Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3276 insertions, 0 deletions

diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
new file mode 100644
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--- /dev/null
+++ b/library/std/src/collections/hash/map.rs
@@ -0,0 +1,3276 @@
+#[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::collections::TryReserveErrorKind;
+use crate::fmt::{self, Debug};
+#[allow(deprecated)]
+use crate::hash::{BuildHasher, Hash, Hasher, SipHasher13};
+use crate::iter::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.
+/// There are many alternative [hashing algorithms available on crates.io].
+///
+/// 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 behavior resulting from such a logic error is not specified, but will
+/// be encapsulated to the `HashMap` that observed the logic error and not
+/// result in undefined behavior. This could include panics, incorrect results,
+/// aborts, memory leaks, and non-termination.
+///
+/// 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.
+///
+/// [hash map]: crate::collections#use-a-hashmap-when
+/// [hashing algorithms available on crates.io]: https://crates.io/keywords/hasher
+/// [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!("{book} is unreviewed.")
+///     }
+/// }
+///
+/// // 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}\"");
+/// }
+/// ```
+///
+/// A `HashMap` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let solar_distance = HashMap::from([
+///     ("Mercury", 0.4),
+///     ("Venus", 0.7),
+///     ("Earth", 1.0),
+///     ("Mars", 1.5),
+/// ]);
+/// ```
+///
+/// `HashMap` implements an [`Entry` API](#method.entry), which allows
+/// for 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();
+///
+/// // modify an entry before an insert with in-place mutation
+/// player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);
+/// ```
+///
+/// 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
+///
+/// ```
+/// 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 vikings = HashMap::from([
+///     (Viking::new("Einar", "Norway"), 25),
+///     (Viking::new("Olaf", "Denmark"), 24),
+///     (Viking::new("Harald", "Iceland"), 12),
+/// ]);
+///
+/// // Use derived implementation to print the status of the vikings.
+/// for (viking, health) in &vikings {
+///     println!("{viking:?} has {health} hp");
+/// }
+/// ```
+
+#[cfg_attr(not(test), rustc_diagnostic_item = "HashMap")]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
+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]
+    #[must_use]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new() -> HashMap<K, V, RandomState> {
+        Default::default()
+    }
+
+    /// Creates an empty `HashMap` with at least the specified capacity.
+    ///
+    /// The hash map will be able to hold at least `capacity` elements without
+    /// reallocating. This method is allowed to allocate for more elements than
+    /// `capacity`. If `capacity` is 0, the hash set will not allocate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    /// let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);
+    /// ```
+    #[inline]
+    #[must_use]
+    #[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 at least the specified capacity, using
+    /// `hasher` to hash the keys.
+    ///
+    /// The hash map will be able to hold at least `capacity` elements without
+    /// reallocating. This method is allowed to allocate for more elements than
+    /// `capacity`. If `capacity` is 0, the hash map will not allocate.
+    ///
+    /// Warning: `hasher` 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 `hasher` 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, hasher: S) -> HashMap<K, V, S> {
+        HashMap { base: base::HashMap::with_capacity_and_hasher(capacity, hasher) }
+    }
+
+    /// 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 map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// for key in map.keys() {
+    ///     println!("{key}");
+    /// }
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over keys takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn keys(&self) -> Keys<'_, K, V> {
+        Keys { inner: self.iter() }
+    }
+
+    /// 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
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// let mut vec: Vec<&str> = map.into_keys().collect();
+    /// // The `IntoKeys` iterator produces keys in arbitrary order, so the
+    /// // keys must be sorted to test them against a sorted array.
+    /// vec.sort_unstable();
+    /// assert_eq!(vec, ["a", "b", "c"]);
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over keys takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[inline]
+    #[rustc_lint_query_instability]
+    #[stable(feature = "map_into_keys_values", since = "1.54.0")]
+    pub fn into_keys(self) -> IntoKeys<K, V> {
+        IntoKeys { inner: self.into_iter() }
+    }
+
+    /// An iterator visiting all values in arbitrary order.
+    /// The iterator element type is `&'a V`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// for val in map.values() {
+    ///     println!("{val}");
+    /// }
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over values takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[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::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// for val in map.values_mut() {
+    ///     *val = *val + 10;
+    /// }
+    ///
+    /// for val in map.values() {
+    ///     println!("{val}");
+    /// }
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over values takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[stable(feature = "map_values_mut", since = "1.10.0")]
+    pub fn values_mut(&mut self) -> ValuesMut<'_, K, V> {
+        ValuesMut { inner: self.iter_mut() }
+    }
+
+    /// 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
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// let mut vec: Vec<i32> = map.into_values().collect();
+    /// // The `IntoValues` iterator produces values in arbitrary order, so
+    /// // the values must be sorted to test them against a sorted array.
+    /// vec.sort_unstable();
+    /// assert_eq!(vec, [1, 2, 3]);
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over values takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[inline]
+    #[rustc_lint_query_instability]
+    #[stable(feature = "map_into_keys_values", since = "1.54.0")]
+    pub fn into_values(self) -> IntoValues<K, V> {
+        IntoValues { inner: self.into_iter() }
+    }
+
+    /// 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 map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// for (key, val) in map.iter() {
+    ///     println!("key: {key} val: {val}");
+    /// }
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over map takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[rustc_lint_query_instability]
+    #[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::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// // Update all values
+    /// for (_, val) in map.iter_mut() {
+    ///     *val *= 2;
+    /// }
+    ///
+    /// for (key, val) in &map {
+    ///     println!("key: {key} val: {val}");
+    /// }
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, iterating over map takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[rustc_lint_query_instability]
+    #[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.
+    ///
+    /// If the returned iterator is dropped before being fully consumed, it
+    /// drops the remaining key-value pairs. The returned iterator keeps a
+    /// mutable borrow on the map to optimize its implementation.
+    ///
+    /// # 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]
+    #[rustc_lint_query_instability]
+    #[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]
+    #[rustc_lint_query_instability]
+    #[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) }
+    }
+
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all pairs `(k, v)` for which `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in unsorted (and unspecified) order.
+    ///
+    /// # 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);
+    /// ```
+    ///
+    /// # Performance
+    ///
+    /// In the current implementation, this operation takes O(capacity) time
+    /// instead of O(len) because it internally visits empty buckets too.
+    #[inline]
+    #[rustc_lint_query_instability]
+    #[stable(feature = "retain_hash_collection", since = "1.18.0")]
+    pub fn retain<F>(&mut self, f: F)
+    where
+        F: FnMut(&K, &mut V) -> bool,
+    {
+        self.base.retain(f)
+    }
+
+    /// 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());
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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 speculatively
+    /// avoid frequent reallocations. After calling `reserve`,
+    /// capacity will be greater than or equal to `self.len() + additional`.
+    /// Does nothing if capacity is already sufficient.
+    ///
+    /// # 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 `HashMap`. The collection may reserve more space to speculatively
+    /// avoid frequent reallocations. After calling `reserve`,
+    /// capacity will be greater than or equal to `self.len() + additional` if
+    /// it returns `Ok(())`.
+    /// Does nothing if capacity is already sufficient.
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let mut map: HashMap<&str, isize> = HashMap::new();
+    /// map.try_reserve(10).expect("why is the test harness OOMing on a handful of bytes?");
+    /// ```
+    #[inline]
+    #[stable(feature = "try_reserve", since = "1.57.0")]
+    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.
+    ///
+    /// If the current capacity is less than the lower limit, this is a no-op.
+    ///
+    /// # 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(10);
+    /// assert!(map.capacity() >= 10);
+    /// map.shrink_to(0);
+    /// assert!(map.capacity() >= 2);
+    /// ```
+    #[inline]
+    #[stable(feature = "shrink_to", since = "1.56.0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        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() {
+    ///     letters.entry(ch).and_modify(|counter| *counter += 1).or_insert(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);
+    /// ```
+    #[inline]
+    #[stable(feature = "map_get_key_value", since = "1.40.0")]
+    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)
+    }
+
+    /// Attempts to get mutable references to `N` values in the map at once.
+    ///
+    /// Returns an array of length `N` with the results of each query. For soundness, at most one
+    /// mutable reference will be returned to any value. `None` will be returned if any of the
+    /// keys are duplicates or missing.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_many_mut)]
+    /// use std::collections::HashMap;
+    ///
+    /// let mut libraries = HashMap::new();
+    /// libraries.insert("Bodleian Library".to_string(), 1602);
+    /// libraries.insert("Athenæum".to_string(), 1807);
+    /// libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
+    /// libraries.insert("Library of Congress".to_string(), 1800);
+    ///
+    /// let got = libraries.get_many_mut([
+    ///     "Athenæum",
+    ///     "Library of Congress",
+    /// ]);
+    /// assert_eq!(
+    ///     got,
+    ///     Some([
+    ///         &mut 1807,
+    ///         &mut 1800,
+    ///     ]),
+    /// );
+    ///
+    /// // Missing keys result in None
+    /// let got = libraries.get_many_mut([
+    ///     "Athenæum",
+    ///     "New York Public Library",
+    /// ]);
+    /// assert_eq!(got, None);
+    ///
+    /// // Duplicate keys result in None
+    /// let got = libraries.get_many_mut([
+    ///     "Athenæum",
+    ///     "Athenæum",
+    /// ]);
+    /// assert_eq!(got, None);
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_many_mut", issue = "97601")]
+    pub fn get_many_mut<Q: ?Sized, const N: usize>(&mut self, ks: [&Q; N]) -> Option<[&'_ mut V; N]>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq,
+    {
+        self.base.get_many_mut(ks)
+    }
+
+    /// Attempts to get mutable references to `N` values in the map at once, without validating that
+    /// the values are unique.
+    ///
+    /// Returns an array of length `N` with the results of each query. `None` will be returned if
+    /// any of the keys are missing.
+    ///
+    /// For a safe alternative see [`get_many_mut`](Self::get_many_mut).
+    ///
+    /// # Safety
+    ///
+    /// Calling this method with overlapping keys is *[undefined behavior]* even if the resulting
+    /// references are not used.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_many_mut)]
+    /// use std::collections::HashMap;
+    ///
+    /// let mut libraries = HashMap::new();
+    /// libraries.insert("Bodleian Library".to_string(), 1602);
+    /// libraries.insert("Athenæum".to_string(), 1807);
+    /// libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
+    /// libraries.insert("Library of Congress".to_string(), 1800);
+    ///
+    /// let got = libraries.get_many_mut([
+    ///     "Athenæum",
+    ///     "Library of Congress",
+    /// ]);
+    /// assert_eq!(
+    ///     got,
+    ///     Some([
+    ///         &mut 1807,
+    ///         &mut 1800,
+    ///     ]),
+    /// );
+    ///
+    /// // Missing keys result in None
+    /// let got = libraries.get_many_mut([
+    ///     "Athenæum",
+    ///     "New York Public Library",
+    /// ]);
+    /// assert_eq!(got, None);
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_many_mut", issue = "97601")]
+    pub unsafe fn get_many_unchecked_mut<Q: ?Sized, const N: usize>(
+        &mut self,
+        ks: [&Q; N],
+    ) -> Option<[&'_ mut V; N]>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq,
+    {
+        self.base.get_many_unchecked_mut(ks)
+    }
+
+    /// 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);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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");
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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");
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn insert(&mut self, k: K, v: V) -> Option<V> {
+        self.base.insert(k, v)
+    }
+
+    /// Tries to insert a key-value pair into the map, and returns
+    /// a mutable reference to the value in the entry.
+    ///
+    /// If the map already had this key present, nothing is updated, and
+    /// an error containing the occupied entry and the value is returned.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(map_try_insert)]
+    ///
+    /// use std::collections::HashMap;
+    ///
+    /// let mut map = HashMap::new();
+    /// assert_eq!(map.try_insert(37, "a").unwrap(), &"a");
+    ///
+    /// let err = map.try_insert(37, "b").unwrap_err();
+    /// assert_eq!(err.entry.key(), &37);
+    /// assert_eq!(err.entry.get(), &"a");
+    /// assert_eq!(err.value, "b");
+    /// ```
+    #[unstable(feature = "map_try_insert", issue = "82766")]
+    pub fn try_insert(&mut self, key: K, value: V) -> Result<&mut V, OccupiedError<'_, K, V>> {
+        match self.entry(key) {
+            Occupied(entry) => Err(OccupiedError { entry, value }),
+            Vacant(entry) => Ok(entry.insert(value)),
+        }
+    }
+
+    /// 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);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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);
+    /// # }
+    /// ```
+    #[inline]
+    #[stable(feature = "hash_map_remove_entry", since = "1.27.0")]
+    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)
+    }
+}
+
+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> Clone for HashMap<K, V, S>
+where
+    K: Clone,
+    V: Clone,
+    S: Clone,
+{
+    #[inline]
+    fn clone(&self) -> Self {
+        Self { base: self.base.clone() }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, other: &Self) {
+        self.base.clone_from(&other.base);
+    }
+}
+
+#[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")
+    }
+}
+
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+// Note: as what is currently the most convenient built-in way to construct
+// a HashMap, a simple usage of this function must not *require* the user
+// to provide a type annotation in order to infer the third type parameter
+// (the hasher parameter, conventionally "S").
+// To that end, this impl is defined using RandomState as the concrete
+// type of S, rather than being generic over `S: BuildHasher + Default`.
+// It is expected that users who want to specify a hasher will manually use
+// `with_capacity_and_hasher`.
+// If type parameter defaults worked on impls, and if type parameter
+// defaults could be mixed with const generics, then perhaps
+// this could be generalized.
+// See also the equivalent impl on HashSet.
+impl<K, V, const N: usize> From<[(K, V); N]> for HashMap<K, V, RandomState>
+where
+    K: Eq + Hash,
+{
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let map1 = HashMap::from([(1, 2), (3, 4)]);
+    /// let map2: HashMap<_, _> = [(1, 2), (3, 4)].into();
+    /// assert_eq!(map1, map2);
+    /// ```
+    fn from(arr: [(K, V); N]) -> Self {
+        Self::from_iter(arr)
+    }
+}
+
+/// 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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter = map.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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let mut map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter = map.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 an 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
+/// [`IntoIterator`]: crate::iter::IntoIterator
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter = map.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 an 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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter_keys = map.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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter_values = map.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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let mut map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter = map.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 an 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
+///
+/// # Example
+///
+/// ```
+/// #![feature(hash_drain_filter)]
+///
+/// use std::collections::HashMap;
+///
+/// let mut map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter = map.drain_filter(|_k, v| *v % 2 == 0);
+/// ```
+#[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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let mut map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter_values = map.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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter_keys = map.into_keys();
+/// ```
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+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
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let map = HashMap::from([
+///     ("a", 1),
+/// ]);
+/// let iter_keys = map.into_values();
+/// ```
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+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.
+#[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.
+#[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]
+    #[must_use]
+    #[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]
+    #[must_use]
+    #[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]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[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]
+    #[must_use]
+    #[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]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[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]
+    #[must_use]
+    #[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]
+    #[must_use]
+    #[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]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[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_non_exhaustive()
+    }
+}
+
+#[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_non_exhaustive()
+    }
+}
+
+#[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_non_exhaustive()
+    }
+}
+
+#[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_non_exhaustive()
+    }
+}
+
+/// 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")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "HashMapEntry")]
+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_non_exhaustive()
+    }
+}
+
+/// 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()
+    }
+}
+
+/// The error returned by [`try_insert`](HashMap::try_insert) when the key already exists.
+///
+/// Contains the occupied entry, and the value that was not inserted.
+#[unstable(feature = "map_try_insert", issue = "82766")]
+pub struct OccupiedError<'a, K: 'a, V: 'a> {
+    /// The entry in the map that was already occupied.
+    pub entry: OccupiedEntry<'a, K, V>,
+    /// The value which was not inserted, because the entry was already occupied.
+    pub value: V,
+}
+
+#[unstable(feature = "map_try_insert", issue = "82766")]
+impl<K: Debug, V: Debug> Debug for OccupiedError<'_, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("OccupiedError")
+            .field("key", self.entry.key())
+            .field("old_value", self.entry.get())
+            .field("new_value", &self.value)
+            .finish_non_exhaustive()
+    }
+}
+
+#[unstable(feature = "map_try_insert", issue = "82766")]
+impl<'a, K: Debug, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "failed to insert {:?}, key {:?} already exists with value {:?}",
+            self.value,
+            self.entry.key(),
+            self.entry.get(),
+        )
+    }
+}
+
+#[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]
+    #[rustc_lint_query_instability]
+    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]
+    #[rustc_lint_query_instability]
+    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 map = HashMap::from([
+    ///     ("a", 1),
+    ///     ("b", 2),
+    ///     ("c", 3),
+    /// ]);
+    ///
+    /// // Not possible with .iter()
+    /// let vec: Vec<(&str, i32)> = map.into_iter().collect();
+    /// ```
+    #[inline]
+    #[rustc_lint_query_instability]
+    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> fmt::Debug for ValuesMut<'_, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
+    }
+}
+
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+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()
+    }
+}
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K, V> FusedIterator for IntoKeys<K, V> {}
+
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K: Debug, V> 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()
+    }
+}
+
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+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()
+    }
+}
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K, V> ExactSizeIterator for IntoValues<K, V> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K, V> FusedIterator for IntoValues<K, V> {}
+
+#[stable(feature = "map_into_keys_values", since = "1.54.0")]
+impl<K, 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.debug_struct("DrainFilter").finish_non_exhaustive()
+    }
+}
+
+impl<'a, K, V> Entry<'a, K, V> {
+    /// 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]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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 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]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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()),
+        }
+    }
+
+    /// Ensures a value is in the entry by inserting, if empty, the result of the default function.
+    /// This method allows for generating key-derived values for insertion by providing the default
+    /// function a reference to the key that was moved during the `.entry(key)` method call.
+    ///
+    /// The reference to the moved key is provided so that cloning or copying the key is
+    /// unnecessary, unlike with `.or_insert_with(|| ... )`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// 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]
+    #[stable(feature = "or_insert_with_key", since = "1.50.0")]
+    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_entry("hoho".to_string());
+    ///
+    /// assert_eq!(entry.key(), &"poneyland");
+    /// ```
+    #[inline]
+    #[unstable(feature = "entry_insert", issue = "65225")]
+    pub fn insert_entry(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> {
+    /// 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]
+    #[stable(feature = "entry_or_default", since = "1.28.0")]
+    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
+    ///
+    /// ```
+    /// #![feature(entry_insert)]
+    /// 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_entry(37);
+    /// }
+    /// assert_eq!(map["poneyland"], 37);
+    /// ```
+    #[inline]
+    #[unstable(feature = "entry_insert", issue = "65225")]
+    pub 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);
+    }
+}
+
+#[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
+    #[must_use]
+    #[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")]
+    #[inline]
+    #[allow(deprecated)]
+    #[must_use]
+    pub fn new() -> DefaultHasher {
+        DefaultHasher(SipHasher13::new_with_keys(0, 0))
+    }
+}
+
+#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
+impl Default for DefaultHasher {
+    /// Creates a new `DefaultHasher` using [`new`].
+    /// See its documentation for more.
+    ///
+    /// [`new`]: DefaultHasher::new
+    #[inline]
+    fn default() -> DefaultHasher {
+        DefaultHasher::new()
+    }
+}
+
+#[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
+impl Hasher for DefaultHasher {
+    // The underlying `SipHasher13` doesn't override the other
+    // `write_*` methods, so it's ok not to forward them here.
+
+    #[inline]
+    fn write(&mut self, msg: &[u8]) {
+        self.0.write(msg)
+    }
+
+    #[inline]
+    fn write_str(&mut self, s: &str) {
+        self.0.write_str(s);
+    }
+
+    #[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.debug_struct("RandomState").finish_non_exhaustive()
+    }
+}
+
+#[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 => {
+            TryReserveErrorKind::CapacityOverflow.into()
+        }
+        hashbrown::TryReserveError::AllocError { layout } => {
+            TryReserveErrorKind::AllocError { layout, non_exhaustive: () }.into()
+        }
+    }
+}
+
+#[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
+    }
+}