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    #![doc(html_root_url = "https://docs.rs/slab/0.4.2")]
    #![deny(warnings, missing_docs, missing_debug_implementations)]
    #![cfg_attr(test, deny(warnings, unreachable_pub))]

    //! Pre-allocated storage for a uniform data type.
    //!
    //! `Slab` provides pre-allocated storage for a single data type. If many values
    //! of a single type are being allocated, it can be more efficient to
    //! pre-allocate the necessary storage. Since the size of the type is uniform,
    //! memory fragmentation can be avoided. Storing, clearing, and lookup
    //! operations become very cheap.
    //!
    //! While `Slab` may look like other Rust collections, it is not intended to be
    //! used as a general purpose collection. The primary difference between `Slab`
    //! and `Vec` is that `Slab` returns the key when storing the value.
    //!
    //! It is important to note that keys may be reused. In other words, once a
    //! value associated with a given key is removed from a slab, that key may be
    //! returned from future calls to `insert`.
    //!
    //! # Examples
    //!
    //! Basic storing and retrieval.
    //!
    //! ```
    //! # use slab::*;
    //! let mut slab = Slab::new();
    //!
    //! let hello = slab.insert("hello");
    //! let world = slab.insert("world");
    //!
    //! assert_eq!(slab[hello], "hello");
    //! assert_eq!(slab[world], "world");
    //!
    //! slab[world] = "earth";
    //! assert_eq!(slab[world], "earth");
    //! ```
    //!
    //! Sometimes it is useful to be able to associate the key with the value being
    //! inserted in the slab. This can be done with the `vacant_entry` API as such:
    //!
    //! ```
    //! # use slab::*;
    //! let mut slab = Slab::new();
    //!
    //! let hello = {
    //!     let entry = slab.vacant_entry();
    //!     let key = entry.key();
    //!
    //!     entry.insert((key, "hello"));
    //!     key
    //! };
    //!
    //! assert_eq!(hello, slab[hello].0);
    //! assert_eq!("hello", slab[hello].1);
    //! ```
    //!
    //! It is generally a good idea to specify the desired capacity of a slab at
    //! creation time. Note that `Slab` will grow the internal capacity when
    //! attempting to insert a new value once the existing capacity has been reached.
    //! To avoid this, add a check.
    //!
    //! ```
    //! # use slab::*;
    //! let mut slab = Slab::with_capacity(1024);
    //!
    //! // ... use the slab
    //!
    //! if slab.len() == slab.capacity() {
    //!     panic!("slab full");
    //! }
    //!
    //! slab.insert("the slab is not at capacity yet");
    //! ```
    //!
    //! # Capacity and reallocation
    //!
    //! The capacity of a slab is the amount of space allocated for any future
    //! values that will be inserted in the slab. This is not to be confused with
    //! the *length* of the slab, which specifies the number of actual values
    //! currently being inserted. If a slab's length is equal to its capacity, the
    //! next value inserted into the slab will require growing the slab by
    //! reallocating.
    //!
    //! For example, a slab with capacity 10 and length 0 would be an empty slab
    //! with space for 10 more stored values. Storing 10 or fewer elements into the
    //! slab will not change its capacity or cause reallocation to occur. However,
    //! if the slab length is increased to 11 (due to another `insert`), it will
    //! have to reallocate, which can be slow. For this reason, it is recommended to
    //! use [`Slab::with_capacity`] whenever possible to specify how many values the
    //! slab is expected to store.
    //!
    //! # Implementation
    //!
    //! `Slab` is backed by a `Vec` of slots. Each slot is either occupied or
    //! vacant. `Slab` maintains a stack of vacant slots using a linked list. To
    //! find a vacant slot, the stack is popped. When a slot is released, it is
    //! pushed onto the stack.
    //!
    //! If there are no more available slots in the stack, then `Vec::reserve(1)` is
    //! called and a new slot is created.
    //!
    //! [`Slab::with_capacity`]: struct.Slab.html#with_capacity

    use std::iter::IntoIterator;
    use std::ops;
    use std::vec;
    use std::{fmt, mem};

    /// Pre-allocated storage for a uniform data type
    ///
    /// See the [module documentation] for more details.
    ///
    /// [module documentation]: index.html
    #[derive(Clone)]
    pub struct Slab<T> {
        // Chunk of memory
        entries: Vec<Entry<T>>,

        // Number of Filled elements currently in the slab
        len: usize,

        // Offset of the next available slot in the slab. Set to the slab's
        // capacity when the slab is full.
        next: usize,
    }

    impl<T> Default for Slab<T> {
        fn default() -> Self {
            Slab::new()
        }
    }

    /// A handle to a vacant entry in a `Slab`.
    ///
    /// `VacantEntry` allows constructing values with the key that they will be
    /// assigned to.
    ///
    /// # Examples
    ///
    /// ```
    /// # use slab::*;
    /// let mut slab = Slab::new();
    ///
    /// let hello = {
    ///     let entry = slab.vacant_entry();
    ///     let key = entry.key();
    ///
    ///     entry.insert((key, "hello"));
    ///     key
    /// };
    ///
    /// assert_eq!(hello, slab[hello].0);
    /// assert_eq!("hello", slab[hello].1);
    /// ```
    #[derive(Debug)]
    pub struct VacantEntry<'a, T: 'a> {
        slab: &'a mut Slab<T>,
        key: usize,
    }

    /// An iterator over the values stored in the `Slab`
    pub struct Iter<'a, T: 'a> {
        entries: std::slice::Iter<'a, Entry<T>>,
        curr: usize,
    }

    /// A mutable iterator over the values stored in the `Slab`
    pub struct IterMut<'a, T: 'a> {
        entries: std::slice::IterMut<'a, Entry<T>>,
        curr: usize,
    }

    /// A draining iterator for `Slab`
    pub struct Drain<'a, T: 'a>(vec::Drain<'a, Entry<T>>);

    #[derive(Clone)]
    enum Entry<T> {
        Vacant(usize),
        Occupied(T),
    }

    impl<T> Slab<T> {
        /// Construct a new, empty `Slab`.
        ///
        /// The function does not allocate and the returned slab will have no
        /// capacity until `insert` is called or capacity is explicitly reserved.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let slab: Slab<i32> = Slab::new();
        /// ```
        pub fn new() -> Slab<T> {
            Slab::with_capacity(0)
        }

        /// Construct a new, empty `Slab` with the specified capacity.
        ///
        /// The returned slab will be able to store exactly `capacity` without
        /// reallocating. If `capacity` is 0, the slab will not allocate.
        ///
        /// It is important to note that this function does not specify the *length*
        /// of the returned slab, but only the capacity. For an explanation of the
        /// difference between length and capacity, see [Capacity and
        /// reallocation](index.html#capacity-and-reallocation).
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::with_capacity(10);
        ///
        /// // The slab contains no values, even though it has capacity for more
        /// assert_eq!(slab.len(), 0);
        ///
        /// // These are all done without reallocating...
        /// for i in 0..10 {
        ///     slab.insert(i);
        /// }
        ///
        /// // ...but this may make the slab reallocate
        /// slab.insert(11);
        /// ```
        pub fn with_capacity(capacity: usize) -> Slab<T> {
            Slab {
                entries: Vec::with_capacity(capacity),
                next: 0,
                len: 0,
            }
        }

        /// Return the number of values the slab can store without reallocating.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let slab: Slab<i32> = Slab::with_capacity(10);
        /// assert_eq!(slab.capacity(), 10);
        /// ```
        pub fn capacity(&self) -> usize {
            self.entries.capacity()
        }

        /// Reserve capacity for at least `additional` more values to be stored
        /// without allocating.
        ///
        /// `reserve` does nothing if the slab already has sufficient capacity for
        /// `additional` more values. If more capacity is required, a new segment of
        /// memory will be allocated and all existing values will be copied into it.
        /// As such, if the slab is already very large, a call to `reserve` can end
        /// up being expensive.
        ///
        /// The slab may reserve more than `additional` extra space in order to
        /// avoid frequent reallocations. Use `reserve_exact` instead to guarantee
        /// that only the requested space is allocated.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// slab.insert("hello");
        /// slab.reserve(10);
        /// assert!(slab.capacity() >= 11);
        /// ```
        pub fn reserve(&mut self, additional: usize) {
            if self.capacity() - self.len >= additional {
                return;
            }
            let need_add = self.len + additional - self.entries.len();
            self.entries.reserve(need_add);
        }

        /// Reserve the minimum capacity required to store exactly `additional`
        /// more values.
        ///
        /// `reserve_exact` does nothing if the slab already has sufficient capacity
        /// for `additional` more valus. If more capacity is required, a new segment
        /// of memory will be allocated and all existing values will be copied into
        /// it.  As such, if the slab is already very large, a call to `reserve` can
        /// end up being expensive.
        ///
        /// Note that the allocator may give the slab more space than it requests.
        /// Therefore capacity can not be relied upon to be precisely minimal.
        /// Prefer `reserve` if future insertions are expected.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// slab.insert("hello");
        /// slab.reserve_exact(10);
        /// assert!(slab.capacity() >= 11);
        /// ```
        pub fn reserve_exact(&mut self, additional: usize) {
            if self.capacity() - self.len >= additional {
                return;
            }
            let need_add = self.len + additional - self.entries.len();
            self.entries.reserve_exact(need_add);
        }

        /// Shrink the capacity of the slab as much as possible.
        ///
        /// It will drop down as close as possible to the length but the allocator
        /// may still inform the vector that there is space for a few more elements.
        /// Also, since values are not moved, the slab cannot shrink past any stored
        /// values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::with_capacity(10);
        ///
        /// for i in 0..3 {
        ///     slab.insert(i);
        /// }
        ///
        /// assert_eq!(slab.capacity(), 10);
        /// slab.shrink_to_fit();
        /// assert!(slab.capacity() >= 3);
        /// ```
        ///
        /// In this case, even though two values are removed, the slab cannot shrink
        /// past the last value.
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::with_capacity(10);
        ///
        /// for i in 0..3 {
        ///     slab.insert(i);
        /// }
        ///
        /// slab.remove(0);
        /// slab.remove(1);
        ///
        /// assert_eq!(slab.capacity(), 10);
        /// slab.shrink_to_fit();
        /// assert!(slab.capacity() >= 3);
        /// ```
        pub fn shrink_to_fit(&mut self) {
            self.entries.shrink_to_fit();
        }

        /// Clear the slab of all values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// for i in 0..3 {
        ///     slab.insert(i);
        /// }
        ///
        /// slab.clear();
        /// assert!(slab.is_empty());
        /// ```
        pub fn clear(&mut self) {
            self.entries.clear();
            self.len = 0;
            self.next = 0;
        }

        /// Return the number of stored values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// for i in 0..3 {
        ///     slab.insert(i);
        /// }
        ///
        /// assert_eq!(3, slab.len());
        /// ```
        pub fn len(&self) -> usize {
            self.len
        }

        /// Return `true` if there are no values stored in the slab.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// assert!(slab.is_empty());
        ///
        /// slab.insert(1);
        /// assert!(!slab.is_empty());
        /// ```
        pub fn is_empty(&self) -> bool {
            self.len == 0
        }

        /// Return an iterator over the slab.
        ///
        /// This function should generally be **avoided** as it is not efficient.
        /// Iterators must iterate over every slot in the slab even if it is
        /// vacant. As such, a slab with a capacity of 1 million but only one
        /// stored value must still iterate the million slots.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// for i in 0..3 {
        ///     slab.insert(i);
        /// }
        ///
        /// let mut iterator = slab.iter();
        ///
        /// assert_eq!(iterator.next(), Some((0, &0)));
        /// assert_eq!(iterator.next(), Some((1, &1)));
        /// assert_eq!(iterator.next(), Some((2, &2)));
        /// assert_eq!(iterator.next(), None);
        /// ```
        pub fn iter(&self) -> Iter<T> {
            Iter {
                entries: self.entries.iter(),
                curr: 0,
            }
        }

        /// Return an iterator that allows modifying each value.
        ///
        /// This function should generally be **avoided** as it is not efficient.
        /// Iterators must iterate over every slot in the slab even if it is
        /// vacant. As such, a slab with a capacity of 1 million but only one
        /// stored value must still iterate the million slots.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let key1 = slab.insert(0);
        /// let key2 = slab.insert(1);
        ///
        /// for (key, val) in slab.iter_mut() {
        ///     if key == key1 {
        ///         *val += 2;
        ///     }
        /// }
        ///
        /// assert_eq!(slab[key1], 2);
        /// assert_eq!(slab[key2], 1);
        /// ```
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut {
                entries: self.entries.iter_mut(),
                curr: 0,
            }
        }

        /// Return a reference to the value associated with the given key.
        ///
        /// If the given key is not associated with a value, then `None` is
        /// returned.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// let key = slab.insert("hello");
        ///
        /// assert_eq!(slab.get(key), Some(&"hello"));
        /// assert_eq!(slab.get(123), None);
        /// ```
        pub fn get(&self, key: usize) -> Option<&T> {
            match self.entries.get(key) {
                Some(&Entry::Occupied(ref val)) => Some(val),
                _ => None,
            }
        }

        /// Return a mutable reference to the value associated with the given key.
        ///
        /// If the given key is not associated with a value, then `None` is
        /// returned.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// let key = slab.insert("hello");
        ///
        /// *slab.get_mut(key).unwrap() = "world";
        ///
        /// assert_eq!(slab[key], "world");
        /// assert_eq!(slab.get_mut(123), None);
        /// ```
        pub fn get_mut(&mut self, key: usize) -> Option<&mut T> {
            match self.entries.get_mut(key) {
                Some(&mut Entry::Occupied(ref mut val)) => Some(val),
                _ => None,
            }
        }

        /// Return a reference to the value associated with the given key without
        /// performing bounds checking.
        ///
        /// This function should be used with care.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// let key = slab.insert(2);
        ///
        /// unsafe {
        ///     assert_eq!(slab.get_unchecked(key), &2);
        /// }
        /// ```
        pub unsafe fn get_unchecked(&self, key: usize) -> &T {
            match *self.entries.get_unchecked(key) {
                Entry::Occupied(ref val) => val,
                _ => unreachable!(),
            }
        }

        /// Return a mutable reference to the value associated with the given key
        /// without performing bounds checking.
        ///
        /// This function should be used with care.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// let key = slab.insert(2);
        ///
        /// unsafe {
        ///     let val = slab.get_unchecked_mut(key);
        ///     *val = 13;
        /// }
        ///
        /// assert_eq!(slab[key], 13);
        /// ```
        pub unsafe fn get_unchecked_mut(&mut self, key: usize) -> &mut T {
            match *self.entries.get_unchecked_mut(key) {
                Entry::Occupied(ref mut val) => val,
                _ => unreachable!(),
            }
        }

        /// Insert a value in the slab, returning key assigned to the value.
        ///
        /// The returned key can later be used to retrieve or remove the value using indexed
        /// lookup and `remove`. Additional capacity is allocated if needed. See
        /// [Capacity and reallocation](index.html#capacity-and-reallocation).
        ///
        /// # Panics
        ///
        /// Panics if the number of elements in the vector overflows a `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        /// let key = slab.insert("hello");
        /// assert_eq!(slab[key], "hello");
        /// ```
        pub fn insert(&mut self, val: T) -> usize {
            let key = self.next;

            self.insert_at(key, val);

            key
        }

        /// Return a handle to a vacant entry allowing for further manipulation.
        ///
        /// This function is useful when creating values that must contain their
        /// slab key. The returned `VacantEntry` reserves a slot in the slab and is
        /// able to query the associated key.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let hello = {
        ///     let entry = slab.vacant_entry();
        ///     let key = entry.key();
        ///
        ///     entry.insert((key, "hello"));
        ///     key
        /// };
        ///
        /// assert_eq!(hello, slab[hello].0);
        /// assert_eq!("hello", slab[hello].1);
        /// ```
        pub fn vacant_entry(&mut self) -> VacantEntry<T> {
            VacantEntry {
                key: self.next,
                slab: self,
            }
        }

        fn insert_at(&mut self, key: usize, val: T) {
            self.len += 1;

            if key == self.entries.len() {
                self.entries.push(Entry::Occupied(val));
                self.next = key + 1;
            } else {
                let prev = mem::replace(&mut self.entries[key], Entry::Occupied(val));

                match prev {
                    Entry::Vacant(next) => {
                        self.next = next;
                    }
                    _ => unreachable!(),
                }
            }
        }

        /// Remove and return the value associated with the given key.
        ///
        /// The key is then released and may be associated with future stored
        /// values.
        ///
        /// # Panics
        ///
        /// Panics if `key` is not associated with a value.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let hello = slab.insert("hello");
        ///
        /// assert_eq!(slab.remove(hello), "hello");
        /// assert!(!slab.contains(hello));
        /// ```
        pub fn remove(&mut self, key: usize) -> T {
            // Swap the entry at the provided value
            let prev = mem::replace(&mut self.entries[key], Entry::Vacant(self.next));

            match prev {
                Entry::Occupied(val) => {
                    self.len -= 1;
                    self.next = key;
                    val
                }
                _ => {
                    // Woops, the entry is actually vacant, restore the state
                    self.entries[key] = prev;
                    panic!("invalid key");
                }
            }
        }

        /// Return `true` if a value is associated with the given key.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let hello = slab.insert("hello");
        /// assert!(slab.contains(hello));
        ///
        /// slab.remove(hello);
        ///
        /// assert!(!slab.contains(hello));
        /// ```
        pub fn contains(&self, key: usize) -> bool {
            self.entries
                .get(key)
                .map(|e| match *e {
                    Entry::Occupied(_) => true,
                    _ => false,
                })
                .unwrap_or(false)
        }

        /// Retain only the elements specified by the predicate.
        ///
        /// In other words, remove all elements `e` such that `f(usize, &mut e)`
        /// returns false. This method operates in place and preserves the key
        /// associated with the retained values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let k1 = slab.insert(0);
        /// let k2 = slab.insert(1);
        /// let k3 = slab.insert(2);
        ///
        /// slab.retain(|key, val| key == k1 || *val == 1);
        ///
        /// assert!(slab.contains(k1));
        /// assert!(slab.contains(k2));
        /// assert!(!slab.contains(k3));
        ///
        /// assert_eq!(2, slab.len());
        /// ```
        pub fn retain<F>(&mut self, mut f: F)
        where
            F: FnMut(usize, &mut T) -> bool,
        {
            for i in 0..self.entries.len() {
                let keep = match self.entries[i] {
                    Entry::Occupied(ref mut v) => f(i, v),
                    _ => true,
                };

                if !keep {
                    self.remove(i);
                }
            }
        }

        /// Return a draining iterator that removes all elements from the slab and
        /// yields the removed items.
        ///
        /// Note: Elements are removed even if the iterator is only partially
        /// consumed or not consumed at all.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let _ = slab.insert(0);
        /// let _ = slab.insert(1);
        /// let _ = slab.insert(2);
        ///
        /// {
        ///     let mut drain = slab.drain();
        ///
        ///     assert_eq!(Some(0), drain.next());
        ///     assert_eq!(Some(1), drain.next());
        ///     assert_eq!(Some(2), drain.next());
        ///     assert_eq!(None, drain.next());
        /// }
        ///
        /// assert!(slab.is_empty());
        /// ```
        pub fn drain(&mut self) -> Drain<T> {
            self.len = 0;
            self.next = 0;
            Drain(self.entries.drain(..))
        }
    }

    impl<T> ops::Index<usize> for Slab<T> {
        type Output = T;

        fn index(&self, key: usize) -> &T {
            match self.entries[key] {
                Entry::Occupied(ref v) => v,
                _ => panic!("invalid key"),
            }
        }
    }

    impl<T> ops::IndexMut<usize> for Slab<T> {
        fn index_mut(&mut self, key: usize) -> &mut T {
            match self.entries[key] {
                Entry::Occupied(ref mut v) => v,
                _ => panic!("invalid key"),
            }
        }
    }

    impl<'a, T> IntoIterator for &'a Slab<T> {
        type Item = (usize, &'a T);
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    impl<'a, T> IntoIterator for &'a mut Slab<T> {
        type Item = (usize, &'a mut T);
        type IntoIter = IterMut<'a, T>;

        fn into_iter(self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

    impl<T> fmt::Debug for Slab<T>
    where
        T: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            write!(
                fmt,
                "Slab {{ len: {}, cap: {} }}",
                self.len,
                self.capacity()
            )
        }
    }

    impl<'a, T: 'a> fmt::Debug for Iter<'a, T>
    where
        T: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Iter")
                .field("curr", &self.curr)
                .field("remaining", &self.entries.len())
                .finish()
        }
    }

    impl<'a, T: 'a> fmt::Debug for IterMut<'a, T>
    where
        T: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("IterMut")
                .field("curr", &self.curr)
                .field("remaining", &self.entries.len())
                .finish()
        }
    }

    impl<'a, T: 'a> fmt::Debug for Drain<'a, T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Drain").finish()
        }
    }

    // ===== VacantEntry =====

    impl<'a, T> VacantEntry<'a, T> {
        /// Insert a value in the entry, returning a mutable reference to the value.
        ///
        /// To get the key associated with the value, use `key` prior to calling
        /// `insert`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let hello = {
        ///     let entry = slab.vacant_entry();
        ///     let key = entry.key();
        ///
        ///     entry.insert((key, "hello"));
        ///     key
        /// };
        ///
        /// assert_eq!(hello, slab[hello].0);
        /// assert_eq!("hello", slab[hello].1);
        /// ```
        pub fn insert(self, val: T) -> &'a mut T {
            self.slab.insert_at(self.key, val);

            match self.slab.entries[self.key] {
                Entry::Occupied(ref mut v) => v,
                _ => unreachable!(),
            }
        }

        /// Return the key associated with this entry.
        ///
        /// A value stored in this entry will be associated with this key.
        ///
        /// # Examples
        ///
        /// ```
        /// # use slab::*;
        /// let mut slab = Slab::new();
        ///
        /// let hello = {
        ///     let entry = slab.vacant_entry();
        ///     let key = entry.key();
        ///
        ///     entry.insert((key, "hello"));
        ///     key
        /// };
        ///
        /// assert_eq!(hello, slab[hello].0);
        /// assert_eq!("hello", slab[hello].1);
        /// ```
        pub fn key(&self) -> usize {
            self.key
        }
    }

    // ===== Iter =====

    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = (usize, &'a T);

        fn next(&mut self) -> Option<(usize, &'a T)> {
            while let Some(entry) = self.entries.next() {
                let curr = self.curr;
                self.curr += 1;

                if let Entry::Occupied(ref v) = *entry {
                    return Some((curr, v));
                }
            }

            None
        }
    }

    // ===== IterMut =====

    impl<'a, T> Iterator for IterMut<'a, T> {
        type Item = (usize, &'a mut T);

        fn next(&mut self) -> Option<(usize, &'a mut T)> {
            while let Some(entry) = self.entries.next() {
                let curr = self.curr;
                self.curr += 1;

                if let Entry::Occupied(ref mut v) = *entry {
                    return Some((curr, v));
                }
            }

            None
        }
    }

    // ===== Drain =====

    impl<'a, T> Iterator for Drain<'a, T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            while let Some(entry) = self.0.next() {
                if let Entry::Occupied(v) = entry {
                    return Some(v);
                }
            }

            None
        }
    }