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    //! A doubly-linked list with owned nodes.
    //!
    //! The `LinkedList` allows pushing and popping elements at either end
    //! in constant time.
    //!
    //! Almost always it is better to use `Vec` or [`VecDeque`] instead of
    //! [`LinkedList`]. In general, array-based containers are faster,
    //! more memory efficient and make better use of CPU cache.
    //!
    //! [`LinkedList`]: ../linked_list/struct.LinkedList.html
    //! [`VecDeque`]: ../vec_deque/struct.VecDeque.html

    #![stable(feature = "rust1", since = "1.0.0")]

    use core::cmp::Ordering;
    use core::fmt;
    use core::hash::{Hasher, Hash};
    use core::iter::{FromIterator, FusedIterator};
    use core::marker::PhantomData;
    use core::mem;
    use core::ptr::NonNull;

    use boxed::Box;
    use super::SpecExtend;

    /// A doubly-linked list with owned nodes.
    ///
    /// The `LinkedList` allows pushing and popping elements at either end
    /// in constant time.
    ///
    /// Almost always it is better to use `Vec` or `VecDeque` instead of
    /// `LinkedList`. In general, array-based containers are faster,
    /// more memory efficient and make better use of CPU cache.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct LinkedList<T> {
        head: Option<NonNull<Node<T>>>,
        tail: Option<NonNull<Node<T>>>,
        len: usize,
        marker: PhantomData<Box<Node<T>>>,
    }

    struct Node<T> {
        next: Option<NonNull<Node<T>>>,
        prev: Option<NonNull<Node<T>>>,
        element: T,
    }

    /// An iterator over the elements of a `LinkedList`.
    ///
    /// This `struct` is created by the [`iter`] method on [`LinkedList`]. See its
    /// documentation for more.
    ///
    /// [`iter`]: struct.LinkedList.html#method.iter
    /// [`LinkedList`]: struct.LinkedList.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        head: Option<NonNull<Node<T>>>,
        tail: Option<NonNull<Node<T>>>,
        len: usize,
        marker: PhantomData<&'a Node<T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Iter<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Iter")
             .field(&self.len)
             .finish()
        }
    }

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Self {
            Iter { ..*self }
        }
    }

    /// A mutable iterator over the elements of a `LinkedList`.
    ///
    /// This `struct` is created by the [`iter_mut`] method on [`LinkedList`]. See its
    /// documentation for more.
    ///
    /// [`iter_mut`]: struct.LinkedList.html#method.iter_mut
    /// [`LinkedList`]: struct.LinkedList.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> {
        list: &'a mut LinkedList<T>,
        head: Option<NonNull<Node<T>>>,
        tail: Option<NonNull<Node<T>>>,
        len: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for IterMut<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("IterMut")
             .field(&self.list)
             .field(&self.len)
             .finish()
        }
    }

    /// An owning iterator over the elements of a `LinkedList`.
    ///
    /// This `struct` is created by the [`into_iter`] method on [`LinkedList`][`LinkedList`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`into_iter`]: struct.LinkedList.html#method.into_iter
    /// [`LinkedList`]: struct.LinkedList.html
    #[derive(Clone)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> {
        list: LinkedList<T>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("IntoIter")
             .field(&self.list)
             .finish()
        }
    }

    impl<T> Node<T> {
        fn new(element: T) -> Self {
            Node {
                next: None,
                prev: None,
                element,
            }
        }

        fn into_element(self: Box<Self>) -> T {
            self.element
        }
    }

    // private methods
    impl<T> LinkedList<T> {
        /// Adds the given node to the front of the list.
        #[inline]
        fn push_front_node(&mut self, mut node: Box<Node<T>>) {
            unsafe {
                node.next = self.head;
                node.prev = None;
                let node = Some(Box::into_raw_non_null(node));

                match self.head {
                    None => self.tail = node,
                    Some(mut head) => head.as_mut().prev = node,
                }

                self.head = node;
                self.len += 1;
            }
        }

        /// Removes and returns the node at the front of the list.
        #[inline]
        fn pop_front_node(&mut self) -> Option<Box<Node<T>>> {
            self.head.map(|node| unsafe {
                let node = Box::from_raw(node.as_ptr());
                self.head = node.next;

                match self.head {
                    None => self.tail = None,
                    Some(mut head) => head.as_mut().prev = None,
                }

                self.len -= 1;
                node
            })
        }

        /// Adds the given node to the back of the list.
        #[inline]
        fn push_back_node(&mut self, mut node: Box<Node<T>>) {
            unsafe {
                node.next = None;
                node.prev = self.tail;
                let node = Some(Box::into_raw_non_null(node));

                match self.tail {
                    None => self.head = node,
                    Some(mut tail) => tail.as_mut().next = node,
                }

                self.tail = node;
                self.len += 1;
            }
        }

        /// Removes and returns the node at the back of the list.
        #[inline]
        fn pop_back_node(&mut self) -> Option<Box<Node<T>>> {
            self.tail.map(|node| unsafe {
                let node = Box::from_raw(node.as_ptr());
                self.tail = node.prev;

                match self.tail {
                    None => self.head = None,
                    Some(mut tail) => tail.as_mut().next = None,
                }

                self.len -= 1;
                node
            })
        }

        /// Unlinks the specified node from the current list.
        ///
        /// Warning: this will not check that the provided node belongs to the current list.
        #[inline]
        unsafe fn unlink_node(&mut self, mut node: NonNull<Node<T>>) {
            let node = node.as_mut();

            match node.prev {
                Some(mut prev) => prev.as_mut().next = node.next.clone(),
                // this node is the head node
                None => self.head = node.next.clone(),
            };

            match node.next {
                Some(mut next) => next.as_mut().prev = node.prev.clone(),
                // this node is the tail node
                None => self.tail = node.prev.clone(),
            };

            self.len -= 1;
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for LinkedList<T> {
        /// Creates an empty `LinkedList<T>`.
        #[inline]
        fn default() -> Self {
            Self::new()
        }
    }

    impl<T> LinkedList<T> {
        /// Creates an empty `LinkedList`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::LinkedList;
        ///
        /// let list: LinkedList<u32> = LinkedList::new();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> Self {
            LinkedList {
                head: None,
                tail: None,
                len: 0,
                marker: PhantomData,
            }
        }

        /// 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 list1 = LinkedList::new();
        /// list1.push_back('a');
        ///
        /// let mut list2 = LinkedList::new();
        /// list2.push_back('b');
        /// list2.push_back('c');
        ///
        /// list1.append(&mut list2);
        ///
        /// let mut iter = list1.iter();
        /// assert_eq!(iter.next(), Some(&'a'));
        /// assert_eq!(iter.next(), Some(&'b'));
        /// assert_eq!(iter.next(), Some(&'c'));
        /// assert!(iter.next().is_none());
        ///
        /// assert!(list2.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn append(&mut self, other: &mut Self) {
            match self.tail {
                None => mem::swap(self, other),
                Some(mut tail) => {
                    if let Some(mut other_head) = other.head.take() {
                        unsafe {
                            tail.as_mut().next = Some(other_head);
                            other_head.as_mut().prev = Some(tail);
                        }

                        self.tail = other.tail.take();
                        self.len += mem::replace(&mut other.len, 0);
                    }
                }
            }
        }

        /// Provides a forward iterator.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::LinkedList;
        ///
        /// let mut list: LinkedList<u32> = LinkedList::new();
        ///
        /// list.push_back(0);
        /// list.push_back(1);
        /// list.push_back(2);
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter {
                head: self.head,
                tail: self.tail,
                len: self.len,
                marker: PhantomData,
            }
        }

        /// Provides a forward iterator with mutable references.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::LinkedList;
        ///
        /// let mut list: LinkedList<u32> = LinkedList::new();
        ///
        /// list.push_back(0);
        /// list.push_back(1);
        /// list.push_back(2);
        ///
        /// for element in list.iter_mut() {
        ///     *element += 10;
        /// }
        ///
        /// let mut iter = list.iter();
        /// assert_eq!(iter.next(), Some(&10));
        /// assert_eq!(iter.next(), Some(&11));
        /// assert_eq!(iter.next(), Some(&12));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut {
                head: self.head,
                tail: self.tail,
                len: self.len,
                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.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.len
        }

        /// 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 = Self::new();
        }

        /// Returns `true` if the `LinkedList` contains an element equal to the
        /// given value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::LinkedList;
        ///
        /// let mut list: LinkedList<u32> = LinkedList::new();
        ///
        /// list.push_back(0);
        /// list.push_back(1);
        /// list.push_back(2);
        ///
        /// assert_eq!(list.contains(&0), true);
        /// assert_eq!(list.contains(&10), false);
        /// ```
        #[stable(feature = "linked_list_contains", since = "1.12.0")]
        pub fn contains(&self, x: &T) -> bool
            where T: PartialEq<T>
        {
            self.iter().any(|e| e == x)
        }

        /// 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> {
            unsafe {
                self.head.as_ref().map(|node| &node.as_ref().element)
            }
        }

        /// 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> {
            unsafe {
                self.head.as_mut().map(|node| &mut node.as_mut().element)
            }
        }

        /// 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.tail.as_ref().map(|node| &node.as_ref().element)
            }
        }

        /// 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.tail.as_mut().map(|node| &mut node.as_mut().element)
            }
        }

        /// 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(Node::into_element)
        }

        /// Appends an element to the back of a list.
        ///
        /// This operation should compute in O(1) time.
        ///
        /// # 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.
        ///
        /// This operation should compute in O(1) time.
        ///
        /// # 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(Node::into_element)
        }

        /// 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.
        ///
        /// # Panics
        ///
        /// Panics if `at > len`.
        ///
        /// # 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, Self::new());
            } else if at == len {
                return Self::new();
            }

            // Below, we iterate towards the `i-1`th node, either from the start or the end,
            // depending on which would be faster.
            let 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 second_part_head;

            unsafe {
                second_part_head = split_node.unwrap().as_mut().next.take();
                if let Some(mut head) = second_part_head {
                    head.as_mut().prev = None;
                }
            }

            let second_part = LinkedList {
                head: second_part_head,
                tail: self.tail,
                len: len - at,
                marker: PhantomData,
            };

            // Fix the tail ptr of the first part
            self.tail = split_node;
            self.len = at;

            second_part
        }

        /// Creates an iterator which uses a closure to determine if an element should be removed.
        ///
        /// If the closure returns true, then the element is removed and yielded.
        /// If the closure returns false, the element will remain in the list and will not be yielded
        /// by the iterator.
        ///
        /// Note that `drain_filter` lets you mutate every element in the filter closure, regardless of
        /// whether you choose to keep or remove it.
        ///
        /// # Examples
        ///
        /// Splitting a list into evens and odds, reusing the original list:
        ///
        /// ```
        /// #![feature(drain_filter)]
        /// use std::collections::LinkedList;
        ///
        /// let mut numbers: LinkedList<u32> = LinkedList::new();
        /// numbers.extend(&[1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15]);
        ///
        /// let evens = numbers.drain_filter(|x| *x % 2 == 0).collect::<LinkedList<_>>();
        /// let odds = numbers;
        ///
        /// assert_eq!(evens.into_iter().collect::<Vec<_>>(), vec![2, 4, 6, 8, 14]);
        /// assert_eq!(odds.into_iter().collect::<Vec<_>>(), vec![1, 3, 5, 9, 11, 13, 15]);
        /// ```
        #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
        pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<T, F>
            where F: FnMut(&mut T) -> bool
        {
            // avoid borrow issues.
            let it = self.head;
            let old_len = self.len;

            DrainFilter {
                list: self,
                it: it,
                pred: filter,
                idx: 0,
                old_len: old_len,
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
        fn drop(&mut self) {
            while let Some(_) = self.pop_front_node() {}
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        #[inline]
        fn next(&mut self) -> Option<&'a T> {
            if self.len == 0 {
                None
            } else {
                self.head.map(|node| unsafe {
                    // Need an unbound lifetime to get 'a
                    let node = &*node.as_ptr();
                    self.len -= 1;
                    self.head = node.next;
                    &node.element
                })
            }
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a T> {
            if self.len == 0 {
                None
            } else {
                self.tail.map(|node| unsafe {
                    // Need an unbound lifetime to get 'a
                    let node = &*node.as_ptr();
                    self.len -= 1;
                    self.tail = node.prev;
                    &node.element
                })
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for Iter<'a, T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T> FusedIterator for Iter<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for IterMut<'a, T> {
        type Item = &'a mut T;

        #[inline]
        fn next(&mut self) -> Option<&'a mut T> {
            if self.len == 0 {
                None
            } else {
                self.head.map(|node| unsafe {
                    // Need an unbound lifetime to get 'a
                    let node = &mut *node.as_ptr();
                    self.len -= 1;
                    self.head = node.next;
                    &mut node.element
                })
            }
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a mut T> {
            if self.len == 0 {
                None
            } else {
                self.tail.map(|node| unsafe {
                    // Need an unbound lifetime to get 'a
                    let node = &mut *node.as_ptr();
                    self.len -= 1;
                    self.tail = node.prev;
                    &mut node.element
                })
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T> FusedIterator for IterMut<'a, T> {}

    impl<'a, T> IterMut<'a, T> {
        /// Inserts the given element 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, element: T) {
            match self.head {
                None => self.list.push_back(element),
                Some(mut head) => unsafe {
                    let mut prev = match head.as_ref().prev {
                        None => return self.list.push_front(element),
                        Some(prev) => prev,
                    };

                    let node = Some(Box::into_raw_non_null(box Node {
                        next: Some(head),
                        prev: Some(prev),
                        element,
                    }));

                    prev.as_mut().next = node;
                    head.as_mut().prev = node;

                    self.list.len += 1;
                },
            }
        }

        /// 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 T> {
            if self.len == 0 {
                None
            } else {
                unsafe {
                    self.head.as_mut().map(|node| &mut node.as_mut().element)
                }
            }
        }
    }

    /// An iterator produced by calling `drain_filter` on LinkedList.
    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    pub struct DrainFilter<'a, T: 'a, F: 'a>
        where F: FnMut(&mut T) -> bool,
    {
        list: &'a mut LinkedList<T>,
        it: Option<NonNull<Node<T>>>,
        pred: F,
        idx: usize,
        old_len: usize,
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
        where F: FnMut(&mut T) -> bool,
    {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            while let Some(mut node) = self.it {
                unsafe {
                    self.it = node.as_ref().next;
                    self.idx += 1;

                    if (self.pred)(&mut node.as_mut().element) {
                        self.list.unlink_node(node);
                        return Some(Box::from_raw(node.as_ptr()).element);
                    }
                }
            }

            None
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, Some(self.old_len - self.idx))
        }
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<'a, T, F> Drop for DrainFilter<'a, T, F>
        where F: FnMut(&mut T) -> bool,
    {
        fn drop(&mut self) {
            self.for_each(drop);
        }
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<'a, T: 'a + fmt::Debug, F> fmt::Debug for DrainFilter<'a, T, F>
        where F: FnMut(&mut T) -> bool
    {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("DrainFilter")
             .field(&self.list)
             .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> DoubleEndedIterator for IntoIter<T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> {
            self.list.pop_back()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for IntoIter<T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for IntoIter<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> FromIterator<T> for LinkedList<T> {
        fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
            let mut list = Self::new();
            list.extend(iter);
            list
        }
    }

    #[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(self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Extend<T> for LinkedList<T> {
        fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
            <Self as SpecExtend<I>>::spec_extend(self, iter);
        }
    }

    impl<I: IntoIterator> SpecExtend<I> for LinkedList<I::Item> {
        default fn spec_extend(&mut self, iter: I) {
            for elt in iter {
                self.push_back(elt);
            }
        }
    }

    impl<T> SpecExtend<LinkedList<T>> for LinkedList<T> {
        fn spec_extend(&mut self, ref mut other: LinkedList<T>) {
            self.append(other);
        }
    }

    #[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<T: PartialEq> PartialEq for LinkedList<T> {
        fn eq(&self, other: &Self) -> bool {
            self.len() == other.len() && self.iter().eq(other)
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Eq> Eq for LinkedList<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: PartialOrd> PartialOrd for LinkedList<T> {
        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
            self.iter().partial_cmp(other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> Ord for LinkedList<T> {
        #[inline]
        fn cmp(&self, other: &Self) -> Ordering {
            self.iter().cmp(other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for LinkedList<T> {
        fn clone(&self) -> Self {
            self.iter().cloned().collect()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Debug> fmt::Debug for LinkedList<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self).finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Hash> Hash for LinkedList<T> {
        fn hash<H: Hasher>(&self, state: &mut H) {
            self.len().hash(state);
            for elt in self {
                elt.hash(state);
            }
        }
    }

    // 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
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for LinkedList<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Sync> Sync for LinkedList<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<'a, T: Sync> Send for Iter<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<'a, T: Sync> Sync for Iter<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<'a, T: Send> Send for IterMut<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<'a, T: Sync> Sync for IterMut<'a, T> {}

    #[cfg(test)]
    mod tests {
        use std::thread;
        use std::vec::Vec;

        use rand::{thread_rng, RngCore};

        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>) {
            unsafe {
                let mut len = 0;
                let mut last_ptr: Option<&Node<T>> = None;
                let mut node_ptr: &Node<T>;
                match list.head {
                    None => {
                        // tail node should also be None.
                        assert!(list.tail.is_none());
                        assert_eq!(0, list.len);
                        return;
                    }
                    Some(node) => node_ptr = &*node.as_ptr(),
                }
                loop {
                    match (last_ptr, node_ptr.prev) {
                        (None, None) => {}
                        (None, _) => panic!("prev link for head"),
                        (Some(p), Some(pptr)) => {
                            assert_eq!(p as *const Node<T>, pptr.as_ptr() as *const Node<T>);
                        }
                        _ => panic!("prev link is none, not good"),
                    }
                    match node_ptr.next {
                        Some(next) => {
                            last_ptr = Some(node_ptr);
                            node_ptr = &*next.as_ptr();
                            len += 1;
                        }
                        None => {
                            len += 1;
                            break;
                        }
                    }
                }

                // verify that the tail node points to the last node.
                let tail = list.tail.as_ref().expect("some tail node").as_ref();
                assert_eq!(tail as *const Node<T>, node_ptr as *const Node<T>);
                // check that len matches interior links.
                assert_eq!(len, list.len);
            }
        }

        #[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]
        #[cfg_attr(target_os = "emscripten", ignore)]
        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() {
            // 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());
        }

        #[test]
        fn drain_filter_test() {
            let mut m: LinkedList<u32> = LinkedList::new();
            m.extend(&[1, 2, 3, 4, 5, 6]);
            let deleted = m.drain_filter(|v| *v < 4).collect::<Vec<_>>();

            check_links(&m);

            assert_eq!(deleted, &[1, 2, 3]);
            assert_eq!(m.into_iter().collect::<Vec<_>>(), &[4, 5, 6]);
        }

        #[test]
        fn drain_to_empty_test() {
            let mut m: LinkedList<u32> = LinkedList::new();
            m.extend(&[1, 2, 3, 4, 5, 6]);
            let deleted = m.drain_filter(|_| true).collect::<Vec<_>>();

            check_links(&m);

            assert_eq!(deleted, &[1, 2, 3, 4, 5, 6]);
            assert_eq!(m.into_iter().collect::<Vec<_>>(), &[]);
        }
    }

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