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    //! A doubly-linked list with owned nodes.
    //!
    //! The `LinkedList` allows pushing and popping elements at either end
    //! in constant time.
    //!
    //! NOTE: It is almost always better to use [`Vec`] or [`VecDeque`] because
    //! array-based containers are generally faster,
    //! more memory efficient, and make better use of CPU cache.
    //!
    //! [`Vec`]: crate::vec::Vec
    //! [`VecDeque`]: super::vec_deque::VecDeque

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

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

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

    #[cfg(test)]
    mod tests;

    /// A doubly-linked list with owned nodes.
    ///
    /// The `LinkedList` allows pushing and popping elements at either end
    /// in constant time.
    ///
    /// NOTE: It is almost always better to use `Vec` or `VecDeque` because
    /// array-based containers are generally 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 [`LinkedList::iter()`]. See its
    /// documentation for more.
    #[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<T: fmt::Debug> fmt::Debug for Iter<'_, 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<T> Clone for Iter<'_, T> {
        fn clone(&self) -> Self {
            Iter { ..*self }
        }
    }

    /// A mutable iterator over the elements of a `LinkedList`.
    ///
    /// This `struct` is created by [`LinkedList::iter_mut()`]. See its
    /// documentation for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> {
        // We do *not* exclusively own the entire list here, references to node's `element`
        // have been handed out by the iterator! So be careful when using this; the methods
        // called must be aware that there can be aliasing pointers to `element`.
        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<T: fmt::Debug> fmt::Debug for IterMut<'_, 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`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`into_iter`]: LinkedList::into_iter
    #[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>>) {
            // This method takes care not to create mutable references to whole nodes,
            // to maintain validity of aliasing pointers into `element`.
            unsafe {
                node.next = self.head;
                node.prev = None;
                let node = Some(Box::leak(node).into());

                match self.head {
                    None => self.tail = node,
                    // Not creating new mutable (unique!) references overlapping `element`.
                    Some(head) => (*head.as_ptr()).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>>> {
            // This method takes care not to create mutable references to whole nodes,
            // to maintain validity of aliasing pointers into `element`.
            self.head.map(|node| unsafe {
                let node = Box::from_raw(node.as_ptr());
                self.head = node.next;

                match self.head {
                    None => self.tail = None,
                    // Not creating new mutable (unique!) references overlapping `element`.
                    Some(head) => (*head.as_ptr()).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>>) {
            // This method takes care not to create mutable references to whole nodes,
            // to maintain validity of aliasing pointers into `element`.
            unsafe {
                node.next = None;
                node.prev = self.tail;
                let node = Some(Box::leak(node).into());

                match self.tail {
                    None => self.head = node,
                    // Not creating new mutable (unique!) references overlapping `element`.
                    Some(tail) => (*tail.as_ptr()).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>>> {
            // This method takes care not to create mutable references to whole nodes,
            // to maintain validity of aliasing pointers into `element`.
            self.tail.map(|node| unsafe {
                let node = Box::from_raw(node.as_ptr());
                self.tail = node.prev;

                match self.tail {
                    None => self.head = None,
                    // Not creating new mutable (unique!) references overlapping `element`.
                    Some(tail) => (*tail.as_ptr()).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.
        ///
        /// This method takes care not to create mutable references to `element`, to
        /// maintain validity of aliasing pointers.
        #[inline]
        unsafe fn unlink_node(&mut self, mut node: NonNull<Node<T>>) {
            let node = unsafe { node.as_mut() }; // this one is ours now, we can create an &mut.

            // Not creating new mutable (unique!) references overlapping `element`.
            match node.prev {
                Some(prev) => unsafe { (*prev.as_ptr()).next = node.next },
                // this node is the head node
                None => self.head = node.next,
            };

            match node.next {
                Some(next) => unsafe { (*next.as_ptr()).prev = node.prev },
                // this node is the tail node
                None => self.tail = node.prev,
            };

            self.len -= 1;
        }

        /// Splices a series of nodes between two existing nodes.
        ///
        /// Warning: this will not check that the provided node belongs to the two existing lists.
        #[inline]
        unsafe fn splice_nodes(
            &mut self,
            existing_prev: Option<NonNull<Node<T>>>,
            existing_next: Option<NonNull<Node<T>>>,
            mut splice_start: NonNull<Node<T>>,
            mut splice_end: NonNull<Node<T>>,
            splice_length: usize,
        ) {
            // This method takes care not to create multiple mutable references to whole nodes at the same time,
            // to maintain validity of aliasing pointers into `element`.
            if let Some(mut existing_prev) = existing_prev {
                unsafe {
                    existing_prev.as_mut().next = Some(splice_start);
                }
            } else {
                self.head = Some(splice_start);
            }
            if let Some(mut existing_next) = existing_next {
                unsafe {
                    existing_next.as_mut().prev = Some(splice_end);
                }
            } else {
                self.tail = Some(splice_end);
            }
            unsafe {
                splice_start.as_mut().prev = existing_prev;
                splice_end.as_mut().next = existing_next;
            }

            self.len += splice_length;
        }

        /// Detaches all nodes from a linked list as a series of nodes.
        #[inline]
        fn detach_all_nodes(mut self) -> Option<(NonNull<Node<T>>, NonNull<Node<T>>, usize)> {
            let head = self.head.take();
            let tail = self.tail.take();
            let len = mem::replace(&mut self.len, 0);
            if let Some(head) = head {
                let tail = tail.unwrap_or_else(|| unsafe { core::hint::unreachable_unchecked() });
                Some((head, tail, len))
            } else {
                None
            }
        }

        #[inline]
        unsafe fn split_off_before_node(
            &mut self,
            split_node: Option<NonNull<Node<T>>>,
            at: usize,
        ) -> Self {
            // The split node is the new head node of the second part
            if let Some(mut split_node) = split_node {
                let first_part_head;
                let first_part_tail;
                unsafe {
                    first_part_tail = split_node.as_mut().prev.take();
                }
                if let Some(mut tail) = first_part_tail {
                    unsafe {
                        tail.as_mut().next = None;
                    }
                    first_part_head = self.head;
                } else {
                    first_part_head = None;
                }

                let first_part = LinkedList {
                    head: first_part_head,
                    tail: first_part_tail,
                    len: at,
                    marker: PhantomData,
                };

                // Fix the head ptr of the second part
                self.head = Some(split_node);
                self.len = self.len - at;

                first_part
            } else {
                mem::replace(self, LinkedList::new())
            }
        }

        #[inline]
        unsafe fn split_off_after_node(
            &mut self,
            split_node: Option<NonNull<Node<T>>>,
            at: usize,
        ) -> Self {
            // The split node is the new tail node of the first part and owns
            // the head of the second part.
            if let Some(mut split_node) = split_node {
                let second_part_head;
                let second_part_tail;
                unsafe {
                    second_part_head = split_node.as_mut().next.take();
                }
                if let Some(mut head) = second_part_head {
                    unsafe {
                        head.as_mut().prev = None;
                    }
                    second_part_tail = self.tail;
                } else {
                    second_part_tail = None;
                }

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

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

                second_part
            } else {
                mem::replace(self, LinkedList::new())
            }
        }
    }

    #[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]
        #[rustc_const_stable(feature = "const_linked_list_new", since = "1.32.0")]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub const 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) => {
                    // `as_mut` is okay here because we have exclusive access to the entirety
                    // of both lists.
                    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);
                    }
                }
            }
        }

        /// Moves all elements from `other` to the begin of the list.
        #[unstable(feature = "linked_list_prepend", issue = "none")]
        pub fn prepend(&mut self, other: &mut Self) {
            match self.head {
                None => mem::swap(self, other),
                Some(mut head) => {
                    // `as_mut` is okay here because we have exclusive access to the entirety
                    // of both lists.
                    if let Some(mut other_tail) = other.tail.take() {
                        unsafe {
                            head.as_mut().prev = Some(other_tail);
                            other_tail.as_mut().next = Some(head);
                        }

                        self.head = other.head.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 }
        }

        /// Provides a cursor at the front element.
        ///
        /// The cursor is pointing to the "ghost" non-element if the list is empty.
        #[inline]
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn cursor_front(&self) -> Cursor<'_, T> {
            Cursor { index: 0, current: self.head, list: self }
        }

        /// Provides a cursor with editing operations at the front element.
        ///
        /// The cursor is pointing to the "ghost" non-element if the list is empty.
        #[inline]
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn cursor_front_mut(&mut self) -> CursorMut<'_, T> {
            CursorMut { index: 0, current: self.head, list: self }
        }

        /// Provides a cursor at the back element.
        ///
        /// The cursor is pointing to the "ghost" non-element if the list is empty.
        #[inline]
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn cursor_back(&self) -> Cursor<'_, T> {
            Cursor { index: self.len.checked_sub(1).unwrap_or(0), current: self.tail, list: self }
        }

        /// Provides a cursor with editing operations at the back element.
        ///
        /// The cursor is pointing to the "ghost" non-element if the list is empty.
        #[inline]
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn cursor_back_mut(&mut self) -> CursorMut<'_, T> {
            CursorMut { index: self.len.checked_sub(1).unwrap_or(0), current: self.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.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);
        /// ```
        #[doc(alias = "length")]
        #[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 split = d.split_off(2);
        ///
        /// assert_eq!(split.pop_front(), Some(1));
        /// assert_eq!(split.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::take(self);
            } 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
            };
            unsafe { self.split_off_after_node(split_node, at) }
        }

        /// Removes the element at the given index and returns it.
        ///
        /// This operation should compute in *O*(*n*) time.
        ///
        /// # Panics
        /// Panics if at >= len
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(linked_list_remove)]
        /// use std::collections::LinkedList;
        ///
        /// let mut d = LinkedList::new();
        ///
        /// d.push_front(1);
        /// d.push_front(2);
        /// d.push_front(3);
        ///
        /// assert_eq!(d.remove(1), 2);
        /// assert_eq!(d.remove(0), 3);
        /// assert_eq!(d.remove(0), 1);
        /// ```
        #[unstable(feature = "linked_list_remove", issue = "69210")]
        pub fn remove(&mut self, at: usize) -> T {
            let len = self.len();
            assert!(at < len, "Cannot remove at an index outside of the list bounds");

            // Below, we iterate towards the node at the given index, either from
            // the start or the end, depending on which would be faster.
            let offset_from_end = len - at - 1;
            if at <= offset_from_end {
                let mut cursor = self.cursor_front_mut();
                for _ in 0..at {
                    cursor.move_next();
                }
                cursor.remove_current().unwrap()
            } else {
                let mut cursor = self.cursor_back_mut();
                for _ in 0..offset_from_end {
                    cursor.move_prev();
                }
                cursor.remove_current().unwrap()
            }
        }

        /// 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, pred: filter, idx: 0, old_len }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
        fn drop(&mut self) {
            struct DropGuard<'a, T>(&'a mut LinkedList<T>);

            impl<'a, T> Drop for DropGuard<'a, T> {
                fn drop(&mut self) {
                    // Continue the same loop we do below. This only runs when a destructor has
                    // panicked. If another one panics this will abort.
                    while self.0.pop_front_node().is_some() {}
                }
            }

            while let Some(node) = self.pop_front_node() {
                let guard = DropGuard(self);
                drop(node);
                mem::forget(guard);
            }
        }
    }

    #[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))
        }

        #[inline]
        fn last(mut self) -> Option<&'a T> {
            self.next_back()
        }
    }

    #[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<T> ExactSizeIterator for Iter<'_, T> {}

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

        #[inline]
        fn last(mut self) -> Option<&'a mut T> {
            self.next_back()
        }
    }

    #[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<T> ExactSizeIterator for IterMut<'_, T> {}

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

    /// A cursor over a `LinkedList`.
    ///
    /// A `Cursor` is like an iterator, except that it can freely seek back-and-forth.
    ///
    /// Cursors always rest between two elements in the list, and index in a logically circular way.
    /// To accommodate this, there is a "ghost" non-element that yields `None` between the head and
    /// tail of the list.
    ///
    /// When created, cursors start at the front of the list, or the "ghost" non-element if the list is empty.
    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    pub struct Cursor<'a, T: 'a> {
        index: usize,
        current: Option<NonNull<Node<T>>>,
        list: &'a LinkedList<T>,
    }

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    impl<T> Clone for Cursor<'_, T> {
        fn clone(&self) -> Self {
            let Cursor { index, current, list } = *self;
            Cursor { index, current, list }
        }
    }

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    impl<T: fmt::Debug> fmt::Debug for Cursor<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_tuple("Cursor").field(&self.list).field(&self.index()).finish()
        }
    }

    /// A cursor over a `LinkedList` with editing operations.
    ///
    /// A `Cursor` is like an iterator, except that it can freely seek back-and-forth, and can
    /// safely mutate the list during iteration. This is because the lifetime of its yielded
    /// references is tied to its own lifetime, instead of just the underlying list. This means
    /// cursors cannot yield multiple elements at once.
    ///
    /// Cursors always rest between two elements in the list, and index in a logically circular way.
    /// To accommodate this, there is a "ghost" non-element that yields `None` between the head and
    /// tail of the list.
    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    pub struct CursorMut<'a, T: 'a> {
        index: usize,
        current: Option<NonNull<Node<T>>>,
        list: &'a mut LinkedList<T>,
    }

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    impl<T: fmt::Debug> fmt::Debug for CursorMut<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_tuple("CursorMut").field(&self.list).field(&self.index()).finish()
        }
    }

    impl<'a, T> Cursor<'a, T> {
        /// Returns the cursor position index within the `LinkedList`.
        ///
        /// This returns `None` if the cursor is currently pointing to the
        /// "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn index(&self) -> Option<usize> {
            let _ = self.current?;
            Some(self.index)
        }

        /// Moves the cursor to the next element of the `LinkedList`.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this will move it to
        /// the first element of the `LinkedList`. If it is pointing to the last
        /// element of the `LinkedList` then this will move it to the "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn move_next(&mut self) {
            match self.current.take() {
                // We had no current element; the cursor was sitting at the start position
                // Next element should be the head of the list
                None => {
                    self.current = self.list.head;
                    self.index = 0;
                }
                // We had a previous element, so let's go to its next
                Some(current) => unsafe {
                    self.current = current.as_ref().next;
                    self.index += 1;
                },
            }
        }

        /// Moves the cursor to the previous element of the `LinkedList`.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this will move it to
        /// the last element of the `LinkedList`. If it is pointing to the first
        /// element of the `LinkedList` then this will move it to the "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn move_prev(&mut self) {
            match self.current.take() {
                // No current. We're at the start of the list. Yield None and jump to the end.
                None => {
                    self.current = self.list.tail;
                    self.index = self.list.len().checked_sub(1).unwrap_or(0);
                }
                // Have a prev. Yield it and go to the previous element.
                Some(current) => unsafe {
                    self.current = current.as_ref().prev;
                    self.index = self.index.checked_sub(1).unwrap_or_else(|| self.list.len());
                },
            }
        }

        /// Returns a reference to the element that the cursor is currently
        /// pointing to.
        ///
        /// This returns `None` if the cursor is currently pointing to the
        /// "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn current(&self) -> Option<&'a T> {
            unsafe { self.current.map(|current| &(*current.as_ptr()).element) }
        }

        /// Returns a reference to the next element.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this returns
        /// the first element of the `LinkedList`. If it is pointing to the last
        /// element of the `LinkedList` then this returns `None`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn peek_next(&self) -> Option<&'a T> {
            unsafe {
                let next = match self.current {
                    None => self.list.head,
                    Some(current) => current.as_ref().next,
                };
                next.map(|next| &(*next.as_ptr()).element)
            }
        }

        /// Returns a reference to the previous element.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this returns
        /// the last element of the `LinkedList`. If it is pointing to the first
        /// element of the `LinkedList` then this returns `None`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn peek_prev(&self) -> Option<&'a T> {
            unsafe {
                let prev = match self.current {
                    None => self.list.tail,
                    Some(current) => current.as_ref().prev,
                };
                prev.map(|prev| &(*prev.as_ptr()).element)
            }
        }
    }

    impl<'a, T> CursorMut<'a, T> {
        /// Returns the cursor position index within the `LinkedList`.
        ///
        /// This returns `None` if the cursor is currently pointing to the
        /// "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn index(&self) -> Option<usize> {
            let _ = self.current?;
            Some(self.index)
        }

        /// Moves the cursor to the next element of the `LinkedList`.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this will move it to
        /// the first element of the `LinkedList`. If it is pointing to the last
        /// element of the `LinkedList` then this will move it to the "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn move_next(&mut self) {
            match self.current.take() {
                // We had no current element; the cursor was sitting at the start position
                // Next element should be the head of the list
                None => {
                    self.current = self.list.head;
                    self.index = 0;
                }
                // We had a previous element, so let's go to its next
                Some(current) => unsafe {
                    self.current = current.as_ref().next;
                    self.index += 1;
                },
            }
        }

        /// Moves the cursor to the previous element of the `LinkedList`.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this will move it to
        /// the last element of the `LinkedList`. If it is pointing to the first
        /// element of the `LinkedList` then this will move it to the "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn move_prev(&mut self) {
            match self.current.take() {
                // No current. We're at the start of the list. Yield None and jump to the end.
                None => {
                    self.current = self.list.tail;
                    self.index = self.list.len().checked_sub(1).unwrap_or(0);
                }
                // Have a prev. Yield it and go to the previous element.
                Some(current) => unsafe {
                    self.current = current.as_ref().prev;
                    self.index = self.index.checked_sub(1).unwrap_or_else(|| self.list.len());
                },
            }
        }

        /// Returns a reference to the element that the cursor is currently
        /// pointing to.
        ///
        /// This returns `None` if the cursor is currently pointing to the
        /// "ghost" non-element.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn current(&mut self) -> Option<&mut T> {
            unsafe { self.current.map(|current| &mut (*current.as_ptr()).element) }
        }

        /// Returns a reference to the next element.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this returns
        /// the first element of the `LinkedList`. If it is pointing to the last
        /// element of the `LinkedList` then this returns `None`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn peek_next(&mut self) -> Option<&mut T> {
            unsafe {
                let next = match self.current {
                    None => self.list.head,
                    Some(current) => current.as_ref().next,
                };
                next.map(|next| &mut (*next.as_ptr()).element)
            }
        }

        /// Returns a reference to the previous element.
        ///
        /// If the cursor is pointing to the "ghost" non-element then this returns
        /// the last element of the `LinkedList`. If it is pointing to the first
        /// element of the `LinkedList` then this returns `None`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn peek_prev(&mut self) -> Option<&mut T> {
            unsafe {
                let prev = match self.current {
                    None => self.list.tail,
                    Some(current) => current.as_ref().prev,
                };
                prev.map(|prev| &mut (*prev.as_ptr()).element)
            }
        }

        /// Returns a read-only cursor pointing to the current element.
        ///
        /// The lifetime of the returned `Cursor` is bound to that of the
        /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
        /// `CursorMut` is frozen for the lifetime of the `Cursor`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn as_cursor(&self) -> Cursor<'_, T> {
            Cursor { list: self.list, current: self.current, index: self.index }
        }
    }

    // Now the list editing operations

    impl<'a, T> CursorMut<'a, T> {
        /// Inserts a new element into the `LinkedList` after the current one.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the new element is
        /// inserted at the front of the `LinkedList`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn insert_after(&mut self, item: T) {
            unsafe {
                let spliced_node = Box::leak(Box::new(Node::new(item))).into();
                let node_next = match self.current {
                    None => self.list.head,
                    Some(node) => node.as_ref().next,
                };
                self.list.splice_nodes(self.current, node_next, spliced_node, spliced_node, 1);
                if self.current.is_none() {
                    // The "ghost" non-element's index has changed.
                    self.index = self.list.len;
                }
            }
        }

        /// Inserts a new element into the `LinkedList` before the current one.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the new element is
        /// inserted at the end of the `LinkedList`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn insert_before(&mut self, item: T) {
            unsafe {
                let spliced_node = Box::leak(Box::new(Node::new(item))).into();
                let node_prev = match self.current {
                    None => self.list.tail,
                    Some(node) => node.as_ref().prev,
                };
                self.list.splice_nodes(node_prev, self.current, spliced_node, spliced_node, 1);
                self.index += 1;
            }
        }

        /// Removes the current element from the `LinkedList`.
        ///
        /// The element that was removed is returned, and the cursor is
        /// moved to point to the next element in the `LinkedList`.
        ///
        /// If the cursor is currently pointing to the "ghost" non-element then no element
        /// is removed and `None` is returned.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn remove_current(&mut self) -> Option<T> {
            let unlinked_node = self.current?;
            unsafe {
                self.current = unlinked_node.as_ref().next;
                self.list.unlink_node(unlinked_node);
                let unlinked_node = Box::from_raw(unlinked_node.as_ptr());
                Some(unlinked_node.element)
            }
        }

        /// Removes the current element from the `LinkedList` without deallocating the list node.
        ///
        /// The node that was removed is returned as a new `LinkedList` containing only this node.
        /// The cursor is moved to point to the next element in the current `LinkedList`.
        ///
        /// If the cursor is currently pointing to the "ghost" non-element then no element
        /// is removed and `None` is returned.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn remove_current_as_list(&mut self) -> Option<LinkedList<T>> {
            let mut unlinked_node = self.current?;
            unsafe {
                self.current = unlinked_node.as_ref().next;
                self.list.unlink_node(unlinked_node);

                unlinked_node.as_mut().prev = None;
                unlinked_node.as_mut().next = None;
                Some(LinkedList {
                    head: Some(unlinked_node),
                    tail: Some(unlinked_node),
                    len: 1,
                    marker: PhantomData,
                })
            }
        }

        /// Inserts the elements from the given `LinkedList` after the current one.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the new elements are
        /// inserted at the start of the `LinkedList`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn splice_after(&mut self, list: LinkedList<T>) {
            unsafe {
                let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
                    Some(parts) => parts,
                    _ => return,
                };
                let node_next = match self.current {
                    None => self.list.head,
                    Some(node) => node.as_ref().next,
                };
                self.list.splice_nodes(self.current, node_next, splice_head, splice_tail, splice_len);
                if self.current.is_none() {
                    // The "ghost" non-element's index has changed.
                    self.index = self.list.len;
                }
            }
        }

        /// Inserts the elements from the given `LinkedList` before the current one.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the new elements are
        /// inserted at the end of the `LinkedList`.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn splice_before(&mut self, list: LinkedList<T>) {
            unsafe {
                let (splice_head, splice_tail, splice_len) = match list.detach_all_nodes() {
                    Some(parts) => parts,
                    _ => return,
                };
                let node_prev = match self.current {
                    None => self.list.tail,
                    Some(node) => node.as_ref().prev,
                };
                self.list.splice_nodes(node_prev, self.current, splice_head, splice_tail, splice_len);
                self.index += splice_len;
            }
        }

        /// Splits the list into two after the current element. This will return a
        /// new list consisting of everything after the cursor, with the original
        /// list retaining everything before.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the entire contents
        /// of the `LinkedList` are moved.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn split_after(&mut self) -> LinkedList<T> {
            let split_off_idx = if self.index == self.list.len { 0 } else { self.index + 1 };
            if self.index == self.list.len {
                // The "ghost" non-element's index has changed to 0.
                self.index = 0;
            }
            unsafe { self.list.split_off_after_node(self.current, split_off_idx) }
        }

        /// Splits the list into two before the current element. This will return a
        /// new list consisting of everything before the cursor, with the original
        /// list retaining everything after.
        ///
        /// If the cursor is pointing at the "ghost" non-element then the entire contents
        /// of the `LinkedList` are moved.
        #[unstable(feature = "linked_list_cursors", issue = "58533")]
        pub fn split_before(&mut self) -> LinkedList<T> {
            let split_off_idx = self.index;
            self.index = 0;
            unsafe { self.list.split_off_before_node(self.current, split_off_idx) }
        }
    }

    /// 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<T, F> Iterator for DrainFilter<'_, 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) {
                        // `unlink_node` is okay with aliasing `element` references.
                        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<T, F> Drop for DrainFilter<'_, T, F>
    where
        F: FnMut(&mut T) -> bool,
    {
        fn drop(&mut self) {
            struct DropGuard<'r, 'a, T, F>(&'r mut DrainFilter<'a, T, F>)
            where
                F: FnMut(&mut T) -> bool;

            impl<'r, 'a, T, F> Drop for DropGuard<'r, 'a, T, F>
            where
                F: FnMut(&mut T) -> bool,
            {
                fn drop(&mut self) {
                    self.0.for_each(drop);
                }
            }

            while let Some(item) = self.next() {
                let guard = DropGuard(self);
                drop(item);
                mem::forget(guard);
            }
        }
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<T: fmt::Debug, F> fmt::Debug for DrainFilter<'_, 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);
        }

        #[inline]
        fn extend_one(&mut self, elem: T) {
            self.push_back(elem);
        }
    }

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

        #[inline]
        fn extend_one(&mut self, &elem: &'a T) {
            self.push_back(elem);
        }
    }

    #[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()
        }

        fn clone_from(&mut self, other: &Self) {
            let mut iter_other = other.iter();
            if self.len() > other.len() {
                self.split_off(other.len());
            }
            for (elem, elem_other) in self.iter_mut().zip(&mut iter_other) {
                elem.clone_from(elem_other);
            }
            if !iter_other.is_empty() {
                self.extend(iter_other.cloned());
            }
        }
    }

    #[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<T: Sync> Send for Iter<'_, T> {}

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

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

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

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    unsafe impl<T: Sync> Send for Cursor<'_, T> {}

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    unsafe impl<T: Sync> Sync for Cursor<'_, T> {}

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    unsafe impl<T: Send> Send for CursorMut<'_, T> {}

    #[unstable(feature = "linked_list_cursors", issue = "58533")]
    unsafe impl<T: Sync> Sync for CursorMut<'_, T> {}