LinkedList::new();
+        v1.push_front(1);
+        v1.push_front(1);
+        v1.push_front(1);
+        v1.push_front(1);
+
+        // test all splits
+        for ix in 0..1 + v1.len() {
+            let mut a = v1.clone();
+            let b = a.split_off(ix);
+            check_links(&a);
+            check_links(&b);
+            a.extend(b);
+            assert_eq!(v1, a);
+        }
+    }
+
+
+    #[cfg(test)]
+    fn fuzz_test(sz: i32) {
+        let mut m: LinkedList<_> = LinkedList::new();
+        let mut v = vec![];
+        for i in 0..sz {
+            check_links(&m);
+            let r: u8 = thread_rng().next_u32() as u8;
+            match r % 6 {
+                0 => {
+                    m.pop_back();
+                    v.pop();
+                }
+                1 => {
+                    if !v.is_empty() {
+                        m.pop_front();
+                        v.remove(0);
+                    }
+                }
+                2 | 4 => {
+                    m.push_front(-i);
+                    v.insert(0, -i);
+                }
+                3 | 5 | _ => {
+                    m.push_back(i);
+                    v.push(i);
+                }
+            }
+        }
+
+        check_links(&m);
+
+        let mut i = 0;
+        for (a, &b) in m.into_iter().zip(&v) {
+            i += 1;
+            assert_eq!(a, b);
+        }
+        assert_eq!(i, v.len());
+    }
+}