#[cfg(test)]
mod tests;

use hashbrown::hash_set as base;

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

use super::map::{map_try_reserve_error, RandomState};

// Future 优化 (FIXME!)
// ============================
//
// 对零大小的值进行迭代是是一个空操作。
// HashSet 不需要 `bucket.val`。
// 我想我们需要 HKT 适当地消除它。

/// [hash set]，实现为 `HashMap`，其中值为 `()`。
///
/// 与 [`HashMap`] 类型一样，`HashSet` 要求元素实现 [`Eq`] 和 [`Hash`] traits。这通常可以通过使用 `#[derive(PartialEq, Eq, Hash)]` 来实现。
/// 如果您自己实现这些，那么拥有以下属性非常重要:
///
/// ```text
/// k1 == k2 -> hash(k1) == hash(k2)
/// ```
///
/// 换句话说，如果两个键相等，则它们的哈希值必须相等。
///
/// 以某种方式修改项目的逻辑错误是由该项目的哈希值 (由 [`Hash`] trait 确定) 或其相等性 (由 [`Eq`] trait 确定) 在其位于集合中时发生变化的。
/// 通常只有通过 [`Cell`]，[`RefCell`]，二进制状态，I/O 或不安全代码才能实现此操作。
/// 未指定由此类逻辑错误导致的行为，但不会导致未定义的行为。
/// 这可能包括 panics，不正确的结果，异常终止，内存泄漏和未终止。
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
/// // 通过类型推断，我们可以省略显式类型签名 (在本示例中为 `HashSet<String>`)。
/////
/// let mut books = HashSet::new();
///
/// // 添加一些书。
/// 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());
///
/// // 检查一个特定的。
/// if !books.contains("The Winds of Winter") {
///     println!("We have {} books, but The Winds of Winter ain't one.",
///              books.len());
/// }
///
/// // 删除一本书。
/// books.remove("The Odyssey");
///
/// // 遍历所有内容。
/// for book in &books {
///     println!("{}", book);
/// }
/// ```
///
/// 将 `HashSet` 与自定义类型一起使用的最简单方法是派生 [`Eq`] 和 [`Hash`]。我们还必须导出 [`PartialEq`]，这将在 [`Eq`] 中隐含在 future 中。
///
/// ```
/// use std::collections::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 });
///
/// // 使用派生的实现来打印 Viking。
/// for x in &vikings {
///     println!("{:?}", x);
/// }
/// ```
///
/// 具有固定元素列表的 `HashSet` 可以从数组中初始化:
///
/// ```
/// use std::collections::HashSet;
///
/// let viking_names: HashSet<&'static str> =
///     [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
/// // 使用存储在集合中的值
/// ```
///
/// [hash set]: crate::collections#use-the-set-variant-of-any-of-these-maps-when
/// [`HashMap`]: crate::collections::HashMap
/// [`RefCell`]: crate::cell::RefCell
/// [`Cell`]: crate::cell::Cell
///
///
///
///
///
///
///
///
#[cfg_attr(not(test), rustc_diagnostic_item = "hashset_type")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct HashSet<T, S = RandomState> {
    base: base::HashSet<T, S>,
}

impl<T> HashSet<T, RandomState> {
    /// 创建一个空的 `HashSet`。
    ///
    /// 哈希集最初创建时的容量为 0，因此只有在首次插入时才分配。
    ///
    ///
    /// # 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> {
        Default::default()
    }

    /// 创建具有指定容量的空 `HashSet`。
    ///
    /// 哈希集将能够至少保留 `capacity` 个元素而无需重新分配。
    /// 如果 `capacity` 为 0，则不会分配哈希集。
    ///
    /// # 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 { base: base::HashSet::with_capacity_and_hasher(capacity, Default::default()) }
    }
}

impl<T, S> HashSet<T, S> {
    /// 返回集合可以容纳而不重新分配的元素数。
    ///
    /// # 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.base.capacity()
    }

    /// 一个迭代器，以任意顺序访问所有元素。
    /// 迭代器元素类型为 `&'a T`。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a");
    /// set.insert("b");
    ///
    /// // 将以任意顺序打印。
    /// for x in set.iter() {
    ///     println!("{}", x);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { base: self.base.iter() }
    }

    /// 返回集合中的元素数。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert_eq!(v.len(), 0);
    /// v.insert(1);
    /// assert_eq!(v.len(), 1);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize {
        self.base.len()
    }

    /// 如果集合不包含任何元素，则返回 `true`。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// assert!(v.is_empty());
    /// v.insert(1);
    /// assert!(!v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.base.is_empty()
    }

    /// 清除集合，返回迭代器中的所有元素。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert!(!set.is_empty());
    ///
    /// // 以任意顺序打印 1、2、3
    /// 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 { base: self.base.drain() }
    }

    /// 创建一个迭代器，该迭代器使用闭包确定是否应删除值。
    ///
    /// 如果闭包返回 true，则该值将被删除并产生。
    /// 如果闭包返回 false，则该值将保留在列表中，并且不会由迭代器产生。
    ///
    /// 如果迭代器仅被部分消耗或根本没有消耗，则其余所有值仍将受到闭包的处理，如果返回 true，则将其删除并丢弃。
    ///
    /// 如果在闭包中出现 panic，或者在丢弃值时出现 panic，或者 `DrainFilter` 本身被泄漏，那么还有多少值将被关闭，这是未指定的。
    ///
    ///
    /// # Examples
    ///
    /// 将一个集合分为偶数和奇数值，重新使用原始集合:
    ///
    /// ```
    /// #![feature(hash_drain_filter)]
    /// use std::collections::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]);
    /// ```
    ///
    ///
    ///
    #[inline]
    #[unstable(feature = "hash_drain_filter", issue = "59618")]
    pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        DrainFilter { base: self.base.drain_filter(pred) }
    }

    /// 清除设置，删除所有值。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut v = HashSet::new();
    /// v.insert(1);
    /// v.clear();
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        self.base.clear()
    }

    /// 创建一个新的空哈希集，它将使用给定的哈希值来哈希键。
    ///
    /// 还使用默认的初始容量创建哈希集。
    ///
    /// 警告: `hasher` 通常是随机生成的，旨在允许 HashSet 抵抗导致许多冲突和非常差的性能的攻击。
    /// 使用此函数手动设置它可能会导致 DoS 攻击 vector。
    ///
    /// 传递的 `hash_builder` 应该为 X0HashMap0Z 实现 [`BuildHasher`] trait 才有用，有关详细信息，请参见其文档。
    ///
    /// # 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 { base: base::HashSet::with_hasher(hasher) }
    }

    /// 创建一个具有指定容量的空 `HashSet`，使用 `hasher` 对键进行散列。
    ///
    /// 哈希集将能够至少保留 `capacity` 个元素而无需重新分配。如果 `capacity` 为 0，则不会分配哈希集。
    ///
    /// 警告: `hasher` 通常是随机生成的，旨在允许 HashSet 抵抗导致许多冲突和非常差的性能的攻击。
    /// 使用此函数手动设置它可能会导致 DoS 攻击 vector。
    ///
    /// 传递的 `hash_builder` 应该为 X0HashMap0Z 实现 [`BuildHasher`] trait 才有用，有关详细信息，请参见其文档。
    ///
    /// # 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 { base: base::HashSet::with_capacity_and_hasher(capacity, hasher) }
    }

    /// 返回集合的 [`BuildHasher`] 的引用。
    ///
    /// # 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();
    /// ```
    #[inline]
    #[stable(feature = "hashmap_public_hasher", since = "1.9.0")]
    pub fn hasher(&self) -> &S {
        self.base.hasher()
    }
}

impl<T, S> HashSet<T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
    /// 保留至少 `additional` 个要插入 `HashSet` 中的更多元素的容量。
    /// 该集合可以保留更多空间，以避免频繁的重新分配。
    ///
    /// # Panics
    ///
    /// 如果新的分配大小溢出 `usize`，则为 Panics。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set: HashSet<i32> = HashSet::new();
    /// set.reserve(10);
    /// assert!(set.capacity() >= 10);
    /// ```
    ///
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        self.base.reserve(additional)
    }

    /// 尝试为给 `HashSet<K, V>` 至少插入 `additional` 个元素保留容量。
    /// 该集合可以保留更多空间，以避免频繁的重新分配。
    ///
    /// # Errors
    ///
    /// 如果容量溢出，或者分配器报告失败，则返回错误。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(try_reserve)]
    /// use std::collections::HashSet;
    /// let mut set: HashSet<i32> = HashSet::new();
    /// set.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");
    /// ```
    ///
    ///
    #[inline]
    #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.base.try_reserve(additional).map_err(map_try_reserve_error)
    }

    /// 尽可能缩小设备的容量。
    /// 它会在保持内部规则的同时尽可能地丢弃，并可能根据调整大小策略留出一些空间。
    ///
    ///
    /// # 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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn shrink_to_fit(&mut self) {
        self.base.shrink_to_fit()
    }

    /// 将集合的容量降低一个下限。
    /// 它将降低不低于提供的限制，同时保持内部规则，并可能根据调整大小策略留下一些空间。
    ///
    /// 如果当前容量小于下限，则为无操作。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(shrink_to)]
    /// use std::collections::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);
    /// ```
    #[inline]
    #[unstable(feature = "shrink_to", reason = "new API", issue = "56431")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.base.shrink_to(min_capacity)
    }

    /// 访问表示差异的值，即，在 `self` 中但不在 `other` 中的值。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // 可以看作是 `a - b`。
    /// for x in a.difference(&b) {
    ///     println!("{}", x); // 打印 1
    /// }
    ///
    /// let diff: HashSet<_> = a.difference(&b).collect();
    /// assert_eq!(diff, [1].iter().collect());
    ///
    /// // 请注意，差异不是对称的，并且 `b - a` 表示其他含义:
    /////
    /// let diff: HashSet<_> = b.difference(&a).collect();
    /// assert_eq!(diff, [4].iter().collect());
    /// ```
    #[inline]
    #[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 }
    }

    /// 访问代表对称差异的值，即 `self` 或 `other` 中的值，但不能同时存在于两者中。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // 以任意顺序打印 1、4。
    /// 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());
    /// ```
    #[inline]
    #[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)) }
    }

    /// 访问表示相交的值，即 `self` 和 `other` 中的值。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // 以任意顺序打印 2，3。
    /// for x in a.intersection(&b) {
    ///     println!("{}", x);
    /// }
    ///
    /// let intersection: HashSet<_> = a.intersection(&b).collect();
    /// assert_eq!(intersection, [2, 3].iter().collect());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S> {
        if self.len() <= other.len() {
            Intersection { iter: self.iter(), other }
        } else {
            Intersection { iter: other.iter(), other: self }
        }
    }

    /// 访问表示并集的值，即 `self` 或 `other` 中的所有值，没有重复项。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
    ///
    /// // 以任意顺序打印 1、2、3、4。
    /// for x in a.union(&b) {
    ///     println!("{}", x);
    /// }
    ///
    /// let union: HashSet<_> = a.union(&b).collect();
    /// assert_eq!(union, [1, 2, 3, 4].iter().collect());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S> {
        if self.len() >= other.len() {
            Union { iter: self.iter().chain(other.difference(self)) }
        } else {
            Union { iter: other.iter().chain(self.difference(other)) }
        }
    }

    /// 如果集合包含值，则返回 `true`。
    ///
    /// 该值可以是集合值类型的任何借用形式，但是借用形式上的 [`Hash`] 和 [`Eq`] 必须与值类型的那些匹配。
    ///
    ///
    /// # 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);
    /// ```
    ///
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
    where
        T: Borrow<Q>,
        Q: Hash + Eq,
    {
        self.base.contains(value)
    }

    /// 返回集合中与给定值相等的值的引用。
    ///
    /// 该值可以是集合值类型的任何借用形式，但是借用形式上的 [`Hash`] 和 [`Eq`] 必须与值类型的那些匹配。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert_eq!(set.get(&2), Some(&2));
    /// assert_eq!(set.get(&4), None);
    /// ```
    ///
    #[inline]
    #[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,
    {
        self.base.get(value)
    }

    /// 如果给定的 `value` 不存在，则将其插入集合中，然后对集合中的值返回引用。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::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); // 插入了 100 个
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    pub fn get_or_insert(&mut self, value: T) -> &T {
        // 尽管原始条目为我们提供了 `&mut T`，但我们仅返回 `&T` 以与 `get` 保持一致。
        // 键可变的为 "raw"，因为您不应该影响 `Eq` 或 `Hash`。
        self.base.get_or_insert(value)
    }

    /// 如果不存在给定的 `value`，则将其拥有的副本插入到集合中，然后对集合中的值返回引用。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::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); // 插入了新的 "fish"
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    pub fn get_or_insert_owned<Q: ?Sized>(&mut self, value: &Q) -> &T
    where
        T: Borrow<Q>,
        Q: Hash + Eq + ToOwned<Owned = T>,
    {
        // 尽管原始条目为我们提供了 `&mut T`，但我们仅返回 `&T` 以与 `get` 保持一致。
        // 键可变的为 "raw"，因为您不应该影响 `Eq` 或 `Hash`。
        self.base.get_or_insert_owned(value)
    }

    /// 如果不存在给定的 `value`，则将从 `f` 计算得出的值插入到集合中，然后对集合中的值返回引用。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(hash_set_entry)]
    ///
    /// use std::collections::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); // 插入了新的 "fish"
    /// ```
    #[inline]
    #[unstable(feature = "hash_set_entry", issue = "60896")]
    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,
    {
        // 尽管原始条目为我们提供了 `&mut T`，但我们仅返回 `&T` 以与 `get` 保持一致。
        // 键可变的为 "raw"，因为您不应该影响 `Eq` 或 `Hash`。
        self.base.get_or_insert_with(value, f)
    }

    /// 如果 `self` 与 `other` 没有共同的元素，则返回 `true`。
    /// 这等效于检查空的交点。
    ///
    /// # 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 {
        if self.len() <= other.len() {
            self.iter().all(|v| !other.contains(v))
        } else {
            other.iter().all(|v| !self.contains(v))
        }
    }

    /// 如果集合是另一个集合的子集，则返回 `true`，即 `other` 至少包含 `self` 中的所有值。
    ///
    ///
    /// # 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 {
        if self.len() <= other.len() { self.iter().all(|v| other.contains(v)) } else { false }
    }

    /// 如果集合是另一个集合的超集，则返回 `true`，即 `self` 至少包含 `other` 中的所有值。
    ///
    ///
    /// # 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)
    }

    /// 向集合中添加一个值。
    ///
    /// 如果集合中不存在该值，则返回 `true`。
    ///
    /// 如果集合中确实存在该值，则返回 `false`。
    ///
    /// # 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);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn insert(&mut self, value: T) -> bool {
        self.base.insert(value)
    }

    /// 向集合中添加一个值，替换等于给定值的现有值 (如果有)。
    /// 返回替换值。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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);
    /// ```
    #[inline]
    #[stable(feature = "set_recovery", since = "1.9.0")]
    pub fn replace(&mut self, value: T) -> Option<T> {
        self.base.replace(value)
    }

    /// 从集合中删除一个值。返回值是否存在于集合中。
    ///
    /// 该值可以是集合值类型的任何借用形式，但是借用形式上的 [`Hash`] 和 [`Eq`] 必须与值类型的那些匹配。
    ///
    ///
    /// # 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);
    /// ```
    ///
    ///
    #[inline]
    #[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.base.remove(value)
    }

    /// 删除并返回集合中等于给定值的值 (如果有)。
    ///
    /// 该值可以是集合值类型的任何借用形式，但是借用形式上的 [`Hash`] 和 [`Eq`] 必须与值类型的那些匹配。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    ///
    /// let mut set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    /// assert_eq!(set.take(&2), Some(2));
    /// assert_eq!(set.take(&2), None);
    /// ```
    ///
    #[inline]
    #[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,
    {
        self.base.take(value)
    }

    /// 仅保留谓词指定的元素。
    ///
    /// 换句话说，删除所有元素 `e`，以使 `f(&e)` 返回 `false`。
    /// 元素以未排序 (和未指定) 的顺序访问。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::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);
    /// ```
    #[stable(feature = "retain_hash_collection", since = "1.18.0")]
    pub fn retain<F>(&mut self, f: F)
    where
        F: FnMut(&T) -> bool,
    {
        self.base.retain(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for HashSet<T, S>
where
    T: Clone,
    S: Clone,
{
    #[inline]
    fn clone(&self) -> Self {
        Self { base: self.base.clone() }
    }

    #[inline]
    fn clone_from(&mut self, other: &Self) {
        self.base.clone_from(&other.base);
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<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: fmt::Debug,
{
    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,
{
    #[inline]
    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,
{
    #[inline]
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        self.base.extend(iter);
    }

    #[inline]
    fn extend_one(&mut self, item: T) {
        self.base.insert(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.base.extend_reserve(additional);
    }
}

#[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,
{
    #[inline]
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }

    #[inline]
    fn extend_one(&mut self, &item: &'a T) {
        self.base.insert(item);
    }

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Default for HashSet<T, S>
where
    S: Default,
{
    /// 用哈希器的 `Default` 值创建一个空的 `HashSet<T, S>`。
    #[inline]
    fn default() -> HashSet<T, S> {
        HashSet { base: Default::default() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> BitOr<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// 返回 `self` 和 `rhs` 的并集作为新的 `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<T, S> BitAnd<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// 将 `self` 和 `rhs` 的交集返回为新的 `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<T, S> BitXor<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// 返回 `self` 和 `rhs` 的对称差作为新的 `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<T, S> Sub<&HashSet<T, S>> for &HashSet<T, S>
where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default,
{
    type Output = HashSet<T, S>;

    /// 将 `self` 和 `rhs` 之差作为新的 `HashSet<T, S>` 返回。
    ///
    /// # 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()
    }
}

/// `HashSet` 项上的迭代器。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`iter`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`iter`]: HashSet::iter
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
///
/// let mut iter = a.iter();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, K: 'a> {
    base: base::Iter<'a, K>,
}

/// `HashSet` 项上的所属迭代器。
///
/// 该 `struct` 是通过 [`HashSet`] (由 `IntoIterator` trait 提供) 上的 [`into_iter`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`into_iter`]: IntoIterator::into_iter
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
///
/// let mut iter = a.into_iter();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<K> {
    base: base::IntoIter<K>,
}

/// `HashSet` 的项上的 draining 迭代器。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`drain`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`drain`]: HashSet::drain
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let mut a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
///
/// let mut drain = a.drain();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Drain<'a, K: 'a> {
    base: base::Drain<'a, K>,
}

/// draining，在 `HashSet` 的项上过滤迭代器。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`drain_filter`] 方法创建的。
///
/// [`drain_filter`]: HashSet::drain_filter
///
/// # Examples
///
/// ```
/// #![feature(hash_drain_filter)]
///
/// use std::collections::HashSet;
///
/// let mut a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
///
/// let mut drain_filtered = a.drain_filter(|v| v % 2 == 0);
/// ```
#[unstable(feature = "hash_drain_filter", issue = "59618")]
pub struct DrainFilter<'a, K, F>
where
    F: FnMut(&K) -> bool,
{
    base: base::DrainFilter<'a, K, F>,
}

/// 一个惰性迭代器，它在 HashSet 的交集中生成元素。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`intersection`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`intersection`]: HashSet::intersection
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
/// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
///
/// let mut intersection = a.intersection(&b);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Intersection<'a, T: 'a, S: 'a> {
    // 第一组的迭代器
    iter: Iter<'a, T>,
    // 第二组
    other: &'a HashSet<T, S>,
}

/// 一个惰性迭代器，它产生与 HashSet 不同的元素。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`difference`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`difference`]: HashSet::difference
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
/// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
///
/// let mut difference = a.difference(&b);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Difference<'a, T: 'a, S: 'a> {
    // 第一组的迭代器
    iter: Iter<'a, T>,
    // 第二组
    other: &'a HashSet<T, S>,
}

/// 一个惰性迭代器，它产生对称差异为 HashSet 的元素。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`symmetric_difference`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`symmetric_difference`]: HashSet::symmetric_difference
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
/// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
///
/// let mut intersection = a.symmetric_difference(&b);
/// ```
#[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>>,
}

/// 一个惰性迭代器，在 `HashSet` 的并集中生成元素。
///
/// 该 `struct` 是通过 [`HashSet`] 上的 [`union`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`union`]: HashSet::union
///
/// # Examples
///
/// ```
/// use std::collections::HashSet;
///
/// let a: HashSet<u32> = vec![1, 2, 3].into_iter().collect();
/// let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();
///
/// let mut union_iter = a.union(&b);
/// ```
#[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> {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> IntoIterator for HashSet<T, S> {
    type Item = T;
    type IntoIter = IntoIter<T>;

    /// 创建一个消耗迭代器，即将每个值以任意顺序移出集合的迭代器。
    /// 调用此设置后将无法使用该设置。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::HashSet;
    /// let mut set = HashSet::new();
    /// set.insert("a".to_string());
    /// set.insert("b".to_string());
    ///
    /// // 不能与常规 `.iter()` 一起收集到 Vec<String>。
    /// let v: Vec<String> = set.into_iter().collect();
    ///
    /// // 将以任意顺序打印。
    /// for x in &v {
    ///     println!("{}", x);
    /// }
    /// ```
    ///
    #[inline]
    fn into_iter(self) -> IntoIter<T> {
        IntoIter { base: self.base.into_iter() }
    }
}

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

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

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

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

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

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

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

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

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

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

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

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for Intersection<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        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;

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

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

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

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Intersection<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for Difference<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        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;

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

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

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Difference<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, S> Clone for SymmetricDifference<'_, T, S> {
    #[inline]
    fn clone(&self) -> Self {
        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;

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

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for SymmetricDifference<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

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

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

#[stable(feature = "fused", since = "1.26.0")]
impl<T, S> FusedIterator for Union<'_, T, S>
where
    T: Eq + Hash,
    S: BuildHasher,
{
}

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

#[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;

    #[inline]
    fn next(&mut self) -> Option<&'a T> {
        self.iter.next()
    }
    #[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, 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
    }
}