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

    use borrow::Borrow;
    use fmt;
    use hash::{Hash, BuildHasher};
    use iter::{Chain, FromIterator, FusedIterator};
    use ops::{BitOr, BitAnd, BitXor, Sub};

    use super::Recover;
    use super::map::{self, HashMap, Keys, RandomState};

    // 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// // Type inference lets us omit an explicit type signature (which
    /// // would be `HashSet<&str>` in this example).
    /// let mut books = HashSet::new();
    ///
    /// // Add some books.
    /// books.insert("A Dance With Dragons");
    /// books.insert("To Kill a Mockingbird");
    /// books.insert("The Odyssey");
    /// books.insert("The Great Gatsby");
    ///
    /// // 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 std::collections::HashSet;
    /// #[derive(Hash, Eq, PartialEq, Debug)]
    /// struct Viking<'a> {
    ///     name: &'a str,
    ///     power: usize,
    /// }
    ///
    /// let mut vikings = HashSet::new();
    ///
    /// vikings.insert(Viking { name: "Einar", power: 9 });
    /// vikings.insert(Viking { name: "Einar", power: 9 });
    /// vikings.insert(Viking { name: "Olaf", power: 4 });
    /// vikings.insert(Viking { name: "Harald", 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 std::collections::HashSet;
    ///
    /// fn main() {
    ///     let viking_names: HashSet<&str> =
    ///         [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
    ///     // use the values stored in the set
    /// }
    /// ```
    ///
    /// [`Cell`]: ../../std/cell/struct.Cell.html
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    /// [`HashMap`]: struct.HashMap.html
    /// [`PartialEq`]: ../../std/cmp/trait.PartialEq.html
    /// [`RefCell`]: ../../std/cell/struct.RefCell.html
    #[derive(Clone)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct HashSet<T, S = RandomState> {
        map: HashMap<T, (), S>,
    }

    impl<T: Hash + Eq> HashSet<T, RandomState> {
        /// 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 std::collections::HashSet;
        /// let set: HashSet<i32> = HashSet::new();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> HashSet<T, RandomState> {
            HashSet { 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 std::collections::HashSet;
        /// let set: HashSet<i32> = HashSet::with_capacity(10);
        /// assert!(set.capacity() >= 10);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize) -> HashSet<T, RandomState> {
            HashSet { map: HashMap::with_capacity(capacity) }
        }
    }

    impl<T, S> HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        /// 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.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        /// use std::collections::hash_map::RandomState;
        ///
        /// let s = RandomState::new();
        /// let mut set = HashSet::with_hasher(s);
        /// set.insert(2);
        /// ```
        #[inline]
        #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
        pub fn with_hasher(hasher: S) -> HashSet<T, S> {
            HashSet { map: HashMap::with_hasher(hasher) }
        }

        /// Creates an empty `HashSet` with 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.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        /// use std::collections::hash_map::RandomState;
        ///
        /// let s = RandomState::new();
        /// let mut set = HashSet::with_capacity_and_hasher(10, s);
        /// set.insert(1);
        /// ```
        #[inline]
        #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
        pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S> {
            HashSet { map: HashMap::with_capacity_and_hasher(capacity, hasher) }
        }

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

        /// Returns the number of elements the set can hold without reallocating.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        /// let set: HashSet<i32> = HashSet::with_capacity(100);
        /// assert!(set.capacity() >= 100);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn capacity(&self) -> usize {
            self.map.capacity()
        }

        /// 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 std::collections::HashSet;
        /// let mut set: HashSet<i32> = HashSet::new();
        /// set.reserve(10);
        /// assert!(set.capacity() >= 10);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve(&mut self, additional: usize) {
            self.map.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 std::collections::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);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn shrink_to_fit(&mut self) {
            self.map.shrink_to_fit()
        }

        /// An iterator visiting all elements in arbitrary order.
        /// The iterator element type is `&'a T`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::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);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { iter: self.map.keys() }
        }

        /// Visits the values representing the difference,
        /// i.e. the values that are in `self` but not in `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn difference<'a>(&'a self, other: &'a HashSet<T, S>) -> 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 std::collections::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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn symmetric_difference<'a>(&'a self,
                                        other: &'a HashSet<T, S>)
                                        -> 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 std::collections::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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S> {
            Intersection {
                iter: self.iter(),
                other,
            }
        }

        /// Visits the values representing the union,
        /// i.e. all the values in `self` or `other`, without duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S> {
            Union { iter: self.iter().chain(other.difference(self)) }
        }

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

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

        /// Clears the set, returning all elements in an iterator.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::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());
        /// ```
        #[inline]
        #[stable(feature = "drain", since = "1.6.0")]
        pub fn drain(&mut self) -> Drain<T> {
            Drain { iter: self.map.drain() }
        }

        /// Clears the set, removing all values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        ///
        /// let mut v = HashSet::new();
        /// v.insert(1);
        /// v.clear();
        /// assert!(v.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            self.map.clear()
        }

        /// 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 std::collections::HashSet;
        ///
        /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.contains(&1), true);
        /// assert_eq!(set.contains(&4), false);
        /// ```
        ///
        /// [`Eq`]: ../../std/cmp/trait.Eq.html
        /// [`Hash`]: ../../std/hash/trait.Hash.html
        #[stable(feature = "rust1", since = "1.0.0")]
        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.
        ///
        /// [`Eq`]: ../../std/cmp/trait.Eq.html
        /// [`Hash`]: ../../std/hash/trait.Hash.html
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T>
            where T: Borrow<Q>,
                  Q: Hash + Eq
        {
            Recover::get(&self.map, value)
        }

        /// Returns `true` if `self` has no elements in common with `other`.
        /// This is equivalent to checking for an empty intersection.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::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);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_disjoint(&self, other: &HashSet<T, S>) -> 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 std::collections::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);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_subset(&self, other: &HashSet<T, S>) -> bool {
            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 std::collections::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);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_superset(&self, other: &HashSet<T, S>) -> 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 std::collections::HashSet;
        ///
        /// let mut set = HashSet::new();
        ///
        /// assert_eq!(set.insert(2), true);
        /// assert_eq!(set.insert(2), false);
        /// assert_eq!(set.len(), 1);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        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.
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn replace(&mut self, value: T) -> Option<T> {
            Recover::replace(&mut self.map, value)
        }

        /// Removes a value from the set. Returns `true` if 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 std::collections::HashSet;
        ///
        /// let mut set = HashSet::new();
        ///
        /// set.insert(2);
        /// assert_eq!(set.remove(&2), true);
        /// assert_eq!(set.remove(&2), false);
        /// ```
        ///
        /// [`Eq`]: ../../std/cmp/trait.Eq.html
        /// [`Hash`]: ../../std/hash/trait.Hash.html
        #[stable(feature = "rust1", since = "1.0.0")]
        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.
        ///
        /// [`Eq`]: ../../std/cmp/trait.Eq.html
        /// [`Hash`]: ../../std/hash/trait.Hash.html
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T>
            where T: Borrow<Q>,
                  Q: Hash + Eq
        {
            Recover::take(&mut self.map, value)
        }

        /// Retains only the elements specified by the predicate.
        ///
        /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        ///
        /// let xs = [1,2,3,4,5,6];
        /// let mut set: HashSet<isize> = xs.iter().cloned().collect();
        /// set.retain(|&k| k % 2 == 0);
        /// assert_eq!(set.len(), 3);
        /// ```
        #[stable(feature = "retain_hash_collection", since = "1.18.0")]
        pub fn retain<F>(&mut self, mut f: F)
            where F: FnMut(&T) -> bool
        {
            self.map.retain(|k, _| f(k));
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, S> PartialEq for HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        fn eq(&self, other: &HashSet<T, S>) -> bool {
            if self.len() != other.len() {
                return false;
            }

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

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

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, S> Extend<T> for HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
            self.map.extend(iter.into_iter().map(|k| (k, ())));
        }
    }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, S> Default for HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher + Default
    {
        /// Creates an empty `HashSet<T, S>` with the `Default` value for the hasher.
        fn default() -> HashSet<T, S> {
            HashSet { map: HashMap::default() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T, S> BitOr<&'b HashSet<T, S>> for &'a 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 std::collections::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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T, S> BitAnd<&'b HashSet<T, S>> for &'a 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 std::collections::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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T, S> BitXor<&'b HashSet<T, S>> for &'a 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 std::collections::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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T, S> Sub<&'b HashSet<T, S>> for &'a 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 std::collections::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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, K: 'a> {
        iter: Keys<'a, K, ()>,
    }

    /// An owning iterator over the items of a `HashSet`.
    ///
    /// This `struct` is created by the [`into_iter`] method on [`HashSet`][`HashSet`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`HashSet`]: struct.HashSet.html
    /// [`into_iter`]: struct.HashSet.html#method.into_iter
    #[stable(feature = "rust1", since = "1.0.0")]
    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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Drain<'a, K: 'a> {
        iter: map::Drain<'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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Intersection<'a, T: 'a, S: 'a> {
        // 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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Difference<'a, T: 'a, S: 'a> {
        // 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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct SymmetricDifference<'a, T: 'a, S: 'a> {
        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
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Union<'a, T: 'a, S: 'a> {
        iter: Chain<Iter<'a, T>, Difference<'a, T, S>>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> IntoIterator for &'a HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, S> IntoIterator for HashSet<T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        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 std::collections::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);
        /// }
        /// ```
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { iter: self.map.into_iter() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K> Clone for Iter<'a, K> {
        fn clone(&self) -> Iter<'a, K> {
            Iter { iter: self.iter.clone() }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K> Iterator for Iter<'a, K> {
        type Item = &'a K;

        fn next(&mut self) -> Option<&'a K> {
            self.iter.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K> ExactSizeIterator for Iter<'a, K> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K> FusedIterator for Iter<'a, K> {}

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, K: fmt::Debug> fmt::Debug for Iter<'a, K> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_list().entries(self.clone()).finish()
        }
    }

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

        fn next(&mut self) -> Option<K> {
            self.iter.next().map(|(k, _)| k)
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<K> ExactSizeIterator for IntoIter<K> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    #[unstable(feature = "fused", issue = "35602")]
    impl<K> FusedIterator for IntoIter<K> {}

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<K: fmt::Debug> fmt::Debug for IntoIter<K> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let entries_iter = self.iter
                .inner
                .iter()
                .map(|(k, _)| k);
            f.debug_list().entries(entries_iter).finish()
        }
    }

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

        fn next(&mut self) -> Option<K> {
            self.iter.next().map(|(k, _)| k)
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, K> ExactSizeIterator for Drain<'a, K> {
        fn len(&self) -> usize {
            self.iter.len()
        }
    }
    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, K> FusedIterator for Drain<'a, K> {}

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, K: fmt::Debug> fmt::Debug for Drain<'a, K> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let entries_iter = self.iter
                .inner
                .iter()
                .map(|(k, _)| k);
            f.debug_list().entries(entries_iter).finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Clone for Intersection<'a, T, S> {
        fn clone(&self) -> Intersection<'a, T, S> {
            Intersection { iter: self.iter.clone(), ..*self }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Iterator for Intersection<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            loop {
                match self.iter.next() {
                    None => return None,
                    Some(elt) => {
                        if self.other.contains(elt) {
                            return Some(elt);
                        }
                    }
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper)
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T, S> fmt::Debug for Intersection<'a, 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()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T, S> FusedIterator for Intersection<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Clone for Difference<'a, T, S> {
        fn clone(&self) -> Difference<'a, T, S> {
            Difference { iter: self.iter.clone(), ..*self }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Iterator for Difference<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            loop {
                match self.iter.next() {
                    None => return None,
                    Some(elt) => {
                        if !self.other.contains(elt) {
                            return Some(elt);
                        }
                    }
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper)
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T, S> FusedIterator for Difference<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T, S> fmt::Debug for Difference<'a, 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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Clone for SymmetricDifference<'a, T, S> {
        fn clone(&self) -> SymmetricDifference<'a, T, S> {
            SymmetricDifference { iter: self.iter.clone() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Iterator for SymmetricDifference<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            self.iter.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T, S> FusedIterator for SymmetricDifference<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T, S> fmt::Debug for SymmetricDifference<'a, 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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Clone for Union<'a, T, S> {
        fn clone(&self) -> Union<'a, T, S> {
            Union { iter: self.iter.clone() }
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T, S> FusedIterator for Union<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<'a, T, S> fmt::Debug for Union<'a, 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()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T, S> Iterator for Union<'a, T, S>
        where T: Eq + Hash,
              S: BuildHasher
    {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            self.iter.next()
        }
        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, RandomState>)
                                -> Difference<'a, &'new str, RandomState> {
            v
        }
        fn symmetric_difference<'a, 'new>(v: SymmetricDifference<'a, &'static str, RandomState>)
                                          -> SymmetricDifference<'a, &'new str, RandomState> {
            v
        }
        fn intersection<'a, 'new>(v: Intersection<'a, &'static str, RandomState>)
                                  -> Intersection<'a, &'new str, RandomState> {
            v
        }
        fn union<'a, 'new>(v: Union<'a, &'static str, RandomState>)
                           -> Union<'a, &'new str, RandomState> {
            v
        }
        fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> {
            d
        }
    }

    #[cfg(test)]
    mod test_set {
        use super::HashSet;
        use super::super::map::RandomState;

        #[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(RandomState::new());
            assert_eq!(s.capacity(), 0);

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

            let s = HS::with_capacity_and_hasher(0, RandomState::new());
            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, 3, 4, 5, 6, 7, 8, 9];

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

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

        #[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 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<isize> = 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));
        }
    }

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