hashbrown 0.12.3

use crate::raw::{Allocator, Bucket, Global, RawDrain, RawIntoIter, RawIter, RawTable};
use crate::TryReserveError;
use core::borrow::Borrow;
use core::fmt::{self, Debug};
use core::hash::{BuildHasher, Hash};
use core::iter::{FromIterator, FusedIterator};
use core::marker::PhantomData;
use core::mem;
use core::ops::Index;

/// Default hasher for `HashMap`.
#[cfg(feature = "ahash")]
pub type DefaultHashBuilder = ahash::RandomState;

/// Dummy default hasher for `HashMap`.
#[cfg(not(feature = "ahash"))]
pub enum DefaultHashBuilder {}

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

impl<K: Clone, V: Clone, S: Clone, A: Allocator + Clone> Clone for HashMap<K, V, S, A> {
    fn clone(&self) -> Self {
        HashMap {
            hash_builder: self.hash_builder.clone(),
            table: self.table.clone(),
        }
    }

    fn clone_from(&mut self, source: &Self) {
        self.table.clone_from(&source.table);

        // Update hash_builder only if we successfully cloned all elements.
        self.hash_builder.clone_from(&source.hash_builder);
    }
}

/// Ensures that a single closure type across uses of this which, in turn prevents multiple
/// instances of any functions like RawTable::reserve from being generated
#[cfg_attr(feature = "inline-more", inline)]
pub(crate) fn make_hasher<K, Q, V, S>(hash_builder: &S) -> impl Fn(&(Q, V)) -> u64 + '_
where
    K: Borrow<Q>,
    Q: Hash,
    S: BuildHasher,
{
    move |val| make_hash::<K, Q, S>(hash_builder, &val.0)
}

/// Ensures that a single closure type across uses of this which, in turn prevents multiple
/// instances of any functions like RawTable::reserve from being generated
#[cfg_attr(feature = "inline-more", inline)]
fn equivalent_key<Q, K, V>(k: &Q) -> impl Fn(&(K, V)) -> bool + '_
where
    K: Borrow<Q>,
    Q: ?Sized + Eq,
{
    move |x| k.eq(x.0.borrow())
}

/// Ensures that a single closure type across uses of this which, in turn prevents multiple
/// instances of any functions like RawTable::reserve from being generated
#[cfg_attr(feature = "inline-more", inline)]
fn equivalent<Q, K>(k: &Q) -> impl Fn(&K) -> bool + '_
where
    K: Borrow<Q>,
    Q: ?Sized + Eq,
{
    move |x| k.eq(x.borrow())
}

#[cfg(not(feature = "nightly"))]
#[cfg_attr(feature = "inline-more", inline)]
pub(crate) fn make_hash<K, Q, S>(hash_builder: &S, val: &Q) -> u64
where
    K: Borrow<Q>,
    Q: Hash + ?Sized,
    S: BuildHasher,
{
    use core::hash::Hasher;
    let mut state = hash_builder.build_hasher();
    val.hash(&mut state);
    state.finish()
}

#[cfg(feature = "nightly")]
#[cfg_attr(feature = "inline-more", inline)]
pub(crate) fn make_hash<K, Q, S>(hash_builder: &S, val: &Q) -> u64
where
    K: Borrow<Q>,
    Q: Hash + ?Sized,
    S: BuildHasher,
{
    hash_builder.hash_one(val)
}

#[cfg(not(feature = "nightly"))]
#[cfg_attr(feature = "inline-more", inline)]
pub(crate) fn make_insert_hash<K, S>(hash_builder: &S, val: &K) -> u64
where
    K: Hash,
    S: BuildHasher,
{
    use core::hash::Hasher;
    let mut state = hash_builder.build_hasher();
    val.hash(&mut state);
    state.finish()
}

#[cfg(feature = "nightly")]
#[cfg_attr(feature = "inline-more", inline)]
pub(crate) fn make_insert_hash<K, S>(hash_builder: &S, val: &K) -> u64
where
    K: Hash,
    S: BuildHasher,
{
    hash_builder.hash_one(val)
}

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

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

#[cfg(feature = "ahash")]
impl<K, V, A: Allocator + Clone> HashMap<K, V, DefaultHashBuilder, A> {
    /// Creates an empty `HashMap` using the given allocator.
    ///
    /// The hash map is initially created with a capacity of 0, so it will not allocate until it
    /// is first inserted into.
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn new_in(alloc: A) -> Self {
        Self::with_hasher_in(DefaultHashBuilder::default(), alloc)
    }

    /// Creates an empty `HashMap` with the specified capacity using the given allocator.
    ///
    /// The hash map will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash map will not allocate.
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
        Self::with_capacity_and_hasher_in(capacity, DefaultHashBuilder::default(), alloc)
    }
}

impl<K, V, S> HashMap<K, V, S> {
    /// Creates an empty `HashMap` which will use the given hash builder to hash
    /// keys.
    ///
    /// The hash map is initially created with a capacity of 0, so it will not
    /// allocate until it is first inserted into.
    ///
    /// 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 hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut map = HashMap::with_hasher(s);
    /// assert_eq!(map.len(), 0);
    /// assert_eq!(map.capacity(), 0);
    ///
    /// map.insert(1, 2);
    /// ```
    ///
    /// [`BuildHasher`]: https://doc.rust-lang.org/std/hash/trait.BuildHasher.html
    #[cfg_attr(feature = "inline-more", inline)]
    pub const fn with_hasher(hash_builder: S) -> Self {
        Self {
            hash_builder,
            table: RawTable::new(),
        }
    }

    /// Creates an empty `HashMap` with the specified capacity, using `hash_builder`
    /// to hash the keys.
    ///
    /// The hash map will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the hash map will not allocate.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow HashMaps to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the HashMap to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut map = HashMap::with_capacity_and_hasher(10, s);
    /// assert_eq!(map.len(), 0);
    /// assert!(map.capacity() >= 10);
    ///
    /// map.insert(1, 2);
    /// ```
    ///
    /// [`BuildHasher`]: https://doc.rust-lang.org/std/hash/trait.BuildHasher.html
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> Self {
        Self {
            hash_builder,
            table: RawTable::with_capacity(capacity),
        }
    }
}

impl<K, V, S, A: Allocator + Clone> HashMap<K, V, S, A> {
    /// Returns a reference to the underlying allocator.
    #[inline]
    pub fn allocator(&self) -> &A {
        self.table.allocator()
    }

    /// Creates an empty `HashMap` which will use the given hash builder to hash
    /// keys. It will be allocated with the given allocator.
    ///
    /// The created map has the default initial capacity.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow HashMaps to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::DefaultHashBuilder;
    ///
    /// let s = DefaultHashBuilder::default();
    /// let mut map = HashMap::with_hasher(s);
    /// map.insert(1, 2);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn with_hasher_in(hash_builder: S, alloc: A) -> Self {
        Self {
            hash_builder,
            table: RawTable::new_in(alloc),
        }
    }

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

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

    /// Returns the number of elements the map can hold without reallocating.
    ///
    /// This number is a lower bound; the `HashMap<K, V>` might be able to hold
    /// more, but is guaranteed to be able to hold at least this many.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// assert_eq!(map.len(), 0);
    /// assert!(map.capacity() >= 100);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn capacity(&self) -> usize {
        self.table.capacity()
    }

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

    /// An iterator visiting all values in arbitrary order.
    /// The iterator element type is `&'a V`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    /// assert_eq!(map.len(), 3);
    /// let mut vec: Vec<i32> = Vec::new();
    ///
    /// for val in map.values() {
    ///     println!("{}", val);
    ///     vec.push(*val);
    /// }
    ///
    /// // The `Values` 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]);
    ///
    /// assert_eq!(map.len(), 3);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn values(&self) -> Values<'_, K, V> {
        Values { inner: self.iter() }
    }

    /// An iterator visiting all values mutably in arbitrary order.
    /// The iterator element type is `&'a mut V`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    ///
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// for val in map.values_mut() {
    ///     *val = *val + 10;
    /// }
    ///
    /// assert_eq!(map.len(), 3);
    /// let mut vec: Vec<i32> = Vec::new();
    ///
    /// for val in map.values() {
    ///     println!("{}", val);
    ///     vec.push(*val);
    /// }
    ///
    /// // The `Values` 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, [11, 12, 13]);
    ///
    /// assert_eq!(map.len(), 3);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn values_mut(&mut self) -> ValuesMut<'_, K, V> {
        ValuesMut {
            inner: self.iter_mut(),
        }
    }

    /// An iterator visiting all key-value pairs in arbitrary order.
    /// The iterator element type is `(&'a K, &'a V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    /// assert_eq!(map.len(), 3);
    /// let mut vec: Vec<(&str, i32)> = Vec::new();
    ///
    /// for (key, val) in map.iter() {
    ///     println!("key: {} val: {}", key, val);
    ///     vec.push((*key, *val));
    /// }
    ///
    /// // The `Iter` iterator produces items in arbitrary order, so the
    /// // items must be sorted to test them against a sorted array.
    /// vec.sort_unstable();
    /// assert_eq!(vec, [("a", 1), ("b", 2), ("c", 3)]);
    ///
    /// assert_eq!(map.len(), 3);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn iter(&self) -> Iter<'_, K, V> {
        // Here we tie the lifetime of self to the iter.
        unsafe {
            Iter {
                inner: self.table.iter(),
                marker: PhantomData,
            }
        }
    }

    /// An iterator visiting all key-value pairs in arbitrary order,
    /// with mutable references to the values.
    /// The iterator element type is `(&'a K, &'a mut V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("c", 3);
    ///
    /// // Update all values
    /// for (_, val) in map.iter_mut() {
    ///     *val *= 2;
    /// }
    ///
    /// assert_eq!(map.len(), 3);
    /// let mut vec: Vec<(&str, i32)> = Vec::new();
    ///
    /// for (key, val) in &map {
    ///     println!("key: {} val: {}", key, val);
    ///     vec.push((*key, *val));
    /// }
    ///
    /// // The `Iter` iterator produces items in arbitrary order, so the
    /// // items must be sorted to test them against a sorted array.
    /// vec.sort_unstable();
    /// assert_eq!(vec, [("a", 2), ("b", 4), ("c", 6)]);
    ///
    /// assert_eq!(map.len(), 3);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
        // Here we tie the lifetime of self to the iter.
        unsafe {
            IterMut {
                inner: self.table.iter(),
                marker: PhantomData,
            }
        }
    }

    #[cfg(test)]
    #[cfg_attr(feature = "inline-more", inline)]
    fn raw_capacity(&self) -> usize {
        self.table.buckets()
    }

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

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

    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
    /// allocated memory for reuse.
    ///
    /// 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 vector to optimize its implementation.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// a.insert(2, "b");
    /// let capacity_before_drain = a.capacity();
    ///
    /// for (k, v) in a.drain().take(1) {
    ///     assert!(k == 1 || k == 2);
    ///     assert!(v == "a" || v == "b");
    /// }
    ///
    /// // As we can see, the map is empty and contains no element.
    /// assert!(a.is_empty() && a.len() == 0);
    /// // But map capacity is equal to old one.
    /// assert_eq!(a.capacity(), capacity_before_drain);
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// a.insert(2, "b");
    ///
    /// {   // Iterator is dropped without being consumed.
    ///     let d = a.drain();
    /// }
    ///
    /// // But the map is empty even if we do not use Drain iterator.
    /// assert!(a.is_empty());
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn drain(&mut self) -> Drain<'_, K, V, A> {
        Drain {
            inner: self.table.drain(),
        }
    }

    /// Retains only the elements specified by the predicate. Keeps the
    /// allocated memory for reuse.
    ///
    /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
    /// The elements are visited in unsorted (and unspecified) order.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
    /// assert_eq!(map.len(), 8);
    /// let capacity_before_retain = map.capacity();
    ///
    /// map.retain(|&k, _| k % 2 == 0);
    ///
    /// // We can see, that the number of elements inside map is changed.
    /// assert_eq!(map.len(), 4);
    /// // But map capacity is equal to old one.
    /// assert_eq!(map.capacity(), capacity_before_retain);
    ///
    /// let mut vec: Vec<(i32, i32)> = map.iter().map(|(&k, &v)| (k, v)).collect();
    /// vec.sort_unstable();
    /// assert_eq!(vec, [(0, 0), (2, 20), (4, 40), (6, 60)]);
    /// ```
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        // Here we only use `iter` as a temporary, preventing use-after-free
        unsafe {
            for item in self.table.iter() {
                let &mut (ref key, ref mut value) = item.as_mut();
                if !f(key, value) {
                    self.table.erase(item);
                }
            }
        }
    }

    /// Drains elements which are true under the given predicate,
    /// and returns an iterator over the removed items.
    ///
    /// In other words, move all pairs `(k, v)` such that `f(&k, &mut v)` returns `true` out
    /// into another iterator.
    ///
    /// Note that `drain_filter` lets you mutate every value in the filter closure, regardless of
    /// whether you choose to keep or remove it.
    ///
    /// When the returned DrainedFilter is dropped, any remaining elements that satisfy
    /// the predicate are dropped from the table.
    ///
    /// 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.
    ///
    /// Keeps the allocated memory for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    /// let capacity_before_drain_filter = map.capacity();
    /// let drained: HashMap<i32, i32> = map.drain_filter(|k, _v| k % 2 == 0).collect();
    ///
    /// let mut evens = drained.keys().cloned().collect::<Vec<_>>();
    /// let mut odds = map.keys().cloned().collect::<Vec<_>>();
    /// evens.sort();
    /// odds.sort();
    ///
    /// assert_eq!(evens, vec![0, 2, 4, 6]);
    /// assert_eq!(odds, vec![1, 3, 5, 7]);
    /// // Map capacity is equal to old one.
    /// assert_eq!(map.capacity(), capacity_before_drain_filter);
    ///
    /// let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    ///
    /// {   // Iterator is dropped without being consumed.
    ///     let d = map.drain_filter(|k, _v| k % 2 != 0);
    /// }
    ///
    /// // But the map lens have been reduced by half
    /// // even if we do not use DrainFilter iterator.
    /// assert_eq!(map.len(), 4);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn drain_filter<F>(&mut self, f: F) -> DrainFilter<'_, K, V, F, A>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        DrainFilter {
            f,
            inner: DrainFilterInner {
                iter: unsafe { self.table.iter() },
                table: &mut self.table,
            },
        }
    }

    /// Clears the map, removing all key-value pairs. Keeps the allocated memory
    /// for reuse.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut a = HashMap::new();
    /// a.insert(1, "a");
    /// let capacity_before_clear = a.capacity();
    ///
    /// a.clear();
    ///
    /// // Map is empty.
    /// assert!(a.is_empty());
    /// // But map capacity is equal to old one.
    /// assert_eq!(a.capacity(), capacity_before_clear);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn clear(&mut self) {
        self.table.clear();
    }

    /// 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 hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("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"]);
    /// ```
    #[inline]
    pub fn into_keys(self) -> IntoKeys<K, V, A> {
        IntoKeys {
            inner: self.into_iter(),
        }
    }

    /// Creates a consuming iterator visiting all the values in arbitrary order.
    /// The map cannot be used after calling this.
    /// The iterator element type is `V`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert("a", 1);
    /// map.insert("b", 2);
    /// map.insert("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]);
    /// ```
    #[inline]
    pub fn into_values(self) -> IntoValues<K, V, A> {
        IntoValues {
            inner: self.into_iter(),
        }
    }
}

impl<K, V, S, A> HashMap<K, V, S, A>
where
    K: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone,
{
    /// Reserves capacity for at least `additional` more elements to be inserted
    /// in the `HashMap`. The collection may reserve more space to avoid
    /// frequent reallocations.
    ///
    /// # Panics
    ///
    /// Panics if the new allocation size overflows [`usize`].
    ///
    /// [`usize`]: https://doc.rust-lang.org/std/primitive.usize.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// let mut map: HashMap<&str, i32> = HashMap::new();
    /// // Map is empty and doesn't allocate memory
    /// assert_eq!(map.capacity(), 0);
    ///
    /// map.reserve(10);
    ///
    /// // And now map can hold at least 10 elements
    /// assert!(map.capacity() >= 10);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn reserve(&mut self, additional: usize) {
        self.table
            .reserve(additional, make_hasher::<K, _, V, S>(&self.hash_builder));
    }

    /// Tries to reserve capacity for at least `additional` more elements to be inserted
    /// in the given `HashMap<K,V>`. The collection may reserve more space to avoid
    /// frequent reallocations.
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<&str, isize> = HashMap::new();
    /// // Map is empty and doesn't allocate memory
    /// assert_eq!(map.capacity(), 0);
    ///
    /// map.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");
    ///
    /// // And now map can hold at least 10 elements
    /// assert!(map.capacity() >= 10);
    /// ```
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned:
    /// ```
    /// # fn test() {
    /// use hashbrown::HashMap;
    /// use hashbrown::TryReserveError;
    /// let mut map: HashMap<i32, i32> = HashMap::new();
    ///
    /// match map.try_reserve(usize::MAX) {
    ///     Err(error) => match error {
    ///         TryReserveError::CapacityOverflow => {}
    ///         _ => panic!("TryReserveError::AllocError ?"),
    ///     },
    ///     _ => panic!(),
    /// }
    /// # }
    /// # fn main() {
    /// #     #[cfg(not(miri))]
    /// #     test()
    /// # }
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.table
            .try_reserve(additional, make_hasher::<K, _, V, S>(&self.hash_builder))
    }

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

    /// 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.
    ///
    /// This function does nothing if the current capacity is smaller than the
    /// supplied minimum capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    /// map.insert(1, 2);
    /// map.insert(3, 4);
    /// assert!(map.capacity() >= 100);
    /// map.shrink_to(10);
    /// assert!(map.capacity() >= 10);
    /// map.shrink_to(0);
    /// assert!(map.capacity() >= 2);
    /// map.shrink_to(10);
    /// assert!(map.capacity() >= 2);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.table
            .shrink_to(min_capacity, make_hasher::<K, _, V, S>(&self.hash_builder));
    }

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

    /// Gets the given key's corresponding entry by reference in the map for in-place manipulation.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut words: HashMap<String, usize> = HashMap::new();
    /// let source = ["poneyland", "horseyland", "poneyland", "poneyland"];
    /// for (i, &s) in source.iter().enumerate() {
    ///     let counter = words.entry_ref(s).or_insert(0);
    ///     *counter += 1;
    /// }
    ///
    /// assert_eq!(words["poneyland"], 3);
    /// assert_eq!(words["horseyland"], 1);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn entry_ref<'a, 'b, Q: ?Sized>(&'a mut self, key: &'b Q) -> EntryRef<'a, 'b, K, Q, V, S, A>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hash = make_hash::<K, Q, S>(&self.hash_builder, key);
        if let Some(elem) = self.table.find(hash, equivalent_key(key)) {
            EntryRef::Occupied(OccupiedEntryRef {
                hash,
                key: Some(KeyOrRef::Borrowed(key)),
                elem,
                table: self,
            })
        } else {
            EntryRef::Vacant(VacantEntryRef {
                hash,
                key: KeyOrRef::Borrowed(key),
                table: self,
            })
        }
    }

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

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

    #[inline]
    fn get_inner<Q: ?Sized>(&self, k: &Q) -> Option<&(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        if self.table.is_empty() {
            None
        } else {
            let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
            self.table.get(hash, equivalent_key(k))
        }
    }

    /// Returns the key-value pair corresponding to the supplied key, with a mutable reference to value.
    ///
    /// The supplied key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
    /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// let (k, v) = map.get_key_value_mut(&1).unwrap();
    /// assert_eq!(k, &1);
    /// assert_eq!(v, &mut "a");
    /// *v = "b";
    /// assert_eq!(map.get_key_value_mut(&1), Some((&1, &mut "b")));
    /// assert_eq!(map.get_key_value_mut(&2), None);
    /// ```
    #[inline]
    pub fn get_key_value_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<(&K, &mut V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        // Avoid `Option::map` because it bloats LLVM IR.
        match self.get_inner_mut(k) {
            Some(&mut (ref key, ref mut value)) => Some((key, value)),
            None => None,
        }
    }

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

    /// Returns a mutable reference to the value corresponding to the key.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
    /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.insert(1, "a");
    /// if let Some(x) = map.get_mut(&1) {
    ///     *x = "b";
    /// }
    /// assert_eq!(map[&1], "b");
    ///
    /// assert_eq!(map.get_mut(&2), None);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        // Avoid `Option::map` because it bloats LLVM IR.
        match self.get_inner_mut(k) {
            Some(&mut (_, ref mut v)) => Some(v),
            None => None,
        }
    }

    #[inline]
    fn get_inner_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut (K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        if self.table.is_empty() {
            None
        } else {
            let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
            self.table.get_mut(hash, equivalent_key(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
    ///
    /// ```
    /// use hashbrown::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);
    /// ```
    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.get_many_mut_inner(ks).map(|res| res.map(|(_, v)| v))
    }

    /// 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`](`HashMap::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
    ///
    /// ```
    /// use hashbrown::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);
    /// ```
    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.get_many_unchecked_mut_inner(ks)
            .map(|res| res.map(|(_, v)| v))
    }

    /// Attempts to get mutable references to `N` values in the map at once, with immutable
    /// references to the corresponding keys.
    ///
    /// 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
    ///
    /// ```
    /// use hashbrown::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_key_value_mut([
    ///     "Bodleian Library",
    ///     "Herzogin-Anna-Amalia-Bibliothek",
    /// ]);
    /// assert_eq!(
    ///     got,
    ///     Some([
    ///         (&"Bodleian Library".to_string(), &mut 1602),
    ///         (&"Herzogin-Anna-Amalia-Bibliothek".to_string(), &mut 1691),
    ///     ]),
    /// );
    /// // Missing keys result in None
    /// let got = libraries.get_many_key_value_mut([
    ///     "Bodleian Library",
    ///     "Gewandhaus",
    /// ]);
    /// assert_eq!(got, None);
    ///
    /// // Duplicate keys result in None
    /// let got = libraries.get_many_key_value_mut([
    ///     "Bodleian Library",
    ///     "Herzogin-Anna-Amalia-Bibliothek",
    ///     "Herzogin-Anna-Amalia-Bibliothek",
    /// ]);
    /// assert_eq!(got, None);
    /// ```
    pub fn get_many_key_value_mut<Q: ?Sized, const N: usize>(
        &mut self,
        ks: [&Q; N],
    ) -> Option<[(&'_ K, &'_ mut V); N]>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.get_many_mut_inner(ks)
            .map(|res| res.map(|(k, v)| (&*k, v)))
    }

    /// Attempts to get mutable references to `N` values in the map at once, with immutable
    /// references to the corresponding keys, 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_key_value_mut`](`HashMap::get_many_key_value_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
    ///
    /// ```
    /// use hashbrown::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_key_value_mut([
    ///     "Bodleian Library",
    ///     "Herzogin-Anna-Amalia-Bibliothek",
    /// ]);
    /// assert_eq!(
    ///     got,
    ///     Some([
    ///         (&"Bodleian Library".to_string(), &mut 1602),
    ///         (&"Herzogin-Anna-Amalia-Bibliothek".to_string(), &mut 1691),
    ///     ]),
    /// );
    /// // Missing keys result in None
    /// let got = libraries.get_many_key_value_mut([
    ///     "Bodleian Library",
    ///     "Gewandhaus",
    /// ]);
    /// assert_eq!(got, None);
    /// ```
    pub unsafe fn get_many_key_value_unchecked_mut<Q: ?Sized, const N: usize>(
        &mut self,
        ks: [&Q; N],
    ) -> Option<[(&'_ K, &'_ mut V); N]>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.get_many_unchecked_mut_inner(ks)
            .map(|res| res.map(|(k, v)| (&*k, v)))
    }

    fn get_many_mut_inner<Q: ?Sized, const N: usize>(
        &mut self,
        ks: [&Q; N],
    ) -> Option<[&'_ mut (K, V); N]>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hashes = self.build_hashes_inner(ks);
        self.table
            .get_many_mut(hashes, |i, (k, _)| ks[i].eq(k.borrow()))
    }

    unsafe fn get_many_unchecked_mut_inner<Q: ?Sized, const N: usize>(
        &mut self,
        ks: [&Q; N],
    ) -> Option<[&'_ mut (K, V); N]>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hashes = self.build_hashes_inner(ks);
        self.table
            .get_many_unchecked_mut(hashes, |i, (k, _)| ks[i].eq(k.borrow()))
    }

    fn build_hashes_inner<Q: ?Sized, const N: usize>(&self, ks: [&Q; N]) -> [u64; N]
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let mut hashes = [0_u64; N];
        for i in 0..N {
            hashes[i] = make_hash::<K, Q, S>(&self.hash_builder, ks[i]);
        }
        hashes
    }

    /// 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 [`std::collections`]
    /// [module-level documentation] for more.
    ///
    /// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
    /// [`std::collections`]: https://doc.rust-lang.org/std/collections/index.html
    /// [module-level documentation]: https://doc.rust-lang.org/std/collections/index.html#insert-and-complex-keys
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// assert_eq!(map.insert(37, "a"), None);
    /// assert_eq!(map.is_empty(), false);
    ///
    /// map.insert(37, "b");
    /// assert_eq!(map.insert(37, "c"), Some("b"));
    /// assert_eq!(map[&37], "c");
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn insert(&mut self, k: K, v: V) -> Option<V> {
        let hash = make_insert_hash::<K, S>(&self.hash_builder, &k);
        if let Some((_, item)) = self.table.get_mut(hash, equivalent_key(&k)) {
            Some(mem::replace(item, v))
        } else {
            self.table
                .insert(hash, (k, v), make_hasher::<K, _, V, S>(&self.hash_builder));
            None
        }
    }

    /// Insert a key-value pair into the map without checking
    /// if the key already exists in the map.
    ///
    /// Returns a reference to the key and value just inserted.
    ///
    /// This operation is safe if a key does not exist in the map.
    ///
    /// However, if a key exists in the map already, the behavior is unspecified:
    /// this operation may panic, loop forever, or any following operation with the map
    /// may panic, loop forever or return arbitrary result.
    ///
    /// That said, this operation (and following operations) are guaranteed to
    /// not violate memory safety.
    ///
    /// This operation is faster than regular insert, because it does not perform
    /// lookup before insertion.
    ///
    /// This operation is useful during initial population of the map.
    /// For example, when constructing a map from another map, we know
    /// that keys are unique.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map1 = HashMap::new();
    /// assert_eq!(map1.insert(1, "a"), None);
    /// assert_eq!(map1.insert(2, "b"), None);
    /// assert_eq!(map1.insert(3, "c"), None);
    /// assert_eq!(map1.len(), 3);
    ///
    /// let mut map2 = HashMap::new();
    ///
    /// for (key, value) in map1.into_iter() {
    ///     map2.insert_unique_unchecked(key, value);
    /// }
    ///
    /// let (key, value) = map2.insert_unique_unchecked(4, "d");
    /// assert_eq!(key, &4);
    /// assert_eq!(value, &mut "d");
    /// *value = "e";
    ///
    /// assert_eq!(map2[&1], "a");
    /// assert_eq!(map2[&2], "b");
    /// assert_eq!(map2[&3], "c");
    /// assert_eq!(map2[&4], "e");
    /// assert_eq!(map2.len(), 4);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn insert_unique_unchecked(&mut self, k: K, v: V) -> (&K, &mut V) {
        let hash = make_insert_hash::<K, S>(&self.hash_builder, &k);
        let bucket = self
            .table
            .insert(hash, (k, v), make_hasher::<K, _, V, S>(&self.hash_builder));
        let (k_ref, v_ref) = unsafe { bucket.as_mut() };
        (k_ref, v_ref)
    }

    /// Tries to insert a key-value pair into the map, and returns
    /// a mutable reference to the value in the entry.
    ///
    /// # Errors
    ///
    /// 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:
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use hashbrown::hash_map::OccupiedError;
    ///
    /// let mut map = HashMap::new();
    /// assert_eq!(map.try_insert(37, "a").unwrap(), &"a");
    ///
    /// match map.try_insert(37, "b") {
    ///     Err(OccupiedError { entry, value }) => {
    ///         assert_eq!(entry.key(), &37);
    ///         assert_eq!(entry.get(), &"a");
    ///         assert_eq!(value, "b");
    ///     }
    ///     _ => panic!()
    /// }
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn try_insert(
        &mut self,
        key: K,
        value: V,
    ) -> Result<&mut V, OccupiedError<'_, K, V, S, A>> {
        match self.entry(key) {
            Entry::Occupied(entry) => Err(OccupiedError { entry, value }),
            Entry::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. Keeps the allocated memory for reuse.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
    /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// // The map is empty
    /// assert!(map.is_empty() && map.capacity() == 0);
    ///
    /// map.insert(1, "a");
    /// let capacity_before_remove = map.capacity();
    ///
    /// assert_eq!(map.remove(&1), Some("a"));
    /// assert_eq!(map.remove(&1), None);
    ///
    /// // Now map holds none elements but capacity is equal to the old one
    /// assert!(map.len() == 0 && map.capacity() == capacity_before_remove);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        // Avoid `Option::map` because it bloats LLVM IR.
        match self.remove_entry(k) {
            Some((_, v)) => Some(v),
            None => None,
        }
    }

    /// Removes a key from the map, returning the stored key and value if the
    /// key was previously in the map. Keeps the allocated memory for reuse.
    ///
    /// The key may be any borrowed form of the map's key type, but
    /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
    /// the key type.
    ///
    /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
    /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// // The map is empty
    /// assert!(map.is_empty() && map.capacity() == 0);
    ///
    /// map.insert(1, "a");
    /// let capacity_before_remove = map.capacity();
    ///
    /// assert_eq!(map.remove_entry(&1), Some((1, "a")));
    /// assert_eq!(map.remove(&1), None);
    ///
    /// // Now map hold none elements but capacity is equal to the old one
    /// assert!(map.len() == 0 && map.capacity() == capacity_before_remove);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn remove_entry<Q: ?Sized>(&mut self, k: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq,
    {
        let hash = make_hash::<K, Q, S>(&self.hash_builder, k);
        self.table.remove_entry(hash, equivalent_key(k))
    }
}

impl<K, V, S, A: Allocator + Clone> HashMap<K, V, S, A> {
    /// 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).
    ///
    /// # Examples
    ///
    /// ```
    /// use core::hash::{BuildHasher, Hash};
    /// use hashbrown::hash_map::{HashMap, RawEntryMut};
    ///
    /// let mut map = HashMap::new();
    /// map.extend([("a", 100), ("b", 200), ("c", 300)]);
    ///
    /// fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
    ///     use core::hash::Hasher;
    ///     let mut state = hash_builder.build_hasher();
    ///     key.hash(&mut state);
    ///     state.finish()
    /// }
    ///
    /// // Existing key (insert and update)
    /// match map.raw_entry_mut().from_key(&"a") {
    ///     RawEntryMut::Vacant(_) => unreachable!(),
    ///     RawEntryMut::Occupied(mut view) => {
    ///         assert_eq!(view.get(), &100);
    ///         let v = view.get_mut();
    ///         let new_v = (*v) * 10;
    ///         *v = new_v;
    ///         assert_eq!(view.insert(1111), 1000);
    ///     }
    /// }
    ///
    /// assert_eq!(map[&"a"], 1111);
    /// assert_eq!(map.len(), 3);
    ///
    /// // Existing key (take)
    /// let hash = compute_hash(map.hasher(), &"c");
    /// match map.raw_entry_mut().from_key_hashed_nocheck(hash, &"c") {
    ///     RawEntryMut::Vacant(_) => unreachable!(),
    ///     RawEntryMut::Occupied(view) => {
    ///         assert_eq!(view.remove_entry(), ("c", 300));
    ///     }
    /// }
    /// assert_eq!(map.raw_entry().from_key(&"c"), None);
    /// assert_eq!(map.len(), 2);
    ///
    /// // Nonexistent key (insert and update)
    /// let key = "d";
    /// let hash = compute_hash(map.hasher(), &key);
    /// match map.raw_entry_mut().from_hash(hash, |q| *q == key) {
    ///     RawEntryMut::Occupied(_) => unreachable!(),
    ///     RawEntryMut::Vacant(view) => {
    ///         let (k, value) = view.insert("d", 4000);
    ///         assert_eq!((*k, *value), ("d", 4000));
    ///         *value = 40000;
    ///     }
    /// }
    /// assert_eq!(map[&"d"], 40000);
    /// assert_eq!(map.len(), 3);
    ///
    /// match map.raw_entry_mut().from_hash(hash, |q| *q == key) {
    ///     RawEntryMut::Vacant(_) => unreachable!(),
    ///     RawEntryMut::Occupied(view) => {
    ///         assert_eq!(view.remove_entry(), ("d", 40000));
    ///     }
    /// }
    /// assert_eq!(map.get(&"d"), None);
    /// assert_eq!(map.len(), 2);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<'_, K, V, S, A> {
        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`.
    ///
    /// # Examples
    ///
    /// ```
    /// use core::hash::{BuildHasher, Hash};
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.extend([("a", 100), ("b", 200), ("c", 300)]);
    ///
    /// fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
    ///     use core::hash::Hasher;
    ///     let mut state = hash_builder.build_hasher();
    ///     key.hash(&mut state);
    ///     state.finish()
    /// }
    ///
    /// for k in ["a", "b", "c", "d", "e", "f"] {
    ///     let hash = compute_hash(map.hasher(), k);
    ///     let v = map.get(&k).cloned();
    ///     let kv = v.as_ref().map(|v| (&k, v));
    ///
    ///     println!("Key: {} and value: {:?}", k, v);
    ///
    ///     assert_eq!(map.raw_entry().from_key(&k), kv);
    ///     assert_eq!(map.raw_entry().from_hash(hash, |q| *q == k), kv);
    ///     assert_eq!(map.raw_entry().from_key_hashed_nocheck(hash, &k), kv);
    /// }
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn raw_entry(&self) -> RawEntryBuilder<'_, K, V, S, A> {
        RawEntryBuilder { map: self }
    }

    /// Returns a mutable reference to the [`RawTable`] used underneath [`HashMap`].
    /// This function is only available if the `raw` feature of the crate is enabled.
    ///
    /// # Note
    ///
    /// Calling the function safe, but using raw hash table API's may require
    /// unsafe functions or blocks.
    ///
    /// `RawTable` API gives the lowest level of control under the map that can be useful
    /// for extending the HashMap's API, but may lead to *[undefined behavior]*.
    ///
    /// [`HashMap`]: struct.HashMap.html
    /// [`RawTable`]: raw/struct.RawTable.html
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// use core::hash::{BuildHasher, Hash};
    /// use hashbrown::HashMap;
    ///
    /// let mut map = HashMap::new();
    /// map.extend([("a", 10), ("b", 20), ("c", 30)]);
    /// assert_eq!(map.len(), 3);
    ///
    /// // Let's imagine that we have a value and a hash of the key, but not the key itself.
    /// // However, if you want to remove the value from the map by hash and value, and you
    /// // know exactly that the value is unique, then you can create a function like this:
    /// fn remove_by_hash<K, V, S, F>(
    ///     map: &mut HashMap<K, V, S>,
    ///     hash: u64,
    ///     is_match: F,
    /// ) -> Option<(K, V)>
    /// where
    ///     F: Fn(&(K, V)) -> bool,
    /// {
    ///     let raw_table = map.raw_table();
    ///     match raw_table.find(hash, is_match) {
    ///         Some(bucket) => Some(unsafe { raw_table.remove(bucket) }),
    ///         None => None,
    ///     }
    /// }
    ///
    /// fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
    ///     use core::hash::Hasher;
    ///     let mut state = hash_builder.build_hasher();
    ///     key.hash(&mut state);
    ///     state.finish()
    /// }
    ///
    /// let hash = compute_hash(map.hasher(), "a");
    /// assert_eq!(remove_by_hash(&mut map, hash, |(_, v)| *v == 10), Some(("a", 10)));
    /// assert_eq!(map.get(&"a"), None);
    /// assert_eq!(map.len(), 2);
    /// ```
    #[cfg(feature = "raw")]
    #[cfg_attr(feature = "inline-more", inline)]
    pub fn raw_table(&mut self) -> &mut RawTable<(K, V), A> {
        &mut self.table
    }
}

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

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

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

impl<K, V, S, A> Debug for HashMap<K, V, S, A>
where
    K: Debug,
    V: Debug,
    A: Allocator + Clone,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_map().entries(self.iter()).finish()
    }
}

impl<K, V, S, A> Default for HashMap<K, V, S, A>
where
    S: Default,
    A: Default + Allocator + Clone,
{
    /// Creates an empty `HashMap<K, V, S, A>`, with the `Default` value for the hasher and allocator.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    /// use std::collections::hash_map::RandomState;
    ///
    /// // You can specify all types of HashMap, including hasher and allocator.
    /// // Created map is empty and don't allocate memory
    /// let map: HashMap<u32, String> = Default::default();
    /// assert_eq!(map.capacity(), 0);
    /// let map: HashMap<u32, String, RandomState> = HashMap::default();
    /// assert_eq!(map.capacity(), 0);
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    fn default() -> Self {
        Self::with_hasher_in(Default::default(), Default::default())
    }
}

impl<K, Q: ?Sized, V, S, A> Index<&Q> for HashMap<K, V, S, A>
where
    K: Eq + Hash + Borrow<Q>,
    Q: Eq + Hash,
    S: BuildHasher,
    A: Allocator + Clone,
{
    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`.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashMap;
    ///
    /// let map: HashMap<_, _> = [("a", "One"), ("b", "Two")].into();
    ///
    /// assert_eq!(map[&"a"], "One");
    /// assert_eq!(map[&"b"], "Two");
    /// ```
    #[cfg_attr(feature = "inline-more", inline)]
    fn index(&self, key: &Q) -> &V {
        self.get(key).expect("no entry found for key")
    }
}

// The default hasher is used to match the std implementation signature
#[cfg(feature = "ahash")]
impl<K, V, A, const N: usize> From<[(K, V); N]> for HashMap<K, V, DefaultHashBuilder, A>
where
    K: Eq + Hash,
    A: Default + Allocator + Clone,
{
    /// # Examples
    ///
    /// ```
    /// use hashbrown::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 {
        arr.into_iter().collect()
    }
}

/// An iterator over the entries of a `HashMap` in arbitrary order.
/// The iterator element type is `(&'a K, &'a V)`.
///
/// This `struct` is created by the [`iter`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`iter`]: struct.HashMap.html#method.iter
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut iter = map.iter();
/// let mut vec = vec![iter.next(), iter.next(), iter.next()];
///
/// // The `Iter` iterator produces items in arbitrary order, so the
/// // items must be sorted to test them against a sorted array.
/// vec.sort_unstable();
/// assert_eq!(vec, [Some((&1, &"a")), Some((&2, &"b")), Some((&3, &"c"))]);
///
/// // It is fused iterator
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), None);
/// ```
pub struct Iter<'a, K, V> {
    inner: RawIter<(K, V)>,
    marker: PhantomData<(&'a K, &'a V)>,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
impl<K, V> Clone for Iter<'_, K, V> {
    #[cfg_attr(feature = "inline-more", inline)]
    fn clone(&self) -> Self {
        Iter {
            inner: self.inner.clone(),
            marker: PhantomData,
        }
    }
}

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` in arbitrary order.
/// The iterator element type is `(&'a K, &'a mut V)`.
///
/// This `struct` is created by the [`iter_mut`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`iter_mut`]: struct.HashMap.html#method.iter_mut
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let mut map: HashMap<_, _> = [(1, "One".to_owned()), (2, "Two".into())].into();
///
/// let mut iter = map.iter_mut();
/// iter.next().map(|(_, v)| v.push_str(" Mississippi"));
/// iter.next().map(|(_, v)| v.push_str(" Mississippi"));
///
/// // It is fused iterator
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), None);
///
/// assert_eq!(map.get(&1).unwrap(), &"One Mississippi".to_owned());
/// assert_eq!(map.get(&2).unwrap(), &"Two Mississippi".to_owned());
/// ```
pub struct IterMut<'a, K, V> {
    inner: RawIter<(K, V)>,
    // To ensure invariance with respect to V
    marker: PhantomData<(&'a K, &'a mut V)>,
}

// We override the default Send impl which has K: Sync instead of K: Send. Both
// are correct, but this one is more general since it allows keys which
// implement Send but not Sync.
unsafe impl<K: Send, V: Send> Send for IterMut<'_, K, V> {}

impl<K, V> IterMut<'_, K, V> {
    /// Returns a iterator of references over the remaining items.
    #[cfg_attr(feature = "inline-more", inline)]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter {
            inner: self.inner.clone(),
            marker: PhantomData,
        }
    }
}

/// An owning iterator over the entries of a `HashMap` in arbitrary order.
/// The iterator element type is `(K, V)`.
///
/// This `struct` is created by the [`into_iter`] method on [`HashMap`]
/// (provided by the [`IntoIterator`] trait). See its documentation for more.
/// The map cannot be used after calling that method.
///
/// [`into_iter`]: struct.HashMap.html#method.into_iter
/// [`HashMap`]: struct.HashMap.html
/// [`IntoIterator`]: https://doc.rust-lang.org/core/iter/trait.IntoIterator.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut iter = map.into_iter();
/// let mut vec = vec![iter.next(), iter.next(), iter.next()];
///
/// // The `IntoIter` iterator produces items in arbitrary order, so the
/// // items must be sorted to test them against a sorted array.
/// vec.sort_unstable();
/// assert_eq!(vec, [Some((1, "a")), Some((2, "b")), Some((3, "c"))]);
///
/// // It is fused iterator
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), None);
/// ```
pub struct IntoIter<K, V, A: Allocator + Clone = Global> {
    inner: RawIntoIter<(K, V), A>,
}

impl<K, V, A: Allocator + Clone> IntoIter<K, V, A> {
    /// Returns a iterator of references over the remaining items.
    #[cfg_attr(feature = "inline-more", inline)]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter {
            inner: self.inner.iter(),
            marker: PhantomData,
        }
    }
}

/// An owning iterator over the keys of a `HashMap` in arbitrary order.
/// The iterator element type is `K`.
///
/// This `struct` is created by the [`into_keys`] method on [`HashMap`].
/// See its documentation for more.
/// The map cannot be used after calling that method.
///
/// [`into_keys`]: struct.HashMap.html#method.into_keys
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut keys = map.into_keys();
/// let mut vec = vec![keys.next(), keys.next(), keys.next()];
///
/// // 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, [Some(1), Some(2), Some(3)]);
///
/// // It is fused iterator
/// assert_eq!(keys.next(), None);
/// assert_eq!(keys.next(), None);
/// ```
pub struct IntoKeys<K, V, A: Allocator + Clone = Global> {
    inner: IntoIter<K, V, A>,
}

impl<K, V, A: Allocator + Clone> Iterator for IntoKeys<K, V, A> {
    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()
    }
}

impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoKeys<K, V, A> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}

impl<K, V, A: Allocator + Clone> FusedIterator for IntoKeys<K, V, A> {}

impl<K: Debug, V: Debug, A: Allocator + Clone> fmt::Debug for IntoKeys<K, V, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list()
            .entries(self.inner.iter().map(|(k, _)| k))
            .finish()
    }
}

/// An owning iterator over the values of a `HashMap` in arbitrary order.
/// The iterator element type is `V`.
///
/// This `struct` is created by the [`into_values`] method on [`HashMap`].
/// See its documentation for more. The map cannot be used after calling that method.
///
/// [`into_values`]: struct.HashMap.html#method.into_values
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut values = map.into_values();
/// let mut vec = vec![values.next(), values.next(), values.next()];
///
/// // 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, [Some("a"), Some("b"), Some("c")]);
///
/// // It is fused iterator
/// assert_eq!(values.next(), None);
/// assert_eq!(values.next(), None);
/// ```
pub struct IntoValues<K, V, A: Allocator + Clone = Global> {
    inner: IntoIter<K, V, A>,
}

impl<K, V, A: Allocator + Clone> Iterator for IntoValues<K, V, A> {
    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()
    }
}

impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoValues<K, V, A> {
    #[inline]
    fn len(&self) -> usize {
        self.inner.len()
    }
}

impl<K, V, A: Allocator + Clone> FusedIterator for IntoValues<K, V, A> {}

impl<K, V: Debug, A: Allocator + Clone> fmt::Debug for IntoValues<K, V, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list()
            .entries(self.inner.iter().map(|(_, v)| v))
            .finish()
    }
}

/// An iterator over the keys of a `HashMap` in arbitrary order.
/// The iterator element type is `&'a K`.
///
/// This `struct` is created by the [`keys`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`keys`]: struct.HashMap.html#method.keys
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut keys = map.keys();
/// let mut vec = vec![keys.next(), keys.next(), keys.next()];
///
/// // The `Keys` 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, [Some(&1), Some(&2), Some(&3)]);
///
/// // It is fused iterator
/// assert_eq!(keys.next(), None);
/// assert_eq!(keys.next(), None);
/// ```
pub struct Keys<'a, K, V> {
    inner: Iter<'a, K, V>,
}

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

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` in arbitrary order.
/// The iterator element type is `&'a V`.
///
/// This `struct` is created by the [`values`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`values`]: struct.HashMap.html#method.values
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut values = map.values();
/// let mut vec = vec![values.next(), values.next(), values.next()];
///
/// // The `Values` 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, [Some(&"a"), Some(&"b"), Some(&"c")]);
///
/// // It is fused iterator
/// assert_eq!(values.next(), None);
/// assert_eq!(values.next(), None);
/// ```
pub struct Values<'a, K, V> {
    inner: Iter<'a, K, V>,
}

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

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` in arbitrary
/// order. The iterator element type is `(K, V)`.
///
/// This `struct` is created by the [`drain`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`drain`]: struct.HashMap.html#method.drain
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let mut map: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut drain_iter = map.drain();
/// let mut vec = vec![drain_iter.next(), drain_iter.next(), drain_iter.next()];
///
/// // The `Drain` iterator produces items in arbitrary order, so the
/// // items must be sorted to test them against a sorted array.
/// vec.sort_unstable();
/// assert_eq!(vec, [Some((1, "a")), Some((2, "b")), Some((3, "c"))]);
///
/// // It is fused iterator
/// assert_eq!(drain_iter.next(), None);
/// assert_eq!(drain_iter.next(), None);
/// ```
pub struct Drain<'a, K, V, A: Allocator + Clone = Global> {
    inner: RawDrain<'a, (K, V), A>,
}

impl<K, V, A: Allocator + Clone> Drain<'_, K, V, A> {
    /// Returns a iterator of references over the remaining items.
    #[cfg_attr(feature = "inline-more", inline)]
    pub(super) fn iter(&self) -> Iter<'_, K, V> {
        Iter {
            inner: self.inner.iter(),
            marker: PhantomData,
        }
    }
}

/// A draining iterator over entries of a `HashMap` which don't satisfy the predicate
/// `f(&k, &mut v)` in arbitrary order. The iterator element type is `(K, V)`.
///
/// This `struct` is created by the [`drain_filter`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`drain_filter`]: struct.HashMap.html#method.drain_filter
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let mut map: HashMap<i32, &str> = [(1, "a"), (2, "b"), (3, "c")].into();
///
/// let mut drain_filter = map.drain_filter(|k, _v| k % 2 != 0);
/// let mut vec = vec![drain_filter.next(), drain_filter.next()];
///
/// // The `DrainFilter` iterator produces items in arbitrary order, so the
/// // items must be sorted to test them against a sorted array.
/// vec.sort_unstable();
/// assert_eq!(vec, [Some((1, "a")),Some((3, "c"))]);
///
/// // It is fused iterator
/// assert_eq!(drain_filter.next(), None);
/// assert_eq!(drain_filter.next(), None);
/// drop(drain_filter);
///
/// assert_eq!(map.len(), 1);
/// ```
pub struct DrainFilter<'a, K, V, F, A: Allocator + Clone = Global>
where
    F: FnMut(&K, &mut V) -> bool,
{
    f: F,
    inner: DrainFilterInner<'a, K, V, A>,
}

impl<'a, K, V, F, A> Drop for DrainFilter<'a, K, V, F, A>
where
    F: FnMut(&K, &mut V) -> bool,
    A: Allocator + Clone,
{
    #[cfg_attr(feature = "inline-more", inline)]
    fn drop(&mut self) {
        while let Some(item) = self.next() {
            let guard = ConsumeAllOnDrop(self);
            drop(item);
            mem::forget(guard);
        }
    }
}

pub(super) struct ConsumeAllOnDrop<'a, T: Iterator>(pub &'a mut T);

impl<T: Iterator> Drop for ConsumeAllOnDrop<'_, T> {
    #[cfg_attr(feature = "inline-more", inline)]
    fn drop(&mut self) {
        self.0.for_each(drop);
    }
}

impl<K, V, F, A> Iterator for DrainFilter<'_, K, V, F, A>
where
    F: FnMut(&K, &mut V) -> bool,
    A: Allocator + Clone,
{
    type Item = (K, V);

    #[cfg_attr(feature = "inline-more", inline)]
    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next(&mut self.f)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, self.inner.iter.size_hint().1)
    }
}

impl<K, V, F> FusedIterator for DrainFilter<'_, K, V, F> where F: FnMut(&K, &mut V) -> bool {}

/// Portions of `DrainFilter` shared with `set::DrainFilter`
pub(super) struct DrainFilterInner<'a, K, V, A: Allocator + Clone> {
    pub iter: RawIter<(K, V)>,
    pub table: &'a mut RawTable<(K, V), A>,
}

impl<K, V, A: Allocator + Clone> DrainFilterInner<'_, K, V, A> {
    #[cfg_attr(feature = "inline-more", inline)]
    pub(super) fn next<F>(&mut self, f: &mut F) -> Option<(K, V)>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        unsafe {
            for item in &mut self.iter {
                let &mut (ref key, ref mut value) = item.as_mut();
                if f(key, value) {
                    return Some(self.table.remove(item));
                }
            }
        }
        None
    }
}

/// A mutable iterator over the values of a `HashMap` in arbitrary order.
/// The iterator element type is `&'a mut V`.
///
/// This `struct` is created by the [`values_mut`] method on [`HashMap`]. See its
/// documentation for more.
///
/// [`values_mut`]: struct.HashMap.html#method.values_mut
/// [`HashMap`]: struct.HashMap.html
///
/// # Examples
///
/// ```
/// use hashbrown::HashMap;
///
/// let mut map: HashMap<_, _> = [(1, "One".to_owned()), (2, "Two".into())].into();
///
/// let mut values = map.values_mut();
/// values.next().map(|v| v.push_str(" Mississippi"));
/// values.next().map(|v| v.push_str(" Mississippi"));
///
/// // It is fused iterator
/// assert_eq!(values.next(), None);
/// assert_eq!(values.next(), None);
///
/// assert_eq!(map.get(&1).unwrap(), &"One Mississippi".to_owned());
/// assert_eq!(map.get(&2).unwrap(), &"Two Mississippi".to_owned());
/// ```
pub struct ValuesMut<'a, K, V> {
    inner: IterMut<'a, K, V>,
}

/// A builder for computing where in a [`HashMap`] a key-value pair would be stored.
///
/// See the [`HashMap::raw_entry_mut`] docs for usage examples.
///
/// [`HashMap::raw_entry_mut`]: struct.HashMap.html#method.raw_entry_mut
///
/// # Examples
///
/// ```
/// use hashbrown::hash_map::{RawEntryBuilderMut, RawEntryMut::Vacant, RawEntryMut::Occupied};
/// use hashbrown::HashMap;
/// use core::hash::{BuildHasher, Hash};
///
/// let mut map = HashMap::new();
/// map.extend([(1, 11), (2, 12), (3, 13), (4, 14), (5, 15), (6, 16)]);
/// assert_eq!(map.len(), 6);
///
/// fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
///     use core::hash::Hasher;
///     let mut state = hash_builder.build_hasher();
///     key.hash(&mut state);
///     state.finish()
/// }
///
/// let builder: RawEntryBuilderMut<_, _, _> = map.raw_entry_mut();
///
/// // Existing key
/// match builder.from_key(&6) {
///     Vacant(_) => unreachable!(),
///     Occupied(view) => assert_eq!(view.get(), &16),
/// }
///
/// for key in 0..12 {
///     let hash = compute_hash(map.hasher(), &key);
///     let value = map.get(&key).cloned();
///     let key_value = value.as_ref().map(|v| (&key, v));
///
///     println!("Key: {} and value: {:?}", key, value);
///
///     match map.raw_entry_mut().from_key(&key) {
///         Occupied(mut o) => assert_eq!(Some(o.get_key_value()), key_value),
///         Vacant(_) => assert_eq!(value, None),
///     }
///     match map.raw_entry_mut().from_key_hashed_nocheck(hash, &key) {
///         Occupied(mut o) => assert_eq!(Some(o.get_key_value()), key_value),
///         Vacant(_) => assert_eq!(value, None),
///     }
///     match map.raw_entry_mut().from_hash(hash, |q| *q == key) {
///         Occupied(mut o) => assert_eq!(Some(o.get_key_value()), key_value),
///         Vacant(_) => assert_eq!(value, None),
///     }
/// }
///
/// assert_eq!(map.len(), 6);
/// ```
pub struct RawEntryBuilderMut<'a, K, V, S, A: Allocator + Clone = Global> {
    map: &'a mut HashMap<K, V, S, A>,
}

/// 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`].
///
/// [`HashMap`]: struct.HashMap.html
/// [`Entry`]: enum.Entry.html
/// [`raw_entry_mut`]: struct.HashMap.html#method.raw_entry_mut
/// [`RawEntryBuilderMut`]: struct.RawEntryBuilderMut.html
///
/// # Examples
///
/// ```
/// use core::hash::{BuildHasher, Hash};
/// use hashbrown::hash_map::{HashMap, RawEntryMut, RawOccupiedEntryMut};
///
/// let mut map = HashMap::new();
/// map.extend([('a', 1), ('b', 2), ('c', 3)]);
/// assert_eq!(map.len(), 3);
///
/// fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
///     use core::hash::Hasher;
///     let mut state = hash_builder.build_hasher();
///     key.hash(&mut state);
///     state.finish()
/// }
///
/// // Existing key (insert)
/// let raw: RawEntryMut<_, _, _> = map.raw_entry_mut().from_key(&'a');
/// let _raw_o: RawOccupiedEntryMut<_, _, _> = raw.insert('a', 10);
/// assert_eq!(map.len(), 3);
///
/// // Nonexistent key (insert)
/// map.raw_entry_mut().from_key(&'d').insert('d', 40);
/// assert_eq!(map.len(), 4);
///
/// // Existing key (or_insert)
/// let hash = compute_hash(map.hasher(), &'b');
/// let kv = map
///     .raw_entry_mut()
///     .from_key_hashed_nocheck(hash, &'b')
///     .or_insert('b', 20);
/// assert_eq!(kv, (&mut 'b', &mut 2));
/// *kv.1 = 20;
/// assert_eq!(map.len(), 4);
///
/// // Nonexistent key (or_insert)
/// let hash = compute_hash(map.hasher(), &'e');
/// let kv = map
///     .raw_entry_mut()
///     .from_key_hashed_nocheck(hash, &'e')
///     .or_insert('e', 50);
/// assert_eq!(kv, (&mut 'e', &mut 50));
/// assert_eq!(map.len(), 5);
///
/// // Existing key (or_insert_with)
/// let hash = compute_hash(map.hasher(), &'c');
/// let kv = map
///     .raw_entry_mut()
///     .from_hash(hash, |q| q == &'c')
///     .or_insert_with(|| ('c', 30));
/// assert_eq!(kv, (&mut 'c', &mut 3));
/// *kv.1 = 30;
/// assert_eq!(map.len(), 5);
///
/// // Nonexistent key (or_insert_with)
/// let hash = compute_hash(map.hasher(), &'f');
/// let kv = map
///     .raw_entry_mut()
///     .from_hash(hash, |q| q == &'f')
///     .or_insert_with(|| ('f', 60));
/// assert_eq!(kv, (&mut 'f', &mut 60));
/// assert_eq!(map.len(), 6);
///
/// println!("Our HashMap: {:?}", map);
///
/// let mut vec: Vec<_> = map.iter().map(|(&k, &v)| (k, v)).collect();
/// // The `Iter` iterator produces items in arbitrary order, so the
/// // items must be sorted to test them against a sorted array.
/// vec.sort_unstable();
/// assert_eq!(vec, [('a', 10), ('b', 20), ('c', 30), ('d', 40), ('e', 50), ('f', 60)]);
/// ```
pub enum RawEntryMut<'a, K, V, S, A: Allocator + Clone = Global> {
    /// An occupied entry.
    ///
    /// # Examples
    ///
    /// ```