hashbrown 0.1.3

// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use self::Entry::*;

use core::borrow::Borrow;
use core::fmt::{self, Debug};
use core::hash::{BuildHasher, Hash, Hasher};
use core::iter::{FromIterator, FusedIterator};
use core::marker::PhantomData;
use core::mem;
use core::ops::Index;
use raw::{Bucket, RawDrain, RawIntoIter, RawIter, RawTable};
#[cfg(feature = "serde")]
use serde::de::{Deserialize, Deserializer, MapAccess, Visitor};
#[cfg(feature = "serde")]
use serde::ser::{Serialize, Serializer};

#[cfg(feature = "serde")]
use super::size_hint;

pub use fx::FxHashBuilder as DefaultHashBuilder;

/// A hash map implemented with quadratic probing and SIMD lookup.
///
/// The default hashing algorithm is currently `fx`, though this is
/// subject to change at any point in the future. This hash function is very
/// fast for all types of keys, but this algorithm will typically *not* protect
/// against attacks such as HashDoS.
///
/// The hashing algorithm can be replaced on a per-`HashMap` basis using the
/// [`default`], [`with_hasher`], and [`with_capacity_and_hasher`] methods. Many
/// alternative algorithms are available on crates.io, such as the [`fnv`] crate.
///
/// It is required that the keys implement the [`Eq`] and [`Hash`] traits, although
/// this can frequently be achieved by using `#[derive(PartialEq, Eq, Hash)]`.
/// If you implement these yourself, it is important that the following
/// property holds:
///
/// ```text
/// k1 == k2 -> hash(k1) == hash(k2)
/// ```
///
/// In other words, if two keys are equal, their hashes must be equal.
///
/// It is a logic error for a key to be modified in such a way that the key's
/// hash, as determined by the [`Hash`] trait, or its equality, as determined by
/// the [`Eq`] trait, changes while it is in the map. This is normally only
/// possible through [`Cell`], [`RefCell`], global state, I/O, or unsafe code.
///
/// It is also a logic error for the [`Hash`] implementation of a key to panic.
/// This is generally only possible if the trait is implemented manually. If a
/// panic does occur then the contents of the `HashMap` may become corrupted and
/// some items may be dropped from the table.
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// // type inference lets us omit an explicit type signature (which
/// // would be `HashMap<&str, &str>` in this example).
/// let mut book_reviews = HashMap::new();
///
/// // review some books.
/// book_reviews.insert("Adventures of Huckleberry Finn",    "My favorite book.");
/// book_reviews.insert("Grimms' Fairy Tales",               "Masterpiece.");
/// book_reviews.insert("Pride and Prejudice",               "Very enjoyable.");
/// book_reviews.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot.");
///
/// // check for a specific one.
/// if !book_reviews.contains_key("Les Misérables") {
///     println!("We've got {} reviews, but Les Misérables ain't one.",
///              book_reviews.len());
/// }
///
/// // oops, this review has a lot of spelling mistakes, let's delete it.
/// book_reviews.remove("The Adventures of Sherlock Holmes");
///
/// // look up the values associated with some keys.
/// let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
/// for book in &to_find {
///     match book_reviews.get(book) {
///         Some(review) => println!("{}: {}", book, review),
///         None => println!("{} is unreviewed.", book)
///     }
/// }
///
/// // iterate over everything.
/// for (book, review) in &book_reviews {
///     println!("{}: \"{}\"", book, review);
/// }
/// ```
///
/// `HashMap` also implements an [`Entry API`](#method.entry), which allows
/// for more complex methods of getting, setting, updating and removing keys and
/// their values:
///
/// ```
/// use hashbrown::HashMap;
///
/// // type inference lets us omit an explicit type signature (which
/// // would be `HashMap<&str, u8>` in this example).
/// let mut player_stats = HashMap::new();
///
/// fn random_stat_buff() -> u8 {
///     // could actually return some random value here - let's just return
///     // some fixed value for now
///     42
/// }
///
/// // insert a key only if it doesn't already exist
/// player_stats.entry("health").or_insert(100);
///
/// // insert a key using a function that provides a new value only if it
/// // doesn't already exist
/// player_stats.entry("defence").or_insert_with(random_stat_buff);
///
/// // update a key, guarding against the key possibly not being set
/// let stat = player_stats.entry("attack").or_insert(100);
/// *stat += random_stat_buff();
/// ```
///
/// The easiest way to use `HashMap` with a custom type as key is to derive [`Eq`] and [`Hash`].
/// We must also derive [`PartialEq`].
///
/// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
/// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
/// [`PartialEq`]: https://doc.rust-lang.org/std/cmp/trait.PartialEq.html
/// [`RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
/// [`Cell`]:  https://doc.rust-lang.org/std/cell/struct.Cell.html
/// [`default`]: #method.default
/// [`with_hasher`]: #method.with_hasher
/// [`with_capacity_and_hasher`]: #method.with_capacity_and_hasher
/// [`fnv`]: https://crates.io/crates/fnv
///
/// ```
/// use hashbrown::HashMap;
///
/// #[derive(Hash, Eq, PartialEq, Debug)]
/// struct Viking {
///     name: String,
///     country: String,
/// }
///
/// impl Viking {
///     /// Create a new Viking.
///     fn new(name: &str, country: &str) -> Viking {
///         Viking { name: name.to_string(), country: country.to_string() }
///     }
/// }
///
/// // Use a HashMap to store the vikings' health points.
/// let mut vikings = HashMap::new();
///
/// vikings.insert(Viking::new("Einar", "Norway"), 25);
/// vikings.insert(Viking::new("Olaf", "Denmark"), 24);
/// vikings.insert(Viking::new("Harald", "Iceland"), 12);
///
/// // Use derived implementation to print the status of the vikings.
/// for (viking, health) in &vikings {
///     println!("{:?} has {} hp", viking, health);
/// }
/// ```
///
/// A `HashMap` with fixed list of elements can be initialized from an array:
///
/// ```
/// use hashbrown::HashMap;
///
/// fn main() {
///     let timber_resources: HashMap<&str, i32> =
///     [("Norway", 100),
///      ("Denmark", 50),
///      ("Iceland", 10)]
///      .iter().cloned().collect();
///     // use the values stored in map
/// }
/// ```

#[derive(Clone)]
pub struct HashMap<K, V, S = DefaultHashBuilder> {
    hash_builder: S,
    table: RawTable<(K, V)>,
}

#[inline]
fn make_hash<K: Hash + ?Sized>(hash_builder: &impl BuildHasher, val: &K) -> u64 {
    let mut state = hash_builder.build_hasher();
    val.hash(&mut state);
    state.finish()
}

impl<K: Hash + Eq, V> HashMap<K, V, DefaultHashBuilder> {
    /// Creates an empty `HashMap`.
    ///
    /// The hash map is initially created with a capacity of 0, so it will not allocate until it
    /// is first inserted into.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::new();
    /// ```
    #[inline]
    pub fn new() -> HashMap<K, V, DefaultHashBuilder> {
        Default::default()
    }

    /// Creates an empty `HashMap` with the specified capacity.
    ///
    /// The hash map will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash map will not allocate.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);
    /// ```
    #[inline]
    pub fn with_capacity(capacity: usize) -> HashMap<K, V, DefaultHashBuilder> {
        HashMap::with_capacity_and_hasher(capacity, Default::default())
    }
}

impl<K, V, S> HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    /// Creates an empty `HashMap` which will use the given hash builder to hash
    /// keys.
    ///
    /// The created map has the default initial capacity.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow HashMaps to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut map = HashMap::with_hasher(s);
    /// map.insert(1, 2);
    /// ```
    #[inline]
    pub fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
        HashMap {
            hash_builder,
            table: RawTable::new(),
        }
    }

    /// Creates an empty `HashMap` with the specified capacity, using `hash_builder`
    /// to hash the keys.
    ///
    /// The hash map will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash map will not allocate.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow HashMaps to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut map = HashMap::with_capacity_and_hasher(10, s);
    /// map.insert(1, 2);
    /// ```
    #[inline]
    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> HashMap<K, V, S> {
        HashMap {
            hash_builder,
            table: RawTable::with_capacity(capacity),
        }
    }

    /// Returns a reference to the map's [`BuildHasher`].
    ///
    /// [`BuildHasher`]: ../../std/hash/trait.BuildHasher.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let hasher = DefaultHashBuilder::default();
    /// let map: HashMap<i32, i32> = HashMap::with_hasher(hasher);
    /// let hasher: &DefaultHashBuilder = map.hasher();
    /// ```
    #[inline]
    pub fn hasher(&self) -> &S {
        &self.hash_builder
    }

    /// Returns the number of elements the map can hold without reallocating.
    ///
    /// This number is a lower bound; the `HashMap<K, V>` might be able to hold
    /// more, but is guaranteed to be able to hold at least this many.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// assert!(map.capacity() >= 100);
    /// ```
    #[inline]
    pub fn capacity(&self) -> usize {
        self.table.capacity()
    }

    /// Reserves capacity for at least `additional` more elements to be inserted
    /// in the `HashMap`. The collection may reserve more space to avoid
    /// frequent reallocations.
    ///
    /// # Panics
    ///
    /// Panics if the new allocation size overflows [`usize`].
    ///
    /// [`usize`]: ../../std/primitive.usize.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::new();
    /// map.reserve(10);
    /// ```
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        let hash_builder = &self.hash_builder;
        self.table
            .reserve(additional, |x| make_hash(hash_builder, &x.0));
    }

    /// Shrinks the capacity of the map as much as possible. It will drop
    /// down as much as possible while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// map.insert(1, 2);
    /// map.insert(3, 4);
    /// assert!(map.capacity() >= 100);
    /// map.shrink_to_fit();
    /// assert!(map.capacity() >= 2);
    /// ```
    #[inline]
    pub fn shrink_to_fit(&mut self) {
        let hash_builder = &self.hash_builder;
        self.table.shrink_to(0, |x| make_hash(hash_builder, &x.0));
    }

    /// Shrinks the capacity of the map with a lower limit. It will drop
    /// down no lower than the supplied limit while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// Panics if the current capacity is smaller than the supplied
    /// minimum capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// map.insert(1, 2);
    /// map.insert(3, 4);
    /// assert!(map.capacity() >= 100);
    /// map.shrink_to(10);
    /// assert!(map.capacity() >= 10);
    /// map.shrink_to(0);
    /// assert!(map.capacity() >= 2);
    /// ```
    #[inline]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        assert!(
            self.capacity() >= min_capacity,
            "Tried to shrink to a larger capacity"
        );

        let hash_builder = &self.hash_builder;
        self.table
            .shrink_to(min_capacity, |x| make_hash(hash_builder, &x.0));
    }

    /// An iterator visiting all keys in arbitrary order.
    /// The iterator element type is `&'a K`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// for key in map.keys() {
    ///     println!("{}", key);
    /// }
    /// ```
    #[inline]
    pub fn keys(&self) -> Keys<K, V> {
        Keys { inner: self.iter() }
    }

    /// An iterator visiting all values in arbitrary order.
    /// The iterator element type is `&'a V`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// for val in map.values() {
    ///     println!("{}", val);
    /// }
    /// ```
    #[inline]
    pub fn values(&self) -> Values<K, V> {
        Values { inner: self.iter() }
    }

    /// An iterator visiting all values mutably in arbitrary order.
    /// The iterator element type is `&'a mut V`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    ///
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// for val in map.values_mut() {
    ///     *val = *val + 10;
    /// }
    ///
    /// for val in map.values() {
    ///     println!("{}", val);
    /// }
    /// ```
    #[inline]
    pub fn values_mut(&mut self) -> ValuesMut<K, V> {
        ValuesMut {
            inner: self.iter_mut(),
        }
    }

    /// An iterator visiting all key-value pairs in arbitrary order.
    /// The iterator element type is `(&'a K, &'a V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// for (key, val) in map.iter() {
    ///     println!("key: {} val: {}", key, val);
    /// }
    /// ```
    #[inline]
    pub fn iter(&self) -> Iter<K, V> {
        // Here we tie the lifetime of self to the iter.
        unsafe {
            Iter {
                inner: self.table.iter(),
                _marker: PhantomData,
            }
        }
    }

    /// An iterator visiting all key-value pairs in arbitrary order,
    /// with mutable references to the values.
    /// The iterator element type is `(&'a K, &'a mut V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// // Update all values
    /// for (_, val) in map.iter_mut() {
    ///     *val *= 2;
    /// }
    ///
    /// for (key, val) in &map {
    ///     println!("key: {} val: {}", key, val);
    /// }
    /// ```
    #[inline]
    pub fn iter_mut(&mut self) -> IterMut<K, V> {
        // Here we tie the lifetime of self to the iter.
        unsafe {
            IterMut {
                inner: self.table.iter(),
                _marker: PhantomData,
            }
        }
    }

    /// Gets the given key's corresponding entry in the map for in-place manipulation.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut letters = HashMap::new();
    ///
    /// for ch in "a short treatise on fungi".chars() {
    ///     let counter = letters.entry(ch).or_insert(0);
    ///     *counter += 1;
    /// }
    ///
    /// assert_eq!(letters[&'s'], 2);
    /// assert_eq!(letters[&'t'], 3);
    /// assert_eq!(letters[&'u'], 1);
    /// assert_eq!(letters.get(&'y'), None);
    /// ```
    #[inline]
    pub fn entry(&mut self, key: K) -> Entry<K, V, S> {
        let hash = make_hash(&self.hash_builder, &key);
        if let Some(elem) = self.table.find(hash, |q| q.0.eq(&key)) {
            Entry::Occupied(OccupiedEntry {
                key: Some(key),
                elem,
                table: self,
            })
        } else {
            Entry::Vacant(VacantEntry {
                hash,
                key,
                table: self,
            })
        }
    }

    /// Returns the number of elements in the map.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// assert_eq!(a.len(), 0);
    /// a.insert(1, "a");
    /// assert_eq!(a.len(), 1);
    /// ```
    #[inline]
    pub fn len(&self) -> usize {
        self.table.len()
    }

    /// Returns true if the map contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// assert!(a.is_empty());
    /// a.insert(1, "a");
    /// assert!(!a.is_empty());
    /// ```
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
    /// allocated memory for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// a.insert(2, "b");
    ///
    /// for (k, v) in a.drain().take(1) {
    ///     assert!(k == 1 || k == 2);
    ///     assert!(v == "a" || v == "b");
    /// }
    ///
    /// assert!(a.is_empty());
    /// ```
    #[inline]
    pub fn drain(&mut self) -> Drain<K, V> {
        // Here we tie the lifetime of self to the iter.
        unsafe {
            Drain {
                inner: self.table.drain(),
            }
        }
    }

    /// Clears the map, removing all key-value pairs. Keeps the allocated memory
    /// for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// a.clear();
    /// assert!(a.is_empty());
    /// ```
    #[inline]
    pub fn clear(&mut self) {
        self.table.clear();
    }

    /// Returns a reference to the value corresponding to the key.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.get(&1), Some(&"a"));
    /// assert_eq!(map.get(&2), None);
    /// ```
    #[inline]
    pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.get_key_value(k).map(|(_, v)| v)
    }

    /// Returns the key-value pair corresponding to the supplied key.
    ///
    /// The supplied key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
    /// assert_eq!(map.get_key_value(&2), None);
    /// ```
    #[inline]
    pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hash = make_hash(&self.hash_builder, k);
        self.table
            .find(hash, |x| k.eq(x.0.borrow()))
            .map(|item| unsafe {
                let &(ref key, ref value) = item.as_ref();
                (key, value)
            })
    }

    /// Returns true if the map contains a value for the specified key.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.contains_key(&1), true);
    /// assert_eq!(map.contains_key(&2), false);
    /// ```
    #[inline]
    pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.get(k).is_some()
    }

    /// Returns a mutable reference to the value corresponding to the key.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// if let Some(x) = map.get_mut(&1) {
    ///     *x = "b";
    /// }
    /// assert_eq!(map[&1], "b");
    /// ```
    #[inline]
    pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hash = make_hash(&self.hash_builder, k);
        self.table
            .find(hash, |x| k.eq(x.0.borrow()))
            .map(|item| unsafe { &mut item.as_mut().1 })
    }

    /// Inserts a key-value pair into the map.
    ///
    /// If the map did not have this key present, [`None`] is returned.
    ///
    /// If the map did have this key present, the value is updated, and the old
    /// value is returned. The key is not updated, though; this matters for
    /// types that can be `==` without being identical. See the [module-level
    /// documentation] for more.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [module-level documentation]: index.html#insert-and-complex-keys
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// assert_eq!(map.insert(37, "a"), None);
    /// assert_eq!(map.is_empty(), false);
    ///
    /// map.insert(37, "b");
    /// assert_eq!(map.insert(37, "c"), Some("b"));
    /// assert_eq!(map[&37], "c");
    /// ```
    #[inline]
    pub fn insert(&mut self, k: K, v: V) -> Option<V> {
        unsafe {
            let hash = make_hash(&self.hash_builder, &k);
            if let Some(item) = self.table.find(hash, |x| k.eq(&x.0)) {
                Some(mem::replace(&mut item.as_mut().1, v))
            } else {
                let hash_builder = &self.hash_builder;
                self.table
                    .insert(hash, (k, v), |x| make_hash(hash_builder, &x.0));
                None
            }
        }
    }

    /// Removes a key from the map, returning the value at the key if the key
    /// was previously in the map.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.remove(&1), Some("a"));
    /// assert_eq!(map.remove(&1), None);
    /// ```
    #[inline]
    pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.remove_entry(k).map(|(_, v)| v)
    }

    /// Removes a key from the map, returning the stored key and value if the
    /// key was previously in the map.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// # fn main() {
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.remove_entry(&1), Some((1, "a")));
    /// assert_eq!(map.remove(&1), None);
    /// # }
    /// ```
    #[inline]
    pub fn remove_entry<Q: ?Sized>(&mut self, k: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        unsafe {
            let hash = make_hash(&self.hash_builder, &k);
            if let Some(item) = self.table.find(hash, |x| k.eq(x.0.borrow())) {
                // Erase the element from the table first since drop might panic.
                self.table.erase_no_drop(&item);
                Some(item.read())
            } else {
                None
            }
        }
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all pairs `(k, v)` such that `f(&k,&mut v)` returns `false`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
    /// map.retain(|&k, _| k % 2 == 0);
    /// assert_eq!(map.len(), 4);
    /// ```
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        // Here we only use `iter` as a temporary, preventing use-after-free
        unsafe {
            for item in self.table.iter() {
                let &mut (ref key, ref mut value) = item.as_mut();
                if !f(key, value) {
                    // Erase the element from the table first since drop might panic.
                    self.table.erase_no_drop(&item);
                    item.drop();
                }
            }
        }
    }
}

impl<K, V, S> PartialEq for HashMap<K, V, S>
where
    K: Eq + Hash,
    V: PartialEq,
    S: BuildHasher,
{
    fn eq(&self, other: &HashMap<K, V, S>) -> bool {
        if self.len() != other.len() {
            return false;
        }

        self.iter()
            .all(|(key, value)| other.get(key).map_or(false, |v| *value == *v))
    }
}

impl<K, V, S> Eq for HashMap<K, V, S>
where
    K: Eq + Hash,
    V: Eq,
    S: BuildHasher,
{
}

impl<K, V, S> Debug for HashMap<K, V, S>
where
    K: Eq + Hash + Debug,
    V: Debug,
    S: BuildHasher,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_map().entries(self.iter()).finish()
    }
}

impl<K, V, S> Default for HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher + Default,
{
    /// Creates an empty `HashMap<K, V, S>`, with the `Default` value for the hasher.
    #[inline]
    fn default() -> HashMap<K, V, S> {
        HashMap::with_hasher(Default::default())
    }
}

impl<'a, K, Q: ?Sized, V, S> Index<&'a Q> for HashMap<K, V, S>
where
    K: Eq + Hash + Borrow<Q>,
    Q: Eq + Hash,
    S: BuildHasher,
{
    type Output = V;

    /// Returns a reference to the value corresponding to the supplied key.
    ///
    /// # Panics
    ///
    /// Panics if the key is not present in the `HashMap`.
    #[inline]
    fn index(&self, key: &Q) -> &V {
        self.get(key).expect("no entry found for key")
    }
}

/// An iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`iter`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`iter`]: struct.HashMap.html#method.iter
/// [`HashMap`]: struct.HashMap.html
pub struct Iter<'a, K: 'a, V: 'a> {
    inner: RawIter<(K, V)>,
    _marker: PhantomData<&'a HashMap<K, V>>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
impl<'a, K, V> Clone for Iter<'a, K, V> {
    #[inline]
    fn clone(&self) -> Iter<'a, K, V> {
        Iter {
            inner: self.inner.clone(),
            _marker: PhantomData,
        }
    }
}

impl<'a, K: Debug, V: Debug> fmt::Debug for Iter<'a, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

/// A mutable iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`iter_mut`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`iter_mut`]: struct.HashMap.html#method.iter_mut
/// [`HashMap`]: struct.HashMap.html
pub struct IterMut<'a, K: 'a, V: 'a> {
    inner: RawIter<(K, V)>,
    _marker: PhantomData<&'a mut HashMap<K, V>>,
}

impl<'a, K, V> IterMut<'a, K, V> {
    /// Returns a iterator of references over the remaining items.
    #[inline]
    pub(crate) fn iter(&self) -> Iter<K, V> {
        Iter {
            inner: self.inner.clone(),
            _marker: PhantomData,
        }
    }
}

/// An owning iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`into_iter`] method on [`HashMap`][`HashMap`]
/// (provided by the `IntoIterator` trait). See its documentation for more.
///
/// [`into_iter`]: struct.HashMap.html#method.into_iter
/// [`HashMap`]: struct.HashMap.html
pub struct IntoIter<K, V> {
    inner: RawIntoIter<(K, V)>,
}

impl<K, V> IntoIter<K, V> {
    /// Returns a iterator of references over the remaining items.
    #[inline]
    pub(crate) fn iter(&self) -> Iter<K, V> {
        Iter {
            inner: self.inner.iter(),
            _marker: PhantomData,
        }
    }
}

/// An iterator over the keys of a `HashMap`.
///
/// This `struct` is created by the [`keys`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`keys`]: struct.HashMap.html#method.keys
/// [`HashMap`]: struct.HashMap.html
pub struct Keys<'a, K: 'a, V: 'a> {
    inner: Iter<'a, K, V>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
impl<'a, K, V> Clone for Keys<'a, K, V> {
    #[inline]
    fn clone(&self) -> Keys<'a, K, V> {
        Keys {
            inner: self.inner.clone(),
        }
    }
}

impl<'a, K: Debug, V> fmt::Debug for Keys<'a, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

/// An iterator over the values of a `HashMap`.
///
/// This `struct` is created by the [`values`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`values`]: struct.HashMap.html#method.values
/// [`HashMap`]: struct.HashMap.html
pub struct Values<'a, K: 'a, V: 'a> {
    inner: Iter<'a, K, V>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
impl<'a, K, V> Clone for Values<'a, K, V> {
    #[inline]
    fn clone(&self) -> Values<'a, K, V> {
        Values {
            inner: self.inner.clone(),
        }
    }
}

impl<'a, K, V: Debug> fmt::Debug for Values<'a, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.clone()).finish()
    }
}

/// A draining iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`drain`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`drain`]: struct.HashMap.html#method.drain
/// [`HashMap`]: struct.HashMap.html
pub struct Drain<'a, K: 'a, V: 'a> {
    pub(super) inner: RawDrain<'a, (K, V)>,
}

impl<'a, K, V> Drain<'a, K, V> {
    /// Returns a iterator of references over the remaining items.
    #[inline]
    pub(crate) fn iter(&self) -> Iter<K, V> {
        Iter {
            inner: self.inner.iter(),
            _marker: PhantomData,
        }
    }
}

/// A mutable iterator over the values of a `HashMap`.
///
/// This `struct` is created by the [`values_mut`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`values_mut`]: struct.HashMap.html#method.values_mut
/// [`HashMap`]: struct.HashMap.html
pub struct ValuesMut<'a, K: 'a, V: 'a> {
    inner: IterMut<'a, K, V>,
}

/// A view into a single entry in a map, which may either be vacant or occupied.
///
/// This `enum` is constructed from the [`entry`] method on [`HashMap`].
///
/// [`HashMap`]: struct.HashMap.html
/// [`entry`]: struct.HashMap.html#method.entry
pub enum Entry<'a, K: 'a, V: 'a, S: 'a> {
    /// An occupied entry.
    Occupied(OccupiedEntry<'a, K, V, S>),

    /// A vacant entry.
    Vacant(VacantEntry<'a, K, V, S>),
}

impl<'a, K: 'a + Debug + Eq + Hash, V: 'a + Debug, S: BuildHasher> Debug for Entry<'a, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            Vacant(ref v) => f.debug_tuple("Entry").field(v).finish(),
            Occupied(ref o) => f.debug_tuple("Entry").field(o).finish(),
        }
    }
}

/// A view into an occupied entry in a `HashMap`.
/// It is part of the [`Entry`] enum.
///
/// [`Entry`]: enum.Entry.html
pub struct OccupiedEntry<'a, K: 'a, V: 'a, S: 'a> {
    key: Option<K>,
    elem: Bucket<(K, V)>,
    table: &'a mut HashMap<K, V, S>,
}

unsafe impl<'a, K, V, S> Send for OccupiedEntry<'a, K, V, S>
where
    K: Send,
    V: Send,
    S: Send,
{
}
unsafe impl<'a, K, V, S> Sync for OccupiedEntry<'a, K, V, S>
where
    K: Sync,
    V: Sync,
    S: Sync,
{
}

impl<'a, K: 'a + Debug, V: 'a + Debug, S> Debug for OccupiedEntry<'a, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("OccupiedEntry")
            .field("key", self.key())
            .field("value", self.get())
            .finish()
    }
}

/// A view into a vacant entry in a `HashMap`.
/// It is part of the [`Entry`] enum.
///
/// [`Entry`]: enum.Entry.html
pub struct VacantEntry<'a, K: 'a, V: 'a, S: 'a> {
    hash: u64,
    key: K,
    table: &'a mut HashMap<K, V, S>,
}

impl<'a, K: 'a + Debug + Eq + Hash, V: 'a, S: 'a + BuildHasher> Debug for VacantEntry<'a, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("VacantEntry").field(self.key()).finish()
    }
}

impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    type Item = (&'a K, &'a V);
    type IntoIter = Iter<'a, K, V>;

    #[inline]
    fn into_iter(self) -> Iter<'a, K, V> {
        self.iter()
    }
}

impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    type Item = (&'a K, &'a mut V);
    type IntoIter = IterMut<'a, K, V>;

    #[inline]
    fn into_iter(self) -> IterMut<'a, K, V> {
        self.iter_mut()
    }
}

impl<K, V, S> IntoIterator for HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    type Item = (K, V);
    type IntoIter = IntoIter<K, V>;

    /// Creates a consuming iterator, that is, one that moves each key-value
    /// pair out of the map in arbitrary order. The map cannot be used after
    /// calling this.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// // Not possible with .iter()
    /// let vec: Vec<(&str, i32)> = map.into_iter().collect();
    /// ```
    #[inline]
    fn into_iter(self) -> IntoIter<K, V> {
        IntoIter {
            inner: self.table.into_iter(),
        }
    }
}

impl<'a, K, V> Iterator for Iter<'a, K, V> {
    type Item = (&'a K, &'a V);

    #[inline]
    fn next(&mut self) -> Option<(&'a K, &'a V)> {
        self.inner.next().map(|x| unsafe {
            let r = x.as_ref();
            (&r.0, &r.1)
        })
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}

impl<'a, K, V> FusedIterator for Iter<'a, K, V> {}

impl<'a, K, V> Iterator for IterMut<'a, K, V> {
    type Item = (&'a K, &'a mut V);

    #[inline]
    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
        self.inner.next().map(|x| unsafe {
            let r = x.as_mut();
            (&r.0, &mut r.1)
        })
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<'a, K, V> FusedIterator for IterMut<'a, K, V> {}

impl<'a, K, V> fmt::Debug for IterMut<'a, K, V>
where
    K: fmt::Debug,
    V: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<K, V> Iterator for IntoIter<K, V> {
    type Item = (K, V);

    #[inline]
    fn next(&mut self) -> Option<(K, V)> {
        self.inner.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<K, V> ExactSizeIterator for IntoIter<K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<K, V> FusedIterator for IntoIter<K, V> {}

impl<K: Debug, V: Debug> fmt::Debug for IntoIter<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<'a, K, V> Iterator for Keys<'a, K, V> {
    type Item = &'a K;

    #[inline]
    fn next(&mut self) -> Option<(&'a K)> {
        self.inner.next().map(|(k, _)| k)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for Keys<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<'a, K, V> FusedIterator for Keys<'a, K, V> {}

impl<'a, K, V> Iterator for Values<'a, K, V> {
    type Item = &'a V;

    #[inline]
    fn next(&mut self) -> Option<(&'a V)> {
        self.inner.next().map(|(_, v)| v)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for Values<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<'a, K, V> FusedIterator for Values<'a, K, V> {}

impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
    type Item = &'a mut V;

    #[inline]
    fn next(&mut self) -> Option<(&'a mut V)> {
        self.inner.next().map(|(_, v)| v)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for ValuesMut<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<'a, K, V> FusedIterator for ValuesMut<'a, K, V> {}

impl<'a, K, V> fmt::Debug for ValuesMut<'a, K, V>
where
    K: fmt::Debug,
    V: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.inner.iter()).finish()
    }
}

impl<'a, K, V> Iterator for Drain<'a, K, V> {
    type Item = (K, V);

    #[inline]
    fn next(&mut self) -> Option<(K, V)> {
        self.inner.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
impl<'a, K, V> FusedIterator for Drain<'a, K, V> {}

impl<'a, K, V> fmt::Debug for Drain<'a, K, V>
where
    K: fmt::Debug,
    V: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> Entry<'a, K, V, S> {
    /// Ensures a value is in the entry by inserting the default if empty, and returns
    /// a mutable reference to the value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// assert_eq!(map["poneyland"], 12);
    ///
    /// *map.entry("poneyland").or_insert(12) += 10;
    /// assert_eq!(map["poneyland"], 22);
    /// ```
    #[inline]
    pub fn or_insert(self, default: V) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => entry.insert(default),
        }
    }

    /// Ensures a value is in the entry by inserting the result of the default function if empty,
    /// and returns a mutable reference to the value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, String> = HashMap::new();
    /// let s = "hoho".to_string();
    ///
    /// map.entry("poneyland").or_insert_with(|| s);
    ///
    /// assert_eq!(map["poneyland"], "hoho".to_string());
    /// ```
    #[inline]
    pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => entry.insert(default()),
        }
    }

    /// Returns a reference to this entry's key.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    pub fn key(&self) -> &K {
        match *self {
            Occupied(ref entry) => entry.key(),
            Vacant(ref entry) => entry.key(),
        }
    }

    /// Provides in-place mutable access to an occupied entry before any
    /// potential inserts into the map.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// map.entry("poneyland")
    ///    .and_modify(|e| { *e += 1 })
    ///    .or_insert(42);
    /// assert_eq!(map["poneyland"], 42);
    ///
    /// map.entry("poneyland")
    ///    .and_modify(|e| { *e += 1 })
    ///    .or_insert(42);
    /// assert_eq!(map["poneyland"], 43);
    /// ```
    #[inline]
    pub fn and_modify<F>(self, f: F) -> Self
    where
        F: FnOnce(&mut V),
    {
        match self {
            Occupied(mut entry) => {
                f(entry.get_mut());
                Occupied(entry)
            }
            Vacant(entry) => Vacant(entry),
        }
    }
}

impl<'a, K: Eq + Hash, V: Default, S: BuildHasher> Entry<'a, K, V, S> {
    /// Ensures a value is in the entry by inserting the default value if empty,
    /// and returns a mutable reference to the value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// # fn main() {
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, Option<u32>> = HashMap::new();
    /// map.entry("poneyland").or_default();
    ///
    /// assert_eq!(map["poneyland"], None);
    /// # }
    /// ```
    #[inline]
    pub fn or_default(self) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => entry.insert(Default::default()),
        }
    }
}

impl<'a, K, V, S> OccupiedEntry<'a, K, V, S> {
    /// Gets a reference to the key in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    pub fn key(&self) -> &K {
        unsafe { &self.elem.as_ref().0 }
    }

    /// Take the ownership of the key and value from the map.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// if let Entry::Occupied(o) = map.entry("poneyland") {
    ///     // We delete the entry from the map.
    ///     o.remove_entry();
    /// }
    ///
    /// assert_eq!(map.contains_key("poneyland"), false);
    /// ```
    #[inline]
    pub fn remove_entry(self) -> (K, V) {
        unsafe {
            // Erase the element from the table first since drop might panic.
            self.table.table.erase_no_drop(&self.elem);
            self.elem.read()
        }
    }

    /// Gets a reference to the value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// if let Entry::Occupied(o) = map.entry("poneyland") {
    ///     assert_eq!(o.get(), &12);
    /// }
    /// ```
    #[inline]
    pub fn get(&self) -> &V {
        unsafe { &self.elem.as_ref().1 }
    }

    /// Gets a mutable reference to the value in the entry.
    ///
    /// If you need a reference to the `OccupiedEntry` which may outlive the
    /// destruction of the `Entry` value, see [`into_mut`].
    ///
    /// [`into_mut`]: #method.into_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// assert_eq!(map["poneyland"], 12);
    /// if let Entry::Occupied(mut o) = map.entry("poneyland") {
    ///     *o.get_mut() += 10;
    ///     assert_eq!(*o.get(), 22);
    ///
    ///     // We can use the same Entry multiple times.
    ///     *o.get_mut() += 2;
    /// }
    ///
    /// assert_eq!(map["poneyland"], 24);
    /// ```
    #[inline]
    pub fn get_mut(&mut self) -> &mut V {
        unsafe { &mut self.elem.as_mut().1 }
    }

    /// Converts the OccupiedEntry into a mutable reference to the value in the entry
    /// with a lifetime bound to the map itself.
    ///
    /// If you need multiple references to the `OccupiedEntry`, see [`get_mut`].
    ///
    /// [`get_mut`]: #method.get_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// assert_eq!(map["poneyland"], 12);
    /// if let Entry::Occupied(o) = map.entry("poneyland") {
    ///     *o.into_mut() += 10;
    /// }
    ///
    /// assert_eq!(map["poneyland"], 22);
    /// ```
    #[inline]
    pub fn into_mut(self) -> &'a mut V {
        unsafe { &mut self.elem.as_mut().1 }
    }

    /// Sets the value of the entry, and returns the entry's old value.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// if let Entry::Occupied(mut o) = map.entry("poneyland") {
    ///     assert_eq!(o.insert(15), 12);
    /// }
    ///
    /// assert_eq!(map["poneyland"], 15);
    /// ```
    #[inline]
    pub fn insert(&mut self, mut value: V) -> V {
        let old_value = self.get_mut();
        mem::swap(&mut value, old_value);
        value
    }

    /// Takes the value out of the entry, and returns it.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    ///
    /// if let Entry::Occupied(o) = map.entry("poneyland") {
    ///     assert_eq!(o.remove(), 12);
    /// }
    ///
    /// assert_eq!(map.contains_key("poneyland"), false);
    /// ```
    #[inline]
    pub fn remove(self) -> V {
        self.remove_entry().1
    }

    /// Replaces the entry, returning the old key and value. The new key in the hash map will be
    /// the key used to create this entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::hash_map::{Entry, HashMap};
    /// use std::rc::Rc;
    ///
    /// let mut map: HashMap<Rc<String>, u32> = HashMap::new();
    /// map.insert(Rc::new("Stringthing".to_string()), 15);
    ///
    /// let my_key = Rc::new("Stringthing".to_string());
    ///
    /// if let Entry::Occupied(entry) = map.entry(my_key) {
    ///     // Also replace the key with a handle to our other key.
    ///     let (old_key, old_value): (Rc<String>, u32) = entry.replace_entry(16);
    /// }
    ///
    /// ```
    #[inline]
    pub fn replace_entry(self, value: V) -> (K, V) {
        let entry = unsafe { self.elem.as_mut() };

        let old_key = mem::replace(&mut entry.0, self.key.unwrap());
        let old_value = mem::replace(&mut entry.1, value);

        (old_key, old_value)
    }

    /// Replaces the key in the hash map with the key used to create this entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::hash_map::{Entry, HashMap};
    /// use std::rc::Rc;
    ///
    /// let mut map: HashMap<Rc<String>, u32> = HashMap::new();
    /// let mut known_strings: Vec<Rc<String>> = Vec::new();
    ///
    /// // Initialise known strings, run program, etc.
    ///
    /// reclaim_memory(&mut map, &known_strings);
    ///
    /// fn reclaim_memory(map: &mut HashMap<Rc<String>, u32>, known_strings: &[Rc<String>] ) {
    ///     for s in known_strings {
    ///         if let Entry::Occupied(entry) = map.entry(s.clone()) {
    ///             // Replaces the entry's key with our version of it in `known_strings`.
    ///             entry.replace_key();
    ///         }
    ///     }
    /// }
    /// ```
    #[inline]
    pub fn replace_key(self) -> K {
        let entry = unsafe { self.elem.as_mut() };
        mem::replace(&mut entry.0, self.key.unwrap())
    }
}

impl<'a, K: 'a + Eq + Hash, V: 'a, S: BuildHasher> VacantEntry<'a, K, V, S> {
    /// Gets a reference to the key that would be used when inserting a value
    /// through the `VacantEntry`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    pub fn key(&self) -> &K {
        &self.key
    }

    /// Take ownership of the key.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// if let Entry::Vacant(v) = map.entry("poneyland") {
    ///     v.into_key();
    /// }
    /// ```
    #[inline]
    pub fn into_key(self) -> K {
        self.key
    }

    /// Sets the value of the entry with the VacantEntry's key,
    /// and returns a mutable reference to it.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// if let Entry::Vacant(o) = map.entry("poneyland") {
    ///     o.insert(37);
    /// }
    /// assert_eq!(map["poneyland"], 37);
    /// ```
    #[inline]
    pub fn insert(self, value: V) -> &'a mut V {
        let hash_builder = &self.table.hash_builder;
        let bucket = self.table.table.insert(self.hash, (self.key, value), |x| {
            make_hash(hash_builder, &x.0)
        });
        unsafe { &mut bucket.as_mut().1 }
    }
}

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher + Default,
{
    #[inline]
    fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> HashMap<K, V, S> {
        let iter = iter.into_iter();
        let mut map = HashMap::with_capacity_and_hasher(iter.size_hint().0, Default::default());
        for (k, v) in iter {
            map.insert(k, v);
        }
        map
    }
}

impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    #[inline]
    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
        // Keys may be already present or show multiple times in the iterator.
        // Reserve the entire hint lower bound if the map is empty.
        // Otherwise reserve half the hint (rounded up), so the map
        // will only resize twice in the worst case.
        let iter = iter.into_iter();
        let reserve = if self.is_empty() {
            iter.size_hint().0
        } else {
            (iter.size_hint().0 + 1) / 2
        };
        self.reserve(reserve);
        for (k, v) in iter {
            self.insert(k, v);
        }
    }
}

impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>
where
    K: Eq + Hash + Copy,
    V: Copy,
    S: BuildHasher,
{
    fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T) {
        self.extend(iter.into_iter().map(|(&key, &value)| (key, value)));
    }
}

#[cfg(feature = "serde")]
impl<K, V, H> Serialize for HashMap<K, V, H>
where
    K: Serialize + Eq + Hash,
    V: Serialize,
    H: BuildHasher,
{
    #[inline]
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serializer.collect_map(self)
    }
}

#[cfg(feature = "serde")]
impl<'de, K, V, S> Deserialize<'de> for HashMap<K, V, S>
where
    K: Deserialize<'de> + Eq + Hash,
    V: Deserialize<'de>,
    S: BuildHasher + Default,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        struct MapVisitor<K, V, S> {
            marker: PhantomData<HashMap<K, V, S>>,
        }

        impl<'de, K, V, S> Visitor<'de> for MapVisitor<K, V, S>
        where
            K: Deserialize<'de> + Eq + Hash,
            V: Deserialize<'de>,
            S: BuildHasher + Default,
        {
            type Value = HashMap<K, V, S>;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("a map")
            }

            #[inline]
            fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
            where
                A: MapAccess<'de>,
            {
                let mut values = HashMap::with_capacity_and_hasher(
                    size_hint::cautious(map.size_hint()),
                    S::default(),
                );

                while let Some((key, value)) = map.next_entry()? {
                    values.insert(key, value);
                }

                Ok(values)
            }
        }

        let visitor = MapVisitor { marker: PhantomData };
        deserializer.deserialize_map(visitor)
    }
}


#[allow(dead_code)]
fn assert_covariance() {
    fn map_key<'new>(v: HashMap<&'static str, u8>) -> HashMap<&'new str, u8> {
        v
    }
    fn map_val<'new>(v: HashMap<u8, &'static str>) -> HashMap<u8, &'new str> {
        v
    }
    fn iter_key<'a, 'new>(v: Iter<'a, &'static str, u8>) -> Iter<'a, &'new str, u8> {
        v
    }
    fn iter_val<'a, 'new>(v: Iter<'a, u8, &'static str>) -> Iter<'a, u8, &'new str> {
        v
    }
    fn into_iter_key<'new>(v: IntoIter<&'static str, u8>) -> IntoIter<&'new str, u8> {
        v
    }
    fn into_iter_val<'new>(v: IntoIter<u8, &'static str>) -> IntoIter<u8, &'new str> {
        v
    }
    fn keys_key<'a, 'new>(v: Keys<'a, &'static str, u8>) -> Keys<'a, &'new str, u8> {
        v
    }
    fn keys_val<'a, 'new>(v: Keys<'a, u8, &'static str>) -> Keys<'a, u8, &'new str> {
        v
    }
    fn values_key<'a, 'new>(v: Values<'a, &'static str, u8>) -> Values<'a, &'new str, u8> {
        v
    }
    fn values_val<'a, 'new>(v: Values<'a, u8, &'static str>) -> Values<'a, u8, &'new str> {
        v
    }
    fn drain<'new>(
        d: Drain<'static, &'static str, &'static str>,
    ) -> Drain<'new, &'new str, &'new str> {
        d
    }
}