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    use crate::TryReserveError;
    use alloc::borrow::ToOwned;
    use core::borrow::Borrow;
    use core::fmt;
    use core::hash::{BuildHasher, Hash};
    use core::iter::{Chain, FromIterator, FusedIterator};
    use core::mem;
    use core::ops::{BitAnd, BitOr, BitXor, Sub};

    use super::map::{self, ConsumeAllOnDrop, DefaultHashBuilder, DrainFilterInner, HashMap, Keys};

    // Future Optimization (FIXME!)
    // =============================
    //
    // Iteration over zero sized values is a noop. There is no need
    // for `bucket.val` in the case of HashSet. I suppose we would need HKT
    // to get rid of it properly.

    /// A hash set implemented as a `HashMap` where the value is `()`.
    ///
    /// As with the [`HashMap`] type, a `HashSet` requires that the elements
    /// implement the [`Eq`] and [`Hash`] traits. 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 an item to be modified in such a way that the
    /// item's hash, as determined by the [`Hash`] trait, or its equality, as
    /// determined by the [`Eq`] trait, changes while it is in the set. 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 `HashSet` may become corrupted and
    /// some items may be dropped from the table.
    ///
    /// # Examples
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// // Type inference lets us omit an explicit type signature (which
    /// // would be `HashSet<String>` in this example).
    /// let mut books = HashSet::new();
    ///
    /// // Add some books.
    /// books.insert("A Dance With Dragons".to_string());
    /// books.insert("To Kill a Mockingbird".to_string());
    /// books.insert("The Odyssey".to_string());
    /// books.insert("The Great Gatsby".to_string());
    ///
    /// // Check for a specific one.
    /// if !books.contains("The Winds of Winter") {
    ///     println!("We have {} books, but The Winds of Winter ain't one.",
    ///              books.len());
    /// }
    ///
    /// // Remove a book.
    /// books.remove("The Odyssey");
    ///
    /// // Iterate over everything.
    /// for book in &books {
    ///     println!("{}", book);
    /// }
    /// ```
    ///
    /// The easiest way to use `HashSet` with a custom type is to derive
    /// [`Eq`] and [`Hash`]. We must also derive [`PartialEq`], this will in the
    /// future be implied by [`Eq`].
    ///
    /// ```
    /// use hashbrown::HashSet;
    /// #[derive(Hash, Eq, PartialEq, Debug)]
    /// struct Viking {
    ///     name: String,
    ///     power: usize,
    /// }
    ///
    /// let mut vikings = HashSet::new();
    ///
    /// vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
    /// vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
    /// vikings.insert(Viking { name: "Olaf".to_string(), power: 4 });
    /// vikings.insert(Viking { name: "Harald".to_string(), power: 8 });
    ///
    /// // Use derived implementation to print the vikings.
    /// for x in &vikings {
    ///     println!("{:?}", x);
    /// }
    /// ```
    ///
    /// A `HashSet` with fixed list of elements can be initialized from an array:
    ///
    /// ```
    /// use hashbrown::HashSet;
    ///
    /// let viking_names: HashSet<&'static str> =
    ///     [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
    /// // use the values stored in the set
    /// ```
    ///
    /// [`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html
    /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
    /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
    /// [`HashMap`]: struct.HashMap.html
    /// [`PartialEq`]: https://doc.rust-lang.org/std/cmp/trait.PartialEq.html
    /// [`RefCell`]: https://doc.rust-lang.org/std/cell/struct.RefCell.html
    pub struct HashSet<T, S = DefaultHashBuilder> {
        pub(crate) map: HashMap<T, (), S>,
    }

    impl<T: Clone, S: Clone> Clone for HashSet<T, S> {
        fn clone(&self) -> Self {
            HashSet {
                map: self.map.clone(),
            }
        }

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

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

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

    impl<T, S> HashSet<T, S> {
        /// Creates a new empty hash set which will use the given hasher to hash
        /// keys.
        ///
        /// The hash set is also created with the default initial capacity.
        ///
        /// Warning: `hasher` is normally randomly generated, and
        /// is designed to allow `HashSet`s 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::HashSet;
        /// use hashbrown::hash_map::DefaultHashBuilder;
        ///
        /// let s = DefaultHashBuilder::default();
        /// let mut set = HashSet::with_hasher(s);
        /// set.insert(2);
        /// ```
        ///
        /// [`BuildHasher`]: ../../std/hash/trait.BuildHasher.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub const fn with_hasher(hasher: S) -> Self {
            Self {
                map: HashMap::with_hasher(hasher),
            }
        }

        /// Creates an empty `HashSet` with the specified capacity, using
        /// `hasher` to hash the keys.
        ///
        /// The hash set will be able to hold at least `capacity` elements without
        /// reallocating. If `capacity` is 0, the hash set will not allocate.
        ///
        /// Warning: `hasher` is normally randomly generated, and
        /// is designed to allow `HashSet`s 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::HashSet;
        /// use hashbrown::hash_map::DefaultHashBuilder;
        ///
        /// let s = DefaultHashBuilder::default();
        /// let mut set = HashSet::with_capacity_and_hasher(10, s);
        /// set.insert(1);
        /// ```
        ///
        /// [`BuildHasher`]: ../../std/hash/trait.BuildHasher.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> Self {
            Self {
                map: HashMap::with_capacity_and_hasher(capacity, hasher),
            }
        }

        /// Returns the number of elements the set can hold without reallocating.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let set: HashSet<i32> = HashSet::with_capacity(100);
        /// assert!(set.capacity() >= 100);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn capacity(&self) -> usize {
            self.map.capacity()
        }

        /// An iterator visiting all elements in arbitrary order.
        /// The iterator element type is `&'a T`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let mut set = HashSet::new();
        /// set.insert("a");
        /// set.insert("b");
        ///
        /// // Will print in an arbitrary order.
        /// for x in set.iter() {
        ///     println!("{}", x);
        /// }
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn iter(&self) -> Iter<'_, T> {
            Iter {
                iter: self.map.keys(),
            }
        }

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

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

        /// Clears the set, returning all elements in an iterator.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert!(!set.is_empty());
        ///
        /// // print 1, 2, 3 in an arbitrary order
        /// for i in set.drain() {
        ///     println!("{}", i);
        /// }
        ///
        /// assert!(set.is_empty());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn drain(&mut self) -> Drain<'_, T> {
            Drain {
                iter: self.map.drain(),
            }
        }

        /// Retains only the elements specified by the predicate.
        ///
        /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let xs = [1,2,3,4,5,6];
        /// let mut set: HashSet<i32> = xs.iter().cloned().collect();
        /// set.retain(|&k| k % 2 == 0);
        /// assert_eq!(set.len(), 3);
        /// ```
        pub fn retain<F>(&mut self, mut f: F)
        where
            F: FnMut(&T) -> bool,
        {
            self.map.retain(|k, _| f(k));
        }

        /// Drains elements which are true under the given predicate,
        /// and returns an iterator over the removed items.
        ///
        /// In other words, move all elements `e` such that `f(&e)` returns `true` out
        /// into another iterator.
        ///
        /// When the returned DrainedFilter is dropped, any remaining elements that satisfy
        /// the predicate are dropped from the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<i32> = (0..8).collect();
        /// let drained: HashSet<i32> = set.drain_filter(|v| v % 2 == 0).collect();
        ///
        /// let mut evens = drained.into_iter().collect::<Vec<_>>();
        /// let mut odds = set.into_iter().collect::<Vec<_>>();
        /// evens.sort();
        /// odds.sort();
        ///
        /// assert_eq!(evens, vec![0, 2, 4, 6]);
        /// assert_eq!(odds, vec![1, 3, 5, 7]);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn drain_filter<F>(&mut self, f: F) -> DrainFilter<'_, T, F>
        where
            F: FnMut(&T) -> bool,
        {
            DrainFilter {
                f,
                inner: DrainFilterInner {
                    iter: unsafe { self.map.table.iter() },
                    table: &mut self.map.table,
                },
            }
        }

        /// Clears the set, removing all values.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut v = HashSet::new();
        /// v.insert(1);
        /// v.clear();
        /// assert!(v.is_empty());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn clear(&mut self) {
            self.map.clear()
        }

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

    impl<T, S> HashSet<T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        /// Reserves capacity for at least `additional` more elements to be inserted
        /// in the `HashSet`. The collection may reserve more space to avoid
        /// frequent reallocations.
        ///
        /// # Panics
        ///
        /// Panics if the new allocation size overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let mut set: HashSet<i32> = HashSet::new();
        /// set.reserve(10);
        /// assert!(set.capacity() >= 10);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn reserve(&mut self, additional: usize) {
            self.map.reserve(additional)
        }

        /// Tries to reserve capacity for at least `additional` more elements to be inserted
        /// in the given `HashSet<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::HashSet;
        /// let mut set: HashSet<i32> = HashSet::new();
        /// set.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
            self.map.try_reserve(additional)
        }

        /// Shrinks the capacity of the set 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::HashSet;
        ///
        /// let mut set = HashSet::with_capacity(100);
        /// set.insert(1);
        /// set.insert(2);
        /// assert!(set.capacity() >= 100);
        /// set.shrink_to_fit();
        /// assert!(set.capacity() >= 2);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn shrink_to_fit(&mut self) {
            self.map.shrink_to_fit()
        }

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

        /// Visits the values representing the difference,
        /// i.e., the values that are in `self` but not in `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Can be seen as `a - b`.
        /// for x in a.difference(&b) {
        ///     println!("{}", x); // Print 1
        /// }
        ///
        /// let diff: HashSet<_> = a.difference(&b).collect();
        /// assert_eq!(diff, [1].iter().collect());
        ///
        /// // Note that difference is not symmetric,
        /// // and `b - a` means something else:
        /// let diff: HashSet<_> = b.difference(&a).collect();
        /// assert_eq!(diff, [4].iter().collect());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn difference<'a>(&'a self, other: &'a Self) -> Difference<'a, T, S> {
            Difference {
                iter: self.iter(),
                other,
            }
        }

        /// Visits the values representing the symmetric difference,
        /// i.e., the values that are in `self` or in `other` but not in both.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 1, 4 in arbitrary order.
        /// for x in a.symmetric_difference(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let diff1: HashSet<_> = a.symmetric_difference(&b).collect();
        /// let diff2: HashSet<_> = b.symmetric_difference(&a).collect();
        ///
        /// assert_eq!(diff1, diff2);
        /// assert_eq!(diff1, [1, 4].iter().collect());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn symmetric_difference<'a>(&'a self, other: &'a Self) -> SymmetricDifference<'a, T, S> {
            SymmetricDifference {
                iter: self.difference(other).chain(other.difference(self)),
            }
        }

        /// Visits the values representing the intersection,
        /// i.e., the values that are both in `self` and `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 2, 3 in arbitrary order.
        /// for x in a.intersection(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let intersection: HashSet<_> = a.intersection(&b).collect();
        /// assert_eq!(intersection, [2, 3].iter().collect());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn intersection<'a>(&'a self, other: &'a Self) -> Intersection<'a, T, S> {
            let (smaller, larger) = if self.len() <= other.len() {
                (self, other)
            } else {
                (other, self)
            };
            Intersection {
                iter: smaller.iter(),
                other: larger,
            }
        }

        /// Visits the values representing the union,
        /// i.e., all the values in `self` or `other`, without duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
        ///
        /// // Print 1, 2, 3, 4 in arbitrary order.
        /// for x in a.union(&b) {
        ///     println!("{}", x);
        /// }
        ///
        /// let union: HashSet<_> = a.union(&b).collect();
        /// assert_eq!(union, [1, 2, 3, 4].iter().collect());
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn union<'a>(&'a self, other: &'a Self) -> Union<'a, T, S> {
            let (smaller, larger) = if self.len() >= other.len() {
                (self, other)
            } else {
                (other, self)
            };
            Union {
                iter: larger.iter().chain(smaller.difference(larger)),
            }
        }

        /// Returns `true` if the set contains a value.
        ///
        /// The value may be any borrowed form of the set's value type, but
        /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
        /// the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.contains(&1), true);
        /// assert_eq!(set.contains(&4), false);
        /// ```
        ///
        /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
        /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
        where
            T: Borrow<Q>,
            Q: Hash + Eq,
        {
            self.map.contains_key(value)
        }

        /// Returns a reference to the value in the set, if any, that is equal to the given value.
        ///
        /// The value may be any borrowed form of the set's value type, but
        /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
        /// the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.get(&2), Some(&2));
        /// assert_eq!(set.get(&4), None);
        /// ```
        ///
        /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
        /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T>
        where
            T: Borrow<Q>,
            Q: Hash + Eq,
        {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.map.get_key_value(value) {
                Some((k, _)) => Some(k),
                None => None,
            }
        }

        /// Inserts the given `value` into the set if it is not present, then
        /// returns a reference to the value in the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.len(), 3);
        /// assert_eq!(set.get_or_insert(2), &2);
        /// assert_eq!(set.get_or_insert(100), &100);
        /// assert_eq!(set.len(), 4); // 100 was inserted
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn get_or_insert(&mut self, value: T) -> &T {
            // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
            // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
            self.map
                .raw_entry_mut()
                .from_key(&value)
                .or_insert(value, ())
                .0
        }

        /// Inserts an owned copy of the given `value` into the set if it is not
        /// present, then returns a reference to the value in the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<String> = ["cat", "dog", "horse"]
        ///     .iter().map(|&pet| pet.to_owned()).collect();
        ///
        /// assert_eq!(set.len(), 3);
        /// for &pet in &["cat", "dog", "fish"] {
        ///     let value = set.get_or_insert_owned(pet);
        ///     assert_eq!(value, pet);
        /// }
        /// assert_eq!(set.len(), 4); // a new "fish" was inserted
        /// ```
        #[inline]
        pub fn get_or_insert_owned<Q: ?Sized>(&mut self, value: &Q) -> &T
        where
            T: Borrow<Q>,
            Q: Hash + Eq + ToOwned<Owned = T>,
        {
            // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
            // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
            self.map
                .raw_entry_mut()
                .from_key(value)
                .or_insert_with(|| (value.to_owned(), ()))
                .0
        }

        /// Inserts a value computed from `f` into the set if the given `value` is
        /// not present, then returns a reference to the value in the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<String> = ["cat", "dog", "horse"]
        ///     .iter().map(|&pet| pet.to_owned()).collect();
        ///
        /// assert_eq!(set.len(), 3);
        /// for &pet in &["cat", "dog", "fish"] {
        ///     let value = set.get_or_insert_with(pet, str::to_owned);
        ///     assert_eq!(value, pet);
        /// }
        /// assert_eq!(set.len(), 4); // a new "fish" was inserted
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn get_or_insert_with<Q: ?Sized, F>(&mut self, value: &Q, f: F) -> &T
        where
            T: Borrow<Q>,
            Q: Hash + Eq,
            F: FnOnce(&Q) -> T,
        {
            // Although the raw entry gives us `&mut T`, we only return `&T` to be consistent with
            // `get`. Key mutation is "raw" because you're not supposed to affect `Eq` or `Hash`.
            self.map
                .raw_entry_mut()
                .from_key(value)
                .or_insert_with(|| (f(value), ()))
                .0
        }

        /// Returns `true` if `self` has no elements in common with `other`.
        /// This is equivalent to checking for an empty intersection.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let mut b = HashSet::new();
        ///
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(4);
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(1);
        /// assert_eq!(a.is_disjoint(&b), false);
        /// ```
        pub fn is_disjoint(&self, other: &Self) -> bool {
            self.iter().all(|v| !other.contains(v))
        }

        /// Returns `true` if the set is a subset of another,
        /// i.e., `other` contains at least all the values in `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let sup: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let mut set = HashSet::new();
        ///
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(2);
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(4);
        /// assert_eq!(set.is_subset(&sup), false);
        /// ```
        pub fn is_subset(&self, other: &Self) -> bool {
            self.len() <= other.len() && self.iter().all(|v| other.contains(v))
        }

        /// Returns `true` if the set is a superset of another,
        /// i.e., `self` contains at least all the values in `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let sub: HashSet<_> = [1, 2].iter().cloned().collect();
        /// let mut set = HashSet::new();
        ///
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(0);
        /// set.insert(1);
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(2);
        /// assert_eq!(set.is_superset(&sub), true);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn is_superset(&self, other: &Self) -> bool {
            other.is_subset(self)
        }

        /// Adds a value to the set.
        ///
        /// If the set did not have this value present, `true` is returned.
        ///
        /// If the set did have this value present, `false` is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set = HashSet::new();
        ///
        /// assert_eq!(set.insert(2), true);
        /// assert_eq!(set.insert(2), false);
        /// assert_eq!(set.len(), 1);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn insert(&mut self, value: T) -> bool {
            self.map.insert(value, ()).is_none()
        }

        /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
        /// one. Returns the replaced value.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set = HashSet::new();
        /// set.insert(Vec::<i32>::new());
        ///
        /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 0);
        /// set.replace(Vec::with_capacity(10));
        /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 10);
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn replace(&mut self, value: T) -> Option<T> {
            match self.map.entry(value) {
                map::Entry::Occupied(occupied) => Some(occupied.replace_key()),
                map::Entry::Vacant(vacant) => {
                    vacant.insert(());
                    None
                }
            }
        }

        /// Removes a value from the set. Returns whether the value was
        /// present in the set.
        ///
        /// The value may be any borrowed form of the set's value type, but
        /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
        /// the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set = HashSet::new();
        ///
        /// set.insert(2);
        /// assert_eq!(set.remove(&2), true);
        /// assert_eq!(set.remove(&2), false);
        /// ```
        ///
        /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
        /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
        where
            T: Borrow<Q>,
            Q: Hash + Eq,
        {
            self.map.remove(value).is_some()
        }

        /// Removes and returns the value in the set, if any, that is equal to the given one.
        ///
        /// The value may be any borrowed form of the set's value type, but
        /// [`Hash`] and [`Eq`] on the borrowed form *must* match those for
        /// the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.take(&2), Some(2));
        /// assert_eq!(set.take(&2), None);
        /// ```
        ///
        /// [`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
        /// [`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T>
        where
            T: Borrow<Q>,
            Q: Hash + Eq,
        {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.map.remove_entry(value) {
                Some((k, _)) => Some(k),
                None => None,
            }
        }
    }

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

            self.iter().all(|key| other.contains(key))
        }
    }

    impl<T, S> Eq for HashSet<T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
    }

    impl<T, S> fmt::Debug for HashSet<T, S>
    where
        T: Eq + Hash + fmt::Debug,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_set().entries(self.iter()).finish()
        }
    }

    impl<T, S> FromIterator<T> for HashSet<T, S>
    where
        T: Eq + Hash,
        S: BuildHasher + Default,
    {
        #[cfg_attr(feature = "inline-more", inline)]
        fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
            let mut set = Self::with_hasher(Default::default());
            set.extend(iter);
            set
        }
    }

    impl<T, S> Extend<T> for HashSet<T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        #[cfg_attr(feature = "inline-more", inline)]
        fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
            self.map.extend(iter.into_iter().map(|k| (k, ())));
        }

        #[inline]
        #[cfg(feature = "nightly")]
        fn extend_one(&mut self, k: T) {
            self.map.insert(k, ());
        }

        #[inline]
        #[cfg(feature = "nightly")]
        fn extend_reserve(&mut self, additional: usize) {
            Extend::<(T, ())>::extend_reserve(&mut self.map, additional);
        }
    }

    impl<'a, T, S> Extend<&'a T> for HashSet<T, S>
    where
        T: 'a + Eq + Hash + Copy,
        S: BuildHasher,
    {
        #[cfg_attr(feature = "inline-more", inline)]
        fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
            self.extend(iter.into_iter().cloned());
        }

        #[inline]
        #[cfg(feature = "nightly")]
        fn extend_one(&mut self, k: &'a T) {
            self.map.insert(*k, ());
        }

        #[inline]
        #[cfg(feature = "nightly")]
        fn extend_reserve(&mut self, additional: usize) {
            Extend::<(T, ())>::extend_reserve(&mut self.map, additional);
        }
    }

    impl<T, S> Default for HashSet<T, S>
    where
        S: Default,
    {
        /// Creates an empty `HashSet<T, S>` with the `Default` value for the hasher.
        #[cfg_attr(feature = "inline-more", inline)]
        fn default() -> Self {
            Self {
                map: HashMap::default(),
            }
        }
    }

    impl<T, S> BitOr<&HashSet<T, S>> for &HashSet<T, S>
    where
        T: Eq + Hash + Clone,
        S: BuildHasher + Default,
    {
        type Output = HashSet<T, S>;

        /// Returns the union of `self` and `rhs` as a new `HashSet<T, S>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
        ///
        /// let set = &a | &b;
        ///
        /// let mut i = 0;
        /// let expected = [1, 2, 3, 4, 5];
        /// for x in &set {
        ///     assert!(expected.contains(x));
        ///     i += 1;
        /// }
        /// assert_eq!(i, expected.len());
        /// ```
        fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
            self.union(rhs).cloned().collect()
        }
    }

    impl<T, S> BitAnd<&HashSet<T, S>> for &HashSet<T, S>
    where
        T: Eq + Hash + Clone,
        S: BuildHasher + Default,
    {
        type Output = HashSet<T, S>;

        /// Returns the intersection of `self` and `rhs` as a new `HashSet<T, S>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: HashSet<_> = vec![2, 3, 4].into_iter().collect();
        ///
        /// let set = &a & &b;
        ///
        /// let mut i = 0;
        /// let expected = [2, 3];
        /// for x in &set {
        ///     assert!(expected.contains(x));
        ///     i += 1;
        /// }
        /// assert_eq!(i, expected.len());
        /// ```
        fn bitand(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
            self.intersection(rhs).cloned().collect()
        }
    }

    impl<T, S> BitXor<&HashSet<T, S>> for &HashSet<T, S>
    where
        T: Eq + Hash + Clone,
        S: BuildHasher + Default,
    {
        type Output = HashSet<T, S>;

        /// Returns the symmetric difference of `self` and `rhs` as a new `HashSet<T, S>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
        ///
        /// let set = &a ^ &b;
        ///
        /// let mut i = 0;
        /// let expected = [1, 2, 4, 5];
        /// for x in &set {
        ///     assert!(expected.contains(x));
        ///     i += 1;
        /// }
        /// assert_eq!(i, expected.len());
        /// ```
        fn bitxor(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
            self.symmetric_difference(rhs).cloned().collect()
        }
    }

    impl<T, S> Sub<&HashSet<T, S>> for &HashSet<T, S>
    where
        T: Eq + Hash + Clone,
        S: BuildHasher + Default,
    {
        type Output = HashSet<T, S>;

        /// Returns the difference of `self` and `rhs` as a new `HashSet<T, S>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        ///
        /// let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();
        ///
        /// let set = &a - &b;
        ///
        /// let mut i = 0;
        /// let expected = [1, 2];
        /// for x in &set {
        ///     assert!(expected.contains(x));
        ///     i += 1;
        /// }
        /// assert_eq!(i, expected.len());
        /// ```
        fn sub(self, rhs: &HashSet<T, S>) -> HashSet<T, S> {
            self.difference(rhs).cloned().collect()
        }
    }

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

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

    /// A draining iterator over the items of a `HashSet`.
    ///
    /// This `struct` is created by the [`drain`] method on [`HashSet`].
    /// See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`drain`]: struct.HashSet.html#method.drain
    pub struct Drain<'a, K> {
        iter: map::Drain<'a, K, ()>,
    }

    /// A draining iterator over entries of a `HashSet` which don't satisfy the predicate `f`.
    ///
    /// This `struct` is created by the [`drain_filter`] method on [`HashSet`]. See its
    /// documentation for more.
    ///
    /// [`drain_filter`]: struct.HashSet.html#method.drain_filter
    /// [`HashSet`]: struct.HashSet.html
    pub struct DrainFilter<'a, K, F>
    where
        F: FnMut(&K) -> bool,
    {
        f: F,
        inner: DrainFilterInner<'a, K, ()>,
    }

    /// A lazy iterator producing elements in the intersection of `HashSet`s.
    ///
    /// This `struct` is created by the [`intersection`] method on [`HashSet`].
    /// See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`intersection`]: struct.HashSet.html#method.intersection
    pub struct Intersection<'a, T, S> {
        // iterator of the first set
        iter: Iter<'a, T>,
        // the second set
        other: &'a HashSet<T, S>,
    }

    /// A lazy iterator producing elements in the difference of `HashSet`s.
    ///
    /// This `struct` is created by the [`difference`] method on [`HashSet`].
    /// See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`difference`]: struct.HashSet.html#method.difference
    pub struct Difference<'a, T, S> {
        // iterator of the first set
        iter: Iter<'a, T>,
        // the second set
        other: &'a HashSet<T, S>,
    }

    /// A lazy iterator producing elements in the symmetric difference of `HashSet`s.
    ///
    /// This `struct` is created by the [`symmetric_difference`] method on
    /// [`HashSet`]. See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`symmetric_difference`]: struct.HashSet.html#method.symmetric_difference
    pub struct SymmetricDifference<'a, T, S> {
        iter: Chain<Difference<'a, T, S>, Difference<'a, T, S>>,
    }

    /// A lazy iterator producing elements in the union of `HashSet`s.
    ///
    /// This `struct` is created by the [`union`] method on [`HashSet`].
    /// See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`union`]: struct.HashSet.html#method.union
    pub struct Union<'a, T, S> {
        iter: Chain<Iter<'a, T>, Difference<'a, T, S>>,
    }

    impl<'a, T, S> IntoIterator for &'a HashSet<T, S> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        #[cfg_attr(feature = "inline-more", inline)]
        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    impl<T, S> IntoIterator for HashSet<T, S> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Creates a consuming iterator, that is, one that moves each value out
        /// of the set in arbitrary order. The set cannot be used after calling
        /// this.
        ///
        /// # Examples
        ///
        /// ```
        /// use hashbrown::HashSet;
        /// let mut set = HashSet::new();
        /// set.insert("a".to_string());
        /// set.insert("b".to_string());
        ///
        /// // Not possible to collect to a Vec<String> with a regular `.iter()`.
        /// let v: Vec<String> = set.into_iter().collect();
        ///
        /// // Will print in an arbitrary order.
        /// for x in &v {
        ///     println!("{}", x);
        /// }
        /// ```
        #[cfg_attr(feature = "inline-more", inline)]
        fn into_iter(self) -> IntoIter<T> {
            IntoIter {
                iter: self.map.into_iter(),
            }
        }
    }

    impl<K> Clone for Iter<'_, K> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Iter {
                iter: self.iter.clone(),
            }
        }
    }
    impl<'a, K> Iterator for Iter<'a, K> {
        type Item = &'a K;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<&'a K> {
            self.iter.next()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    impl<'a, K> ExactSizeIterator for Iter<'a, K> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    impl<K> FusedIterator for Iter<'_, K> {}

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

    impl<K> Iterator for IntoIter<K> {
        type Item = K;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<K> {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.iter.next() {
                Some((k, _)) => Some(k),
                None => None,
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    impl<K> ExactSizeIterator for IntoIter<K> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    impl<K> FusedIterator for IntoIter<K> {}

    impl<K: fmt::Debug> fmt::Debug for IntoIter<K> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let entries_iter = self.iter.iter().map(|(k, _)| k);
            f.debug_list().entries(entries_iter).finish()
        }
    }

    impl<K> Iterator for Drain<'_, K> {
        type Item = K;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<K> {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.iter.next() {
                Some((k, _)) => Some(k),
                None => None,
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    impl<K> ExactSizeIterator for Drain<'_, K> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    impl<K> FusedIterator for Drain<'_, K> {}

    impl<K: fmt::Debug> fmt::Debug for Drain<'_, K> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let entries_iter = self.iter.iter().map(|(k, _)| k);
            f.debug_list().entries(entries_iter).finish()
        }
    }

    impl<'a, K, F> Drop for DrainFilter<'a, K, F>
    where
        F: FnMut(&K) -> bool,
    {
        #[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);
            }
        }
    }

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

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

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

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

    impl<T, S> Clone for Intersection<'_, T, S> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Intersection {
                iter: self.iter.clone(),
                ..*self
            }
        }
    }

    impl<'a, T, S> Iterator for Intersection<'a, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        type Item = &'a T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<&'a T> {
            loop {
                let elt = self.iter.next()?;
                if self.other.contains(elt) {
                    return Some(elt);
                }
            }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper)
        }
    }

    impl<T, S> fmt::Debug for Intersection<'_, T, S>
    where
        T: fmt::Debug + Eq + Hash,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    impl<T, S> FusedIterator for Intersection<'_, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
    }

    impl<T, S> Clone for Difference<'_, T, S> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Difference {
                iter: self.iter.clone(),
                ..*self
            }
        }
    }

    impl<'a, T, S> Iterator for Difference<'a, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        type Item = &'a T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<&'a T> {
            loop {
                let elt = self.iter.next()?;
                if !self.other.contains(elt) {
                    return Some(elt);
                }
            }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper)
        }
    }

    impl<T, S> FusedIterator for Difference<'_, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
    }

    impl<T, S> fmt::Debug for Difference<'_, T, S>
    where
        T: fmt::Debug + Eq + Hash,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    impl<T, S> Clone for SymmetricDifference<'_, T, S> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            SymmetricDifference {
                iter: self.iter.clone(),
            }
        }
    }

    impl<'a, T, S> Iterator for SymmetricDifference<'a, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        type Item = &'a T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<&'a T> {
            self.iter.next()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<T, S> FusedIterator for SymmetricDifference<'_, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
    }

    impl<T, S> fmt::Debug for SymmetricDifference<'_, T, S>
    where
        T: fmt::Debug + Eq + Hash,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    impl<T, S> Clone for Union<'_, T, S> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Union {
                iter: self.iter.clone(),
            }
        }
    }

    impl<T, S> FusedIterator for Union<'_, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
    }

    impl<T, S> fmt::Debug for Union<'_, T, S>
    where
        T: fmt::Debug + Eq + Hash,
        S: BuildHasher,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

    impl<'a, T, S> Iterator for Union<'a, T, S>
    where
        T: Eq + Hash,
        S: BuildHasher,
    {
        type Item = &'a T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<&'a T> {
            self.iter.next()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[allow(dead_code)]
    fn assert_covariance() {
        fn set<'new>(v: HashSet<&'static str>) -> HashSet<&'new str> {
            v
        }
        fn iter<'a, 'new>(v: Iter<'a, &'static str>) -> Iter<'a, &'new str> {
            v
        }
        fn into_iter<'new>(v: IntoIter<&'static str>) -> IntoIter<&'new str> {
            v
        }
        fn difference<'a, 'new>(
            v: Difference<'a, &'static str, DefaultHashBuilder>,
        ) -> Difference<'a, &'new str, DefaultHashBuilder> {
            v
        }
        fn symmetric_difference<'a, 'new>(
            v: SymmetricDifference<'a, &'static str, DefaultHashBuilder>,
        ) -> SymmetricDifference<'a, &'new str, DefaultHashBuilder> {
            v
        }
        fn intersection<'a, 'new>(
            v: Intersection<'a, &'static str, DefaultHashBuilder>,
        ) -> Intersection<'a, &'new str, DefaultHashBuilder> {
            v
        }
        fn union<'a, 'new>(
            v: Union<'a, &'static str, DefaultHashBuilder>,
        ) -> Union<'a, &'new str, DefaultHashBuilder> {
            v
        }
        fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> {
            d
        }
    }

    #[cfg(test)]
    mod test_set {
        use super::super::map::DefaultHashBuilder;
        use super::HashSet;
        use std::vec::Vec;

        #[test]
        fn test_zero_capacities() {
            type HS = HashSet<i32>;

            let s = HS::new();
            assert_eq!(s.capacity(), 0);

            let s = HS::default();
            assert_eq!(s.capacity(), 0);

            let s = HS::with_hasher(DefaultHashBuilder::default());
            assert_eq!(s.capacity(), 0);

            let s = HS::with_capacity(0);
            assert_eq!(s.capacity(), 0);

            let s = HS::with_capacity_and_hasher(0, DefaultHashBuilder::default());
            assert_eq!(s.capacity(), 0);

            let mut s = HS::new();
            s.insert(1);
            s.insert(2);
            s.remove(&1);
            s.remove(&2);
            s.shrink_to_fit();
            assert_eq!(s.capacity(), 0);

            let mut s = HS::new();
            s.reserve(0);
            assert_eq!(s.capacity(), 0);
        }

        #[test]
        fn test_disjoint() {
            let mut xs = HashSet::new();
            let mut ys = HashSet::new();
            assert!(xs.is_disjoint(&ys));
            assert!(ys.is_disjoint(&xs));
            assert!(xs.insert(5));
            assert!(ys.insert(11));
            assert!(xs.is_disjoint(&ys));
            assert!(ys.is_disjoint(&xs));
            assert!(xs.insert(7));
            assert!(xs.insert(19));
            assert!(xs.insert(4));
            assert!(ys.insert(2));
            assert!(ys.insert(-11));
            assert!(xs.is_disjoint(&ys));
            assert!(ys.is_disjoint(&xs));
            assert!(ys.insert(7));
            assert!(!xs.is_disjoint(&ys));
            assert!(!ys.is_disjoint(&xs));
        }

        #[test]
        fn test_subset_and_superset() {
            let mut a = HashSet::new();
            assert!(a.insert(0));
            assert!(a.insert(5));
            assert!(a.insert(11));
            assert!(a.insert(7));

            let mut b = HashSet::new();
            assert!(b.insert(0));
            assert!(b.insert(7));
            assert!(b.insert(19));
            assert!(b.insert(250));
            assert!(b.insert(11));
            assert!(b.insert(200));

            assert!(!a.is_subset(&b));
            assert!(!a.is_superset(&b));
            assert!(!b.is_subset(&a));
            assert!(!b.is_superset(&a));

            assert!(b.insert(5));

            assert!(a.is_subset(&b));
            assert!(!a.is_superset(&b));
            assert!(!b.is_subset(&a));
            assert!(b.is_superset(&a));
        }

        #[test]
        fn test_iterate() {
            let mut a = HashSet::new();
            for i in 0..32 {
                assert!(a.insert(i));
            }
            let mut observed: u32 = 0;
            for k in &a {
                observed |= 1 << *k;
            }
            assert_eq!(observed, 0xFFFF_FFFF);
        }

        #[test]
        fn test_intersection() {
            let mut a = HashSet::new();
            let mut b = HashSet::new();

            assert!(a.insert(11));
            assert!(a.insert(1));
            assert!(a.insert(3));
            assert!(a.insert(77));
            assert!(a.insert(103));
            assert!(a.insert(5));
            assert!(a.insert(-5));

            assert!(b.insert(2));
            assert!(b.insert(11));
            assert!(b.insert(77));
            assert!(b.insert(-9));
            assert!(b.insert(-42));
            assert!(b.insert(5));
            assert!(b.insert(3));

            let mut i = 0;
            let expected = [3, 5, 11, 77];
            for x in a.intersection(&b) {
                assert!(expected.contains(x));
                i += 1
            }
            assert_eq!(i, expected.len());
        }

        #[test]
        fn test_difference() {
            let mut a = HashSet::new();
            let mut b = HashSet::new();

            assert!(a.insert(1));
            assert!(a.insert(3));
            assert!(a.insert(5));
            assert!(a.insert(9));
            assert!(a.insert(11));

            assert!(b.insert(3));
            assert!(b.insert(9));

            let mut i = 0;
            let expected = [1, 5, 11];
            for x in a.difference(&b) {
                assert!(expected.contains(x));
                i += 1
            }
            assert_eq!(i, expected.len());
        }

        #[test]
        fn test_symmetric_difference() {
            let mut a = HashSet::new();
            let mut b = HashSet::new();

            assert!(a.insert(1));
            assert!(a.insert(3));
            assert!(a.insert(5));
            assert!(a.insert(9));
            assert!(a.insert(11));

            assert!(b.insert(-2));
            assert!(b.insert(3));
            assert!(b.insert(9));
            assert!(b.insert(14));
            assert!(b.insert(22));

            let mut i = 0;
            let expected = [-2, 1, 5, 11, 14, 22];
            for x in a.symmetric_difference(&b) {
                assert!(expected.contains(x));
                i += 1
            }
            assert_eq!(i, expected.len());
        }

        #[test]
        fn test_union() {
            let mut a = HashSet::new();
            let mut b = HashSet::new();

            assert!(a.insert(1));
            assert!(a.insert(3));
            assert!(a.insert(5));
            assert!(a.insert(9));
            assert!(a.insert(11));
            assert!(a.insert(16));
            assert!(a.insert(19));
            assert!(a.insert(24));

            assert!(b.insert(-2));
            assert!(b.insert(1));
            assert!(b.insert(5));
            assert!(b.insert(9));
            assert!(b.insert(13));
            assert!(b.insert(19));

            let mut i = 0;
            let expected = [-2, 1, 3, 5, 9, 11, 13, 16, 19, 24];
            for x in a.union(&b) {
                assert!(expected.contains(x));
                i += 1
            }
            assert_eq!(i, expected.len());
        }

        #[test]
        fn test_from_iter() {
            let xs = [1, 2, 2, 3, 4, 5, 6, 7, 8, 9];

            let set: HashSet<_> = xs.iter().cloned().collect();

            for x in &xs {
                assert!(set.contains(x));
            }

            assert_eq!(set.iter().len(), xs.len() - 1);
        }

        #[test]
        fn test_move_iter() {
            let hs = {
                let mut hs = HashSet::new();

                hs.insert('a');
                hs.insert('b');

                hs
            };

            let v = hs.into_iter().collect::<Vec<char>>();
            assert!(v == ['a', 'b'] || v == ['b', 'a']);
        }

        #[test]
        fn test_eq() {
            // These constants once happened to expose a bug in insert().
            // I'm keeping them around to prevent a regression.
            let mut s1 = HashSet::new();

            s1.insert(1);
            s1.insert(2);
            s1.insert(3);

            let mut s2 = HashSet::new();

            s2.insert(1);
            s2.insert(2);

            assert!(s1 != s2);

            s2.insert(3);

            assert_eq!(s1, s2);
        }

        #[test]
        fn test_show() {
            let mut set = HashSet::new();
            let empty = HashSet::<i32>::new();

            set.insert(1);
            set.insert(2);

            let set_str = format!("{:?}", set);

            assert!(set_str == "{1, 2}" || set_str == "{2, 1}");
            assert_eq!(format!("{:?}", empty), "{}");
        }

        #[test]
        fn test_trivial_drain() {
            let mut s = HashSet::<i32>::new();
            for _ in s.drain() {}
            assert!(s.is_empty());
            drop(s);

            let mut s = HashSet::<i32>::new();
            drop(s.drain());
            assert!(s.is_empty());
        }

        #[test]
        fn test_drain() {
            let mut s: HashSet<_> = (1..100).collect();

            // try this a bunch of times to make sure we don't screw up internal state.
            for _ in 0..20 {
                assert_eq!(s.len(), 99);

                {
                    let mut last_i = 0;
                    let mut d = s.drain();
                    for (i, x) in d.by_ref().take(50).enumerate() {
                        last_i = i;
                        assert!(x != 0);
                    }
                    assert_eq!(last_i, 49);
                }

                for _ in &s {
                    panic!("s should be empty!");
                }

                // reset to try again.
                s.extend(1..100);
            }
        }

        #[test]
        fn test_replace() {
            use core::hash;

            #[derive(Debug)]
            struct Foo(&'static str, i32);

            impl PartialEq for Foo {
                fn eq(&self, other: &Self) -> bool {
                    self.0 == other.0
                }
            }

            impl Eq for Foo {}

            impl hash::Hash for Foo {
                fn hash<H: hash::Hasher>(&self, h: &mut H) {
                    self.0.hash(h);
                }
            }

            let mut s = HashSet::new();
            assert_eq!(s.replace(Foo("a", 1)), None);
            assert_eq!(s.len(), 1);
            assert_eq!(s.replace(Foo("a", 2)), Some(Foo("a", 1)));
            assert_eq!(s.len(), 1);

            let mut it = s.iter();
            assert_eq!(it.next(), Some(&Foo("a", 2)));
            assert_eq!(it.next(), None);
        }

        #[test]
        fn test_extend_ref() {
            let mut a = HashSet::new();
            a.insert(1);

            a.extend(&[2, 3, 4]);

            assert_eq!(a.len(), 4);
            assert!(a.contains(&1));
            assert!(a.contains(&2));
            assert!(a.contains(&3));
            assert!(a.contains(&4));

            let mut b = HashSet::new();
            b.insert(5);
            b.insert(6);

            a.extend(&b);

            assert_eq!(a.len(), 6);
            assert!(a.contains(&1));
            assert!(a.contains(&2));
            assert!(a.contains(&3));
            assert!(a.contains(&4));
            assert!(a.contains(&5));
            assert!(a.contains(&6));
        }

        #[test]
        fn test_retain() {
            let xs = [1, 2, 3, 4, 5, 6];
            let mut set: HashSet<i32> = xs.iter().cloned().collect();
            set.retain(|&k| k % 2 == 0);
            assert_eq!(set.len(), 3);
            assert!(set.contains(&2));
            assert!(set.contains(&4));
            assert!(set.contains(&6));
        }

        #[test]
        fn test_drain_filter() {
            {
                let mut set: HashSet<i32> = (0..8).collect();
                let drained = set.drain_filter(|&k| k % 2 == 0);
                let mut out = drained.collect::<Vec<_>>();
                out.sort_unstable();
                assert_eq!(vec![0, 2, 4, 6], out);
                assert_eq!(set.len(), 4);
            }
            {
                let mut set: HashSet<i32> = (0..8).collect();
                drop(set.drain_filter(|&k| k % 2 == 0));
                assert_eq!(set.len(), 4, "Removes non-matching items on drop");
            }
        }

        #[test]
        fn test_const_with_hasher() {
            use core::hash::BuildHasher;
            use std::collections::hash_map::DefaultHasher;

            #[derive(Clone)]
            struct MyHasher;
            impl BuildHasher for MyHasher {
                type Hasher = DefaultHasher;

                fn build_hasher(&self) -> DefaultHasher {
                    DefaultHasher::new()
                }
            }

            const EMPTY_SET: HashSet<u32, MyHasher> = HashSet::with_hasher(MyHasher);

            let mut set = EMPTY_SET.clone();
            set.insert(19);
            assert!(set.contains(&19));
        }
    }