Import 'slab' rust crate version 0.4.2

Bug: b/151760391
Test: m crosvm.experimental
Change-Id: Ifdc5a0fdd0b226050d45ea80a6e60d9295e65ba1

1 files changed, 977 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..a3638ca
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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
+    }
+}