Rollup merge of #76867 - poliorcetics:intra-doc-core-iter, r=jyn514

[rust.git] / library / std / src / collections / hash / map.rs

#[cfg(test)]
mod tests;

use self::Entry::*;

use hashbrown::hash_map as base;

use crate::borrow::Borrow;
use crate::cell::Cell;
use crate::collections::TryReserveError;
use crate::fmt::{self, Debug};
#[allow(deprecated)]
use crate::hash::{BuildHasher, Hash, Hasher, SipHasher13};
use crate::iter::{FromIterator, FusedIterator};
use crate::ops::Index;
use crate::sys;

/// A hash map implemented with quadratic probing and SIMD lookup.
///
/// By default, `HashMap` uses a hashing algorithm selected to provide
/// resistance against HashDoS attacks. The algorithm is randomly seeded, and a
/// reasonable best-effort is made to generate this seed from a high quality,
/// secure source of randomness provided by the host without blocking the
/// program. Because of this, the randomness of the seed depends on the output
/// quality of the system's random number generator when the seed is created.
/// In particular, seeds generated when the system's entropy pool is abnormally
/// low such as during system boot may be of a lower quality.
///
/// The default hashing algorithm is currently SipHash 1-3, though this is
/// subject to change at any point in the future. While its performance is very
/// competitive for medium sized keys, other hashing algorithms will outperform
/// it for small keys such as integers as well as large keys such as long
/// strings, though those algorithms 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.
///
/// The hash table implementation is a Rust port of Google's [SwissTable].
/// The original C++ version of SwissTable can be found [here], and this
/// [CppCon talk] gives an overview of how the algorithm works.
///
/// [SwissTable]: https://abseil.io/blog/20180927-swisstables
/// [here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h
/// [CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4
///
/// # Examples
///
/// ```
/// use std::collections::HashMap;
///
/// // Type inference lets us omit an explicit type signature (which
/// // would be `HashMap<String, String>` in this example).
/// let mut book_reviews = HashMap::new();
///
/// // Review some books.
/// book_reviews.insert(
///     "Adventures of Huckleberry Finn".to_string(),
///     "My favorite book.".to_string(),
/// );
/// book_reviews.insert(
///     "Grimms' Fairy Tales".to_string(),
///     "Masterpiece.".to_string(),
/// );
/// book_reviews.insert(
///     "Pride and Prejudice".to_string(),
///     "Very enjoyable.".to_string(),
/// );
/// book_reviews.insert(
///     "The Adventures of Sherlock Holmes".to_string(),
///     "Eye lyked it alot.".to_string(),
/// );
///
/// // Check for a specific one.
/// // When collections store owned values (String), they can still be
/// // queried using references (&str).
/// 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)
///     }
/// }
///
/// // Look up the value for a key (will panic if the key is not found).
/// println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);
///
/// // 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 std::collections::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 key type is to derive [`Eq`] and [`Hash`].
/// We must also derive [`PartialEq`].
///
/// [`RefCell`]: crate::cell::RefCell
/// [`Cell`]: crate::cell::Cell
/// [`default`]: Default::default
/// [`with_hasher`]: Self::with_hasher
/// [`with_capacity_and_hasher`]: Self::with_capacity_and_hasher
/// [`fnv`]: https://crates.io/crates/fnv
///
/// ```
/// use std::collections::HashMap;
///
/// #[derive(Hash, Eq, PartialEq, Debug)]
/// struct Viking {
///     name: String,
///     country: String,
/// }
///
/// impl Viking {
///     /// Creates 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 std::collections::HashMap;
///
/// let timber_resources: HashMap<&str, i32> = [("Norway", 100), ("Denmark", 50), ("Iceland", 10)]
///     .iter().cloned().collect();
/// // use the values stored in map
/// ```

#[derive(Clone)]
#[cfg_attr(not(test), rustc_diagnostic_item = "hashmap_type")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct HashMap<K, V, S = RandomState> {
    base: base::HashMap<K, V, S>,
}

impl<K, V> HashMap<K, V, RandomState> {
    /// 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 std::collections::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::new();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> HashMap<K, V, RandomState> {
        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 std::collections::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn with_capacity(capacity: usize) -> HashMap<K, V, RandomState> {
        HashMap::with_capacity_and_hasher(capacity, Default::default())
    }
}

impl<K, V, S> HashMap<K, V, S> {
    /// 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.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the HashMap to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::hash_map::RandomState;
    ///
    /// let s = RandomState::new();
    /// let mut map = HashMap::with_hasher(s);
    /// map.insert(1, 2);
    /// ```
    #[inline]
    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
    pub fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
        HashMap { base: base::HashMap::with_hasher(hash_builder) }
    }

    /// 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.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the HashMap to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::hash_map::RandomState;
    ///
    /// let s = RandomState::new();
    /// let mut map = HashMap::with_capacity_and_hasher(10, s);
    /// map.insert(1, 2);
    /// ```
    #[inline]
    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> HashMap<K, V, S> {
        HashMap { base: base::HashMap::with_capacity_and_hasher(capacity, 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 std::collections::HashMap;
    /// let map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// assert!(map.capacity() >= 100);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize {
        self.base.capacity()
    }

    /// An iterator visiting all keys in arbitrary order.
    /// The iterator element type is `&'a K`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    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 std::collections::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);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    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 std::collections::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);
    /// }
    /// ```
    #[stable(feature = "map_values_mut", since = "1.10.0")]
    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 std::collections::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);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, K, V> {
        Iter { base: self.base.iter() }
    }

    /// 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 std::collections::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);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
        IterMut { base: self.base.iter_mut() }
    }

    /// Returns the number of elements in the map.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// assert_eq!(a.len(), 0);
    /// a.insert(1, "a");
    /// assert_eq!(a.len(), 1);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize {
        self.base.len()
    }

    /// Returns `true` if the map contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// assert!(a.is_empty());
    /// a.insert(1, "a");
    /// assert!(!a.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.base.is_empty()
    }

    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
    /// allocated memory for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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]
    #[stable(feature = "drain", since = "1.6.0")]
    pub fn drain(&mut self) -> Drain<'_, K, V> {
        Drain { base: self.base.drain() }
    }

    /// Creates an iterator which uses a closure to determine if an element should be removed.
    ///
    /// If the closure returns true, the element is removed from the map and yielded.
    /// If the closure returns false, or panics, the element remains in the map and will not be
    /// yielded.
    ///
    /// Note that `drain_filter` lets you mutate every value in the filter closure, regardless of
    /// whether you choose to keep or remove it.
    ///
    /// If the iterator is only partially consumed or not consumed at all, each of the remaining
    /// elements will still be subjected to the closure and removed and dropped if it returns true.
    ///
    /// It is unspecified how many more elements will be subjected to the closure
    /// if a panic occurs in the closure, or a panic occurs while dropping an element,
    /// or if the `DrainFilter` value is leaked.
    ///
    /// # Examples
    ///
    /// Splitting a map into even and odd keys, reusing the original map:
    ///
    /// ```
    /// #![feature(hash_drain_filter)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    /// let drained: HashMap<i32, i32> = map.drain_filter(|k, _v| k % 2 == 0).collect();
    ///
    /// let mut evens = drained.keys().copied().collect::<Vec<_>>();
    /// let mut odds = map.keys().copied().collect::<Vec<_>>();
    /// evens.sort();
    /// odds.sort();
    ///
    /// assert_eq!(evens, vec![0, 2, 4, 6]);
    /// assert_eq!(odds, vec![1, 3, 5, 7]);
    /// ```
    #[inline]
    #[unstable(feature = "hash_drain_filter", issue = "59618")]
    pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, K, V, F>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        DrainFilter { base: self.base.drain_filter(pred) }
    }

    /// Clears the map, removing all key-value pairs. Keeps the allocated memory
    /// for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// a.clear();
    /// assert!(a.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn clear(&mut self) {
        self.base.clear();
    }

    /// Returns a reference to the map's [`BuildHasher`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::hash_map::RandomState;
    ///
    /// let hasher = RandomState::new();
    /// let map: HashMap<i32, i32> = HashMap::with_hasher(hasher);
    /// let hasher: &RandomState = map.hasher();
    /// ```
    #[inline]
    #[stable(feature = "hashmap_public_hasher", since = "1.9.0")]
    pub fn hasher(&self) -> &S {
        self.base.hasher()
    }
}

impl<K, V, S> HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher,
{
    /// 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`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::new();
    /// map.reserve(10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        self.base.reserve(additional)
    }

    /// Tries to reserve capacity for at least `additional` more elements to be inserted
    /// in the given `HashMap<K,V>`. The collection may reserve more space to avoid
    /// frequent reallocations.
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_reserve)]
    /// use std::collections::HashMap;
    /// let mut map: HashMap<&str, isize> = HashMap::new();
    /// map.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");
    /// ```
    #[inline]
    #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.base.try_reserve(additional).map_err(map_try_reserve_error)
    }

    /// 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 std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn shrink_to_fit(&mut self) {
        self.base.shrink_to_fit();
    }

    /// 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
    ///
    /// ```
    /// #![feature(shrink_to)]
    /// use std::collections::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]
    #[unstable(feature = "shrink_to", reason = "new API", issue = "56431")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        assert!(self.capacity() >= min_capacity, "Tried to shrink to a larger capacity");
        self.base.shrink_to(min_capacity);
    }

    /// Gets the given key's corresponding entry in the map for in-place manipulation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
        map_entry(self.base.rustc_entry(key))
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.get(&1), Some(&"a"));
    /// assert_eq!(map.get(&2), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.get(k)
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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);
    /// ```
    #[stable(feature = "map_get_key_value", since = "1.40.0")]
    #[inline]
    pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.get_key_value(k)
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.contains_key(&1), true);
    /// assert_eq!(map.contains_key(&2), false);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.contains_key(k)
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.get_mut(k)
    }

    /// 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.
    ///
    /// [module-level documentation]: crate::collections#insert-and-complex-keys
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn insert(&mut self, k: K, v: V) -> Option<V> {
        self.base.insert(k, v)
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// assert_eq!(map.remove(&1), Some("a"));
    /// assert_eq!(map.remove(&1), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.remove(k)
    }

    /// 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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);
    /// # }
    /// ```
    #[stable(feature = "hash_map_remove_entry", since = "1.27.0")]
    #[inline]
    pub fn remove_entry<Q: ?Sized>(&mut self, k: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.remove_entry(k)
    }

    /// 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 std::collections::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);
    /// ```
    #[stable(feature = "retain_hash_collection", since = "1.18.0")]
    #[inline]
    pub fn retain<F>(&mut self, f: F)
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        self.base.retain(f)
    }

    /// Creates a consuming iterator visiting all the keys in arbitrary order.
    /// The map cannot be used after calling this.
    /// The iterator element type is `K`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_into_keys_values)]
    /// use std::collections::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// let vec: Vec<&str> = map.into_keys().collect();
    /// ```
    #[inline]
    #[unstable(feature = "map_into_keys_values", issue = "75294")]
    pub fn into_keys(self) -> IntoKeys<K, V> {
        IntoKeys { inner: self.into_iter() }
    }

    /// Creates a consuming iterator visiting all the values in arbitrary order.
    /// The map cannot be used after calling this.
    /// The iterator element type is `V`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_into_keys_values)]
    /// use std::collections::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// let vec: Vec<i32> = map.into_values().collect();
    /// ```
    #[inline]
    #[unstable(feature = "map_into_keys_values", issue = "75294")]
    pub fn into_values(self) -> IntoValues<K, V> {
        IntoValues { inner: self.into_iter() }
    }
}

impl<K, V, S> HashMap<K, V, S>
where
    S: BuildHasher,
{
    /// Creates a raw entry builder for the HashMap.
    ///
    /// Raw entries provide the lowest level of control for searching and
    /// manipulating a map. They must be manually initialized with a hash and
    /// then manually searched. After this, insertions into a vacant entry
    /// still require an owned key to be provided.
    ///
    /// Raw entries are useful for such exotic situations as:
    ///
    /// * Hash memoization
    /// * Deferring the creation of an owned key until it is known to be required
    /// * Using a search key that doesn't work with the Borrow trait
    /// * Using custom comparison logic without newtype wrappers
    ///
    /// Because raw entries provide much more low-level control, it's much easier
    /// to put the HashMap into an inconsistent state which, while memory-safe,
    /// will cause the map to produce seemingly random results. Higher-level and
    /// more foolproof APIs like `entry` should be preferred when possible.
    ///
    /// In particular, the hash used to initialized the raw entry must still be
    /// consistent with the hash of the key that is ultimately stored in the entry.
    /// This is because implementations of HashMap may need to recompute hashes
    /// when resizing, at which point only the keys are available.
    ///
    /// Raw entries give mutable access to the keys. This must not be used
    /// to modify how the key would compare or hash, as the map will not re-evaluate
    /// where the key should go, meaning the keys may become "lost" if their
    /// location does not reflect their state. For instance, if you change a key
    /// so that the map now contains keys which compare equal, search may start
    /// acting erratically, with two keys randomly masking each other. Implementations
    /// are free to assume this doesn't happen (within the limits of memory-safety).
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<'_, K, V, S> {
        RawEntryBuilderMut { map: self }
    }

    /// Creates a raw immutable entry builder for the HashMap.
    ///
    /// Raw entries provide the lowest level of control for searching and
    /// manipulating a map. They must be manually initialized with a hash and
    /// then manually searched.
    ///
    /// This is useful for
    /// * Hash memoization
    /// * Using a search key that doesn't work with the Borrow trait
    /// * Using custom comparison logic without newtype wrappers
    ///
    /// Unless you are in such a situation, higher-level and more foolproof APIs like
    /// `get` should be preferred.
    ///
    /// Immutable raw entries have very limited use; you might instead want `raw_entry_mut`.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn raw_entry(&self) -> RawEntryBuilder<'_, K, V, S> {
        RawEntryBuilder { map: self }
    }
}

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V, S> Eq for HashMap<K, V, S>
where
    K: Eq + Hash,
    V: Eq,
    S: BuildHasher,
{
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V, S> Debug for HashMap<K, V, S>
where
    K: Debug,
    V: Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_map().entries(self.iter()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V, S> Default for HashMap<K, V, S>
where
    S: 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())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, Q: ?Sized, V, S> Index<&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`]: HashMap::iter
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, K: 'a, V: 'a> {
    base: base::Iter<'a, K, V>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> Clone for Iter<'_, K, V> {
    #[inline]
    fn clone(&self) -> Self {
        Iter { base: self.base.clone() }
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K: Debug, V: Debug> fmt::Debug for Iter<'_, 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`]: HashMap::iter_mut
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IterMut<'a, K: 'a, V: 'a> {
    base: base::IterMut<'a, K, V>,
}

impl<'a, K, V> IterMut<'a, K, V> {
    /// Returns a iterator of references over the remaining items.
    #[inline]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter { base: self.base.rustc_iter() }
    }
}

/// An owning iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`into_iter`] method on [`HashMap`]
/// (provided by the `IntoIterator` trait). See its documentation for more.
///
/// [`into_iter`]: IntoIterator::into_iter
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<K, V> {
    base: base::IntoIter<K, V>,
}

impl<K, V> IntoIter<K, V> {
    /// Returns a iterator of references over the remaining items.
    #[inline]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter { base: self.base.rustc_iter() }
    }
}

/// An iterator over the keys of a `HashMap`.
///
/// This `struct` is created by the [`keys`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`keys`]: HashMap::keys
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Keys<'a, K: 'a, V: 'a> {
    inner: Iter<'a, K, V>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> Clone for Keys<'_, K, V> {
    #[inline]
    fn clone(&self) -> Self {
        Keys { inner: self.inner.clone() }
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K: Debug, V> fmt::Debug for Keys<'_, 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`]: HashMap::values
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Values<'a, K: 'a, V: 'a> {
    inner: Iter<'a, K, V>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> Clone for Values<'_, K, V> {
    #[inline]
    fn clone(&self) -> Self {
        Values { inner: self.inner.clone() }
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K, V: Debug> fmt::Debug for Values<'_, 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`]: HashMap::drain
#[stable(feature = "drain", since = "1.6.0")]
pub struct Drain<'a, K: 'a, V: 'a> {
    base: base::Drain<'a, K, V>,
}

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

/// A draining, filtering iterator over the entries of a `HashMap`.
///
/// This `struct` is created by the [`drain_filter`] method on [`HashMap`].
///
/// [`drain_filter`]: HashMap::drain_filter
#[unstable(feature = "hash_drain_filter", issue = "59618")]
pub struct DrainFilter<'a, K, V, F>
where
    F: FnMut(&K, &mut V) -> bool,
{
    base: base::DrainFilter<'a, K, V, F>,
}

/// 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`]: HashMap::values_mut
#[stable(feature = "map_values_mut", since = "1.10.0")]
pub struct ValuesMut<'a, K: 'a, V: 'a> {
    inner: IterMut<'a, K, V>,
}

/// An owning iterator over the keys of a `HashMap`.
///
/// This `struct` is created by the [`into_keys`] method on [`HashMap`].
/// See its documentation for more.
///
/// [`into_keys`]: HashMap::into_keys
#[unstable(feature = "map_into_keys_values", issue = "75294")]
pub struct IntoKeys<K, V> {
    inner: IntoIter<K, V>,
}

/// An owning iterator over the values of a `HashMap`.
///
/// This `struct` is created by the [`into_values`] method on [`HashMap`].
/// See its documentation for more.
///
/// [`into_values`]: HashMap::into_values
#[unstable(feature = "map_into_keys_values", issue = "75294")]
pub struct IntoValues<K, V> {
    inner: IntoIter<K, V>,
}

/// A builder for computing where in a HashMap a key-value pair would be stored.
///
/// See the [`HashMap::raw_entry_mut`] docs for usage examples.
///
/// [`HashMap::raw_entry_mut`]: HashMap::raw_entry_mut

#[unstable(feature = "hash_raw_entry", issue = "56167")]
pub struct RawEntryBuilderMut<'a, K: 'a, V: 'a, S: 'a> {
    map: &'a mut HashMap<K, V, S>,
}

/// A view into a single entry in a map, which may either be vacant or occupied.
///
/// This is a lower-level version of [`Entry`].
///
/// This `enum` is constructed through the [`raw_entry_mut`] method on [`HashMap`],
/// then calling one of the methods of that [`RawEntryBuilderMut`].
///
/// [`raw_entry_mut`]: HashMap::raw_entry_mut
#[unstable(feature = "hash_raw_entry", issue = "56167")]
pub enum RawEntryMut<'a, K: 'a, V: 'a, S: 'a> {
    /// An occupied entry.
    Occupied(RawOccupiedEntryMut<'a, K, V, S>),
    /// A vacant entry.
    Vacant(RawVacantEntryMut<'a, K, V, S>),
}

/// A view into an occupied entry in a `HashMap`.
/// It is part of the [`RawEntryMut`] enum.
#[unstable(feature = "hash_raw_entry", issue = "56167")]
pub struct RawOccupiedEntryMut<'a, K: 'a, V: 'a, S: 'a> {
    base: base::RawOccupiedEntryMut<'a, K, V, S>,
}

/// A view into a vacant entry in a `HashMap`.
/// It is part of the [`RawEntryMut`] enum.
#[unstable(feature = "hash_raw_entry", issue = "56167")]
pub struct RawVacantEntryMut<'a, K: 'a, V: 'a, S: 'a> {
    base: base::RawVacantEntryMut<'a, K, V, S>,
}

/// A builder for computing where in a HashMap a key-value pair would be stored.
///
/// See the [`HashMap::raw_entry`] docs for usage examples.
///
/// [`HashMap::raw_entry`]: HashMap::raw_entry
#[unstable(feature = "hash_raw_entry", issue = "56167")]
pub struct RawEntryBuilder<'a, K: 'a, V: 'a, S: 'a> {
    map: &'a HashMap<K, V, S>,
}

impl<'a, K, V, S> RawEntryBuilderMut<'a, K, V, S>
where
    S: BuildHasher,
{
    /// Creates a `RawEntryMut` from the given key.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_key<Q: ?Sized>(self, k: &Q) -> RawEntryMut<'a, K, V, S>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        map_raw_entry(self.map.base.raw_entry_mut().from_key(k))
    }

    /// Creates a `RawEntryMut` from the given key and its hash.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_key_hashed_nocheck<Q: ?Sized>(self, hash: u64, k: &Q) -> RawEntryMut<'a, K, V, S>
    where
        K: Borrow<Q>,
        Q: Eq,
    {
        map_raw_entry(self.map.base.raw_entry_mut().from_key_hashed_nocheck(hash, k))
    }

    /// Creates a `RawEntryMut` from the given hash.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_hash<F>(self, hash: u64, is_match: F) -> RawEntryMut<'a, K, V, S>
    where
        for<'b> F: FnMut(&'b K) -> bool,
    {
        map_raw_entry(self.map.base.raw_entry_mut().from_hash(hash, is_match))
    }
}

impl<'a, K, V, S> RawEntryBuilder<'a, K, V, S>
where
    S: BuildHasher,
{
    /// Access an entry by key.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_key<Q: ?Sized>(self, k: &Q) -> Option<(&'a K, &'a V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.base.raw_entry().from_key(k)
    }

    /// Access an entry by a key and its hash.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_key_hashed_nocheck<Q: ?Sized>(self, hash: u64, k: &Q) -> Option<(&'a K, &'a V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.map.base.raw_entry().from_key_hashed_nocheck(hash, k)
    }

    /// Access an entry by hash.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn from_hash<F>(self, hash: u64, is_match: F) -> Option<(&'a K, &'a V)>
    where
        F: FnMut(&K) -> bool,
    {
        self.map.base.raw_entry().from_hash(hash, is_match)
    }
}

impl<'a, K, V, S> RawEntryMut<'a, K, V, S> {
    /// Ensures a value is in the entry by inserting the default if empty, and returns
    /// mutable references to the key and value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_raw_entry)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// map.raw_entry_mut().from_key("poneyland").or_insert("poneyland", 3);
    /// assert_eq!(map["poneyland"], 3);
    ///
    /// *map.raw_entry_mut().from_key("poneyland").or_insert("poneyland", 10).1 *= 2;
    /// assert_eq!(map["poneyland"], 6);
    /// ```
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn or_insert(self, default_key: K, default_val: V) -> (&'a mut K, &'a mut V)
    where
        K: Hash,
        S: BuildHasher,
    {
        match self {
            RawEntryMut::Occupied(entry) => entry.into_key_value(),
            RawEntryMut::Vacant(entry) => entry.insert(default_key, default_val),
        }
    }

    /// Ensures a value is in the entry by inserting the result of the default function if empty,
    /// and returns mutable references to the key and value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_raw_entry)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, String> = HashMap::new();
    ///
    /// map.raw_entry_mut().from_key("poneyland").or_insert_with(|| {
    ///     ("poneyland", "hoho".to_string())
    /// });
    ///
    /// assert_eq!(map["poneyland"], "hoho".to_string());
    /// ```
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn or_insert_with<F>(self, default: F) -> (&'a mut K, &'a mut V)
    where
        F: FnOnce() -> (K, V),
        K: Hash,
        S: BuildHasher,
    {
        match self {
            RawEntryMut::Occupied(entry) => entry.into_key_value(),
            RawEntryMut::Vacant(entry) => {
                let (k, v) = default();
                entry.insert(k, v)
            }
        }
    }

    /// Provides in-place mutable access to an occupied entry before any
    /// potential inserts into the map.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_raw_entry)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// map.raw_entry_mut()
    ///    .from_key("poneyland")
    ///    .and_modify(|_k, v| { *v += 1 })
    ///    .or_insert("poneyland", 42);
    /// assert_eq!(map["poneyland"], 42);
    ///
    /// map.raw_entry_mut()
    ///    .from_key("poneyland")
    ///    .and_modify(|_k, v| { *v += 1 })
    ///    .or_insert("poneyland", 0);
    /// assert_eq!(map["poneyland"], 43);
    /// ```
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn and_modify<F>(self, f: F) -> Self
    where
        F: FnOnce(&mut K, &mut V),
    {
        match self {
            RawEntryMut::Occupied(mut entry) => {
                {
                    let (k, v) = entry.get_key_value_mut();
                    f(k, v);
                }
                RawEntryMut::Occupied(entry)
            }
            RawEntryMut::Vacant(entry) => RawEntryMut::Vacant(entry),
        }
    }
}

impl<'a, K, V, S> RawOccupiedEntryMut<'a, K, V, S> {
    /// Gets a reference to the key in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn key(&self) -> &K {
        self.base.key()
    }

    /// Gets a mutable reference to the key in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn key_mut(&mut self) -> &mut K {
        self.base.key_mut()
    }

    /// Converts the entry into a mutable reference to the key in the entry
    /// with a lifetime bound to the map itself.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn into_key(self) -> &'a mut K {
        self.base.into_key()
    }

    /// Gets a reference to the value in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn get(&self) -> &V {
        self.base.get()
    }

    /// Converts the OccupiedEntry into a mutable reference to the value in the entry
    /// with a lifetime bound to the map itself.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn into_mut(self) -> &'a mut V {
        self.base.into_mut()
    }

    /// Gets a mutable reference to the value in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn get_mut(&mut self) -> &mut V {
        self.base.get_mut()
    }

    /// Gets a reference to the key and value in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn get_key_value(&mut self) -> (&K, &V) {
        self.base.get_key_value()
    }

    /// Gets a mutable reference to the key and value in the entry.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn get_key_value_mut(&mut self) -> (&mut K, &mut V) {
        self.base.get_key_value_mut()
    }

    /// Converts the OccupiedEntry into a mutable reference to the key and value in the entry
    /// with a lifetime bound to the map itself.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn into_key_value(self) -> (&'a mut K, &'a mut V) {
        self.base.into_key_value()
    }

    /// Sets the value of the entry, and returns the entry's old value.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn insert(&mut self, value: V) -> V {
        self.base.insert(value)
    }

    /// Sets the value of the entry, and returns the entry's old value.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn insert_key(&mut self, key: K) -> K {
        self.base.insert_key(key)
    }

    /// Takes the value out of the entry, and returns it.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn remove(self) -> V {
        self.base.remove()
    }

    /// Take the ownership of the key and value from the map.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn remove_entry(self) -> (K, V) {
        self.base.remove_entry()
    }
}

impl<'a, K, V, S> RawVacantEntryMut<'a, K, V, S> {
    /// Sets the value of the entry with the VacantEntry's key,
    /// and returns a mutable reference to it.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn insert(self, key: K, value: V) -> (&'a mut K, &'a mut V)
    where
        K: Hash,
        S: BuildHasher,
    {
        self.base.insert(key, value)
    }

    /// Sets the value of the entry with the VacantEntry's key,
    /// and returns a mutable reference to it.
    #[inline]
    #[unstable(feature = "hash_raw_entry", issue = "56167")]
    pub fn insert_hashed_nocheck(self, hash: u64, key: K, value: V) -> (&'a mut K, &'a mut V)
    where
        K: Hash,
        S: BuildHasher,
    {
        self.base.insert_hashed_nocheck(hash, key, value)
    }
}

#[unstable(feature = "hash_raw_entry", issue = "56167")]
impl<K, V, S> Debug for RawEntryBuilderMut<'_, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RawEntryBuilder").finish()
    }
}

#[unstable(feature = "hash_raw_entry", issue = "56167")]
impl<K: Debug, V: Debug, S> Debug for RawEntryMut<'_, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            RawEntryMut::Vacant(ref v) => f.debug_tuple("RawEntry").field(v).finish(),
            RawEntryMut::Occupied(ref o) => f.debug_tuple("RawEntry").field(o).finish(),
        }
    }
}

#[unstable(feature = "hash_raw_entry", issue = "56167")]
impl<K: Debug, V: Debug, S> Debug for RawOccupiedEntryMut<'_, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RawOccupiedEntryMut")
            .field("key", self.key())
            .field("value", self.get())
            .finish()
    }
}

#[unstable(feature = "hash_raw_entry", issue = "56167")]
impl<K, V, S> Debug for RawVacantEntryMut<'_, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RawVacantEntryMut").finish()
    }
}

#[unstable(feature = "hash_raw_entry", issue = "56167")]
impl<K, V, S> Debug for RawEntryBuilder<'_, K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RawEntryBuilder").finish()
    }
}

/// 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`].
///
/// [`entry`]: HashMap::entry
#[stable(feature = "rust1", since = "1.0.0")]
pub enum Entry<'a, K: 'a, V: 'a> {
    /// An occupied entry.
    #[stable(feature = "rust1", since = "1.0.0")]
    Occupied(#[stable(feature = "rust1", since = "1.0.0")] OccupiedEntry<'a, K, V>),

    /// A vacant entry.
    #[stable(feature = "rust1", since = "1.0.0")]
    Vacant(#[stable(feature = "rust1", since = "1.0.0")] VacantEntry<'a, K, V>),
}

#[stable(feature = "debug_hash_map", since = "1.12.0")]
impl<K: Debug, V: Debug> Debug for Entry<'_, K, V> {
    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.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
    base: base::RustcOccupiedEntry<'a, K, V>,
}

#[stable(feature = "debug_hash_map", since = "1.12.0")]
impl<K: Debug, V: Debug> Debug for OccupiedEntry<'_, K, V> {
    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.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct VacantEntry<'a, K: 'a, V: 'a> {
    base: base::RustcVacantEntry<'a, K, V>,
}

#[stable(feature = "debug_hash_map", since = "1.12.0")]
impl<K: Debug, V> Debug for VacantEntry<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("VacantEntry").field(self.key()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> {
    type Item = (&'a K, &'a V);
    type IntoIter = Iter<'a, K, V>;

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S> {
    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()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V, S> IntoIterator for HashMap<K, V, S> {
    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 std::collections::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 { base: self.base.into_iter() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
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.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> ExactSizeIterator for Iter<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Iter<'_, K, V> {}

#[stable(feature = "rust1", since = "1.0.0")]
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.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> ExactSizeIterator for IterMut<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for IterMut<'_, K, V> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K, V> fmt::Debug for IterMut<'_, 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()
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<(K, V)> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> ExactSizeIterator for IntoIter<K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for IntoIter<K, V> {}

#[stable(feature = "std_debug", since = "1.16.0")]
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()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
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()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> ExactSizeIterator for Keys<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Keys<'_, K, V> {}

#[stable(feature = "rust1", since = "1.0.0")]
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()
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V> ExactSizeIterator for Values<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Values<'_, K, V> {}

#[stable(feature = "map_values_mut", since = "1.10.0")]
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()
    }
}
#[stable(feature = "map_values_mut", since = "1.10.0")]
impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for ValuesMut<'_, K, V> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K, V> fmt::Debug for ValuesMut<'_, 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()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> Iterator for IntoKeys<K, V> {
    type Item = K;

    #[inline]
    fn next(&mut self) -> Option<K> {
        self.inner.next().map(|(k, _)| k)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> FusedIterator for IntoKeys<K, V> {}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K: Debug, V: Debug> fmt::Debug for IntoKeys<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.inner.iter().map(|(k, _)| k)).finish()
    }
}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> Iterator for IntoValues<K, V> {
    type Item = V;

    #[inline]
    fn next(&mut self) -> Option<V> {
        self.inner.next().map(|(_, v)| v)
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}
#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> ExactSizeIterator for IntoValues<K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}
#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K, V> FusedIterator for IntoValues<K, V> {}

#[unstable(feature = "map_into_keys_values", issue = "75294")]
impl<K: Debug, V: Debug> fmt::Debug for IntoValues<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.inner.iter().map(|(_, v)| v)).finish()
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl<'a, K, V> Iterator for Drain<'a, K, V> {
    type Item = (K, V);

    #[inline]
    fn next(&mut self) -> Option<(K, V)> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}
#[stable(feature = "drain", since = "1.6.0")]
impl<K, V> ExactSizeIterator for Drain<'_, K, V> {
    #[inline]
    fn len(&self) -> usize {
        self.base.len()
    }
}
#[stable(feature = "fused", since = "1.26.0")]
impl<K, V> FusedIterator for Drain<'_, K, V> {}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<K, V> fmt::Debug for Drain<'_, 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()
    }
}

#[unstable(feature = "hash_drain_filter", issue = "59618")]
impl<K, V, F> Iterator for DrainFilter<'_, K, V, F>
where
    F: FnMut(&K, &mut V) -> bool,
{
    type Item = (K, V);

    #[inline]
    fn next(&mut self) -> Option<(K, V)> {
        self.base.next()
    }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}

#[unstable(feature = "hash_drain_filter", issue = "59618")]
impl<K, V, F> FusedIterator for DrainFilter<'_, K, V, F> where F: FnMut(&K, &mut V) -> bool {}

#[unstable(feature = "hash_drain_filter", issue = "59618")]
impl<'a, K, V, F> fmt::Debug for DrainFilter<'a, K, V, F>
where
    F: FnMut(&K, &mut V) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("DrainFilter { .. }")
    }
}

impl<'a, K, V> Entry<'a, K, V> {
    #[stable(feature = "rust1", since = "1.0.0")]
    /// 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 std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// map.entry("poneyland").or_insert(3);
    /// assert_eq!(map["poneyland"], 3);
    ///
    /// *map.entry("poneyland").or_insert(10) *= 2;
    /// assert_eq!(map["poneyland"], 6);
    /// ```
    #[inline]
    pub fn or_insert(self, default: V) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => entry.insert(default),
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    /// 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 std::collections::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()),
        }
    }

    #[unstable(feature = "or_insert_with_key", issue = "71024")]
    /// Ensures a value is in the entry by inserting, if empty, the result of the default function,
    /// which takes the key as its argument, and returns a mutable reference to the value in the
    /// entry.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(or_insert_with_key)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, usize> = HashMap::new();
    ///
    /// map.entry("poneyland").or_insert_with_key(|key| key.chars().count());
    ///
    /// assert_eq!(map["poneyland"], 9);
    /// ```
    #[inline]
    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => {
                let value = default(entry.key());
                entry.insert(value)
            }
        }
    }

    /// Returns a reference to this entry's key.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    #[stable(feature = "map_entry_keys", since = "1.10.0")]
    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 std::collections::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]
    #[stable(feature = "entry_and_modify", since = "1.26.0")]
    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),
        }
    }

    /// Sets the value of the entry, and returns an OccupiedEntry.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(entry_insert)]
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, String> = HashMap::new();
    /// let entry = map.entry("poneyland").insert("hoho".to_string());
    ///
    /// assert_eq!(entry.key(), &"poneyland");
    /// ```
    #[inline]
    #[unstable(feature = "entry_insert", issue = "65225")]
    pub fn insert(self, value: V) -> OccupiedEntry<'a, K, V> {
        match self {
            Occupied(mut entry) => {
                entry.insert(value);
                entry
            }
            Vacant(entry) => entry.insert_entry(value),
        }
    }
}

impl<'a, K, V: Default> Entry<'a, K, V> {
    #[stable(feature = "entry_or_default", since = "1.28.0")]
    /// 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 std::collections::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> OccupiedEntry<'a, K, V> {
    /// Gets a reference to the key in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// map.entry("poneyland").or_insert(12);
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    #[stable(feature = "map_entry_keys", since = "1.10.0")]
    pub fn key(&self) -> &K {
        self.base.key()
    }

    /// Take the ownership of the key and value from the map.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "map_entry_recover_keys2", since = "1.12.0")]
    pub fn remove_entry(self) -> (K, V) {
        self.base.remove_entry()
    }

    /// Gets a reference to the value in the entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get(&self) -> &V {
        self.base.get()
    }

    /// 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`]: Self::into_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get_mut(&mut self) -> &mut V {
        self.base.get_mut()
    }

    /// 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`]: Self::get_mut
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_mut(self) -> &'a mut V {
        self.base.into_mut()
    }

    /// Sets the value of the entry, and returns the entry's old value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn insert(&mut self, value: V) -> V {
        self.base.insert(value)
    }

    /// Takes the value out of the entry, and returns it.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn remove(self) -> V {
        self.base.remove()
    }

    /// 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
    ///
    /// ```
    /// #![feature(map_entry_replace)]
    /// use std::collections::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]
    #[unstable(feature = "map_entry_replace", issue = "44286")]
    pub fn replace_entry(self, value: V) -> (K, V) {
        self.base.replace_entry(value)
    }

    /// Replaces the key in the hash map with the key used to create this entry.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(map_entry_replace)]
    /// use std::collections::hash_map::{Entry, HashMap};
    /// use std::rc::Rc;
    ///
    /// let mut map: HashMap<Rc<String>, u32> = HashMap::new();
    /// let 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(Rc::clone(s)) {
    ///             // Replaces the entry's key with our version of it in `known_strings`.
    ///             entry.replace_key();
    ///         }
    ///     }
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "map_entry_replace", issue = "44286")]
    pub fn replace_key(self) -> K {
        self.base.replace_key()
    }
}

impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
    /// Gets a reference to the key that would be used when inserting a value
    /// through the `VacantEntry`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    /// assert_eq!(map.entry("poneyland").key(), &"poneyland");
    /// ```
    #[inline]
    #[stable(feature = "map_entry_keys", since = "1.10.0")]
    pub fn key(&self) -> &K {
        self.base.key()
    }

    /// Take ownership of the key.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::hash_map::Entry;
    ///
    /// let mut map: HashMap<&str, u32> = HashMap::new();
    ///
    /// if let Entry::Vacant(v) = map.entry("poneyland") {
    ///     v.into_key();
    /// }
    /// ```
    #[inline]
    #[stable(feature = "map_entry_recover_keys2", since = "1.12.0")]
    pub fn into_key(self) -> K {
        self.base.into_key()
    }

    /// Sets the value of the entry with the VacantEntry's key,
    /// and returns a mutable reference to it.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn insert(self, value: V) -> &'a mut V {
        self.base.insert(value)
    }

    /// Sets the value of the entry with the VacantEntry's key,
    /// and returns an OccupiedEntry.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashMap;
    /// use std::collections::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]
    fn insert_entry(self, value: V) -> OccupiedEntry<'a, K, V> {
        let base = self.base.insert_entry(value);
        OccupiedEntry { base }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>
where
    K: Eq + Hash,
    S: BuildHasher + Default,
{
    fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> HashMap<K, V, S> {
        let mut map = HashMap::with_hasher(Default::default());
        map.extend(iter);
        map
    }
}

/// Inserts all new key-values from the iterator and replaces values with existing
/// keys with new values returned from the iterator.
#[stable(feature = "rust1", since = "1.0.0")]
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) {
        self.base.extend(iter)
    }

    #[inline]
    fn extend_one(&mut self, (k, v): (K, V)) {
        self.base.insert(k, v);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        // self.base.extend_reserve(additional);
        // FIXME: hashbrown should implement this method.
        // But until then, use the same reservation logic:

        // 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 reserve = if self.is_empty() { additional } else { (additional + 1) / 2 };
        self.base.reserve(reserve);
    }
}

#[stable(feature = "hash_extend_copy", since = "1.4.0")]
impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>
where
    K: Eq + Hash + Copy,
    V: Copy,
    S: BuildHasher,
{
    #[inline]
    fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T) {
        self.base.extend(iter)
    }

    #[inline]
    fn extend_one(&mut self, (&k, &v): (&'a K, &'a V)) {
        self.base.insert(k, v);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        Extend::<(K, V)>::extend_reserve(self, additional)
    }
}

/// `RandomState` is the default state for [`HashMap`] types.
///
/// A particular instance `RandomState` will create the same instances of
/// [`Hasher`], but the hashers created by two different `RandomState`
/// instances are unlikely to produce the same result for the same values.
///
/// # Examples
///
/// ```
/// use std::collections::HashMap;
/// use std::collections::hash_map::RandomState;
///
/// let s = RandomState::new();
/// let mut map = HashMap::with_hasher(s);
/// map.insert(1, 2);
/// ```
#[derive(Clone)]
#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
pub struct RandomState {
    k0: u64,
    k1: u64,
}

impl RandomState {
    /// Constructs a new `RandomState` that is initialized with random keys.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::hash_map::RandomState;
    ///
    /// let s = RandomState::new();
    /// ```
    #[inline]
    #[allow(deprecated)]
    // rand
    #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
    pub fn new() -> RandomState {
        // Historically this function did not cache keys from the OS and instead
        // simply always called `rand::thread_rng().gen()` twice. In #31356 it
        // was discovered, however, that because we re-seed the thread-local RNG
        // from the OS periodically that this can cause excessive slowdown when
        // many hash maps are created on a thread. To solve this performance
        // trap we cache the first set of randomly generated keys per-thread.
        //
        // Later in #36481 it was discovered that exposing a deterministic
        // iteration order allows a form of DOS attack. To counter that we
        // increment one of the seeds on every RandomState creation, giving
        // every corresponding HashMap a different iteration order.
        thread_local!(static KEYS: Cell<(u64, u64)> = {
            Cell::new(sys::hashmap_random_keys())
        });

        KEYS.with(|keys| {
            let (k0, k1) = keys.get();
            keys.set((k0.wrapping_add(1), k1));
            RandomState { k0, k1 }
        })
    }
}

#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
impl BuildHasher for RandomState {
    type Hasher = DefaultHasher;
    #[inline]
    #[allow(deprecated)]
    fn build_hasher(&self) -> DefaultHasher {
        DefaultHasher(SipHasher13::new_with_keys(self.k0, self.k1))
    }
}

/// The default [`Hasher`] used by [`RandomState`].
///
/// The internal algorithm is not specified, and so it and its hashes should
/// not be relied upon over releases.
#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
#[allow(deprecated)]
#[derive(Clone, Debug)]
pub struct DefaultHasher(SipHasher13);

impl DefaultHasher {
    /// Creates a new `DefaultHasher`.
    ///
    /// This hasher is not guaranteed to be the same as all other
    /// `DefaultHasher` instances, but is the same as all other `DefaultHasher`
    /// instances created through `new` or `default`.
    #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
    #[allow(deprecated)]
    pub fn new() -> DefaultHasher {
        DefaultHasher(SipHasher13::new_with_keys(0, 0))
    }
}

#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
impl Default for DefaultHasher {
    // FIXME: here should link `new` to [DefaultHasher::new], but it occurs intra-doc link
    // resolution failure when re-exporting libstd items. When #56922 fixed,
    // link `new` to [DefaultHasher::new] again.
    /// Creates a new `DefaultHasher` using `new`.
    /// See its documentation for more.
    fn default() -> DefaultHasher {
        DefaultHasher::new()
    }
}

#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
impl Hasher for DefaultHasher {
    #[inline]
    fn write(&mut self, msg: &[u8]) {
        self.0.write(msg)
    }

    #[inline]
    fn finish(&self) -> u64 {
        self.0.finish()
    }
}

#[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
impl Default for RandomState {
    /// Constructs a new `RandomState`.
    #[inline]
    fn default() -> RandomState {
        RandomState::new()
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl fmt::Debug for RandomState {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("RandomState { .. }")
    }
}

#[inline]
fn map_entry<'a, K: 'a, V: 'a>(raw: base::RustcEntry<'a, K, V>) -> Entry<'a, K, V> {
    match raw {
        base::RustcEntry::Occupied(base) => Entry::Occupied(OccupiedEntry { base }),
        base::RustcEntry::Vacant(base) => Entry::Vacant(VacantEntry { base }),
    }
}

#[inline]
pub(super) fn map_try_reserve_error(err: hashbrown::TryReserveError) -> TryReserveError {
    match err {
        hashbrown::TryReserveError::CapacityOverflow => TryReserveError::CapacityOverflow,
        hashbrown::TryReserveError::AllocError { layout } => {
            TryReserveError::AllocError { layout, non_exhaustive: () }
        }
    }
}

#[inline]
fn map_raw_entry<'a, K: 'a, V: 'a, S: 'a>(
    raw: base::RawEntryMut<'a, K, V, S>,
) -> RawEntryMut<'a, K, V, S> {
    match raw {
        base::RawEntryMut::Occupied(base) => RawEntryMut::Occupied(RawOccupiedEntryMut { base }),
        base::RawEntryMut::Vacant(base) => RawEntryMut::Vacant(RawVacantEntryMut { base }),
    }
}

#[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
    }
}