Struct std::collections::HashSet

pub struct HashSet<T, S = RandomState> { /* private fields */ }

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:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must be equal.

It is a logic error for a key to be modified in such a way that the key’s hash, as determined by the Hash trait, or its equality, as determined by the Eq trait, changes while it is in the map. This is normally only possible through Cell, RefCell, global state, I/O, or unsafe code. The behavior resulting from such a logic error is not specified, but will be encapsulated to the HashSet that observed the logic error and not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

Examples

use std::collections::HashSet;
// Type inference lets us omit an explicit type signature (which
// would be `HashSet<String>` in this example).
let mut books = HashSet::new();

// Add some books.
books.insert("A Dance With Dragons".to_string());
books.insert("To Kill a Mockingbird".to_string());
books.insert("The Odyssey".to_string());
books.insert("The Great Gatsby".to_string());

// Check for a specific one.
if !books.contains("The Winds of Winter") {
    println!("We have {} books, but The Winds of Winter ain't one.",
             books.len());
}

// Remove a book.
books.remove("The Odyssey");

// Iterate over everything.
for book in &books {
    println!("{book}");
}

The easiest way to use HashSet with a custom type is to derive Eq and Hash. We must also derive PartialEq, this will in the future be implied by Eq.

use 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 });

// Use derived implementation to print the vikings.
for x in &vikings {
    println!("{x:?}");
}

A HashSet with a known list of items can be initialized from an array:

use std::collections::HashSet;

let viking_names = HashSet::from(["Einar", "Olaf", "Harald"]);

Implementations

impl<T> HashSet<T, RandomState>

pub fn new() -> 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();

pub fn with_capacity(capacity: usize) -> HashSet<T, RandomState>

Creates an empty HashSet with at least the specified capacity.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is 0, the hash set will not allocate.

Examples

use std::collections::HashSet;
let set: HashSet<i32> = HashSet::with_capacity(10);
assert!(set.capacity() >= 10);

impl<T, S> HashSet<T, S>

pub fn capacity(&self) -> usize

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);

pub fn iter(&self) -> Iter<'_, T>

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}");
}

Performance

In the current implementation, iterating over set takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

pub fn len(&self) -> usize

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);

pub fn is_empty(&self) -> bool

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());

pub fn drain(&mut self) -> Drain<'_, T>

Clears the set, returning all elements as an iterator. Keeps the allocated memory for reuse.

If the returned iterator is dropped before being fully consumed, it drops the remaining elements. The returned iterator keeps a mutable borrow on the vector to optimize its implementation.

Examples

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3]);
assert!(!set.is_empty());

// print 1, 2, 3 in an arbitrary order
for i in set.drain() {
    println!("{i}");
}

assert!(set.is_empty());

pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, T, F> where
    F: FnMut(&T) -> bool, 

🔬 This is a nightly-only experimental API. (hash_drain_filter #59618)

Creates an iterator which uses a closure to determine if a value should be removed.

If the closure returns true, then the value is removed and yielded. If the closure returns false, the value will remain in the list and will not be yielded by the iterator.

If the iterator is only partially consumed or not consumed at all, each of the remaining values will still be subjected to the closure and removed and dropped if it returns true.

It is unspecified how many more values will be subjected to the closure if a panic occurs in the closure, or if a panic occurs while dropping a value, or if the DrainFilter itself is leaked.

Examples

Splitting a set into even and odd values, reusing the original set:

#![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]);

pub fn retain<F>(&mut self, f: F) where
    F: FnMut(&T) -> bool, 

Retains only the elements specified by the predicate.

In other words, remove all elements e for which f(&e) returns false. The elements are visited in unsorted (and unspecified) order.

Examples

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3, 4, 5, 6]);
set.retain(|&k| k % 2 == 0);
assert_eq!(set.len(), 3);

Performance

In the current implementation, this operation takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

pub fn clear(&mut self)

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());

pub fn with_hasher(hasher: S) -> HashSet<T, S>

Creates a new empty hash set which will use the given hasher to hash keys.

The hash set is also created with the default initial capacity.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashMap to be useful, see its documentation for details.

Examples

use std::collections::HashSet;
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let mut set = HashSet::with_hasher(s);
set.insert(2);

pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S>

Creates an empty HashSet with at least the specified capacity, using hasher to hash the keys.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is 0, the hash set will not allocate.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashMap to be useful, see its documentation for details.

Examples

use std::collections::HashSet;
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let mut set = HashSet::with_capacity_and_hasher(10, s);
set.insert(1);

pub fn hasher(&self) -> &S

Returns a reference to the set’s BuildHasher.

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();

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

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional more elements to be inserted in the HashSet. The collection may reserve more space to speculatively avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to self.len() + additional. Does nothing if capacity is already sufficient.

Panics

Panics if the new allocation size overflows usize.

Examples

use std::collections::HashSet;
let mut set: HashSet<i32> = HashSet::new();
set.reserve(10);
assert!(set.capacity() >= 10);

pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least additional more elements to be inserted in the HashSet. The collection may reserve more space to speculatively avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if capacity is already sufficient.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::HashSet;
let mut set: HashSet<i32> = HashSet::new();
set.try_reserve(10).expect("why is the test harness OOMing on a handful of bytes?");

pub fn shrink_to_fit(&mut self)

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);

pub fn shrink_to(&mut self, min_capacity: usize)

Shrinks the capacity of the set with a lower limit. It will drop down no lower than the supplied limit while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

If the current capacity is less than the lower limit, this is a no-op.

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(10);
assert!(set.capacity() >= 10);
set.shrink_to(0);
assert!(set.capacity() >= 2);

pub fn difference<'a>(
    &'a self,
    other: &'a HashSet<T, S>
) -> Difference<'a, T, S>

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::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// 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());

pub fn symmetric_difference<'a>(
    &'a self,
    other: &'a HashSet<T, S>
) -> SymmetricDifference<'a, T, S>

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::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// 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());

pub fn intersection<'a>(
    &'a self,
    other: &'a HashSet<T, S>
) -> Intersection<'a, T, S>

Visits the values representing the intersection, i.e., the values that are both in self and other.

When an equal element is present in self and other then the resulting Intersection may yield references to one or the other. This can be relevant if T contains fields which are not compared by its Eq implementation, and may hold different value between the two equal copies of T in the two sets.

Examples

use std::collections::HashSet;
let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// 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());

pub fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S>

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::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// 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());

pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool where
    T: Borrow<Q>,
    Q: Hash + Eq, 

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::from([1, 2, 3]);
assert_eq!(set.contains(&1), true);
assert_eq!(set.contains(&4), false);

pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T> where
    T: Borrow<Q>,
    Q: Hash + Eq, 

Returns a reference to the value in the set, if any, that is equal to the given value.

The value may be any borrowed form of the set’s value type, but Hash and Eq on the borrowed form must match those for the value type.

Examples

use std::collections::HashSet;

let set = HashSet::from([1, 2, 3]);
assert_eq!(set.get(&2), Some(&2));
assert_eq!(set.get(&4), None);

pub fn get_or_insert(&mut self, value: T) -> &T

🔬 This is a nightly-only experimental API. (hash_set_entry #60896)

Inserts the given value into the set if it is not present, then returns a reference to the value in the set.

Examples

#![feature(hash_set_entry)]

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3]);
assert_eq!(set.len(), 3);
assert_eq!(set.get_or_insert(2), &2);
assert_eq!(set.get_or_insert(100), &100);
assert_eq!(set.len(), 4); // 100 was inserted

pub fn get_or_insert_owned<Q: ?Sized>(&mut self, value: &Q) -> &T where
    T: Borrow<Q>,
    Q: Hash + Eq + ToOwned<Owned = T>, 

🔬 This is a nightly-only experimental API. (hash_set_entry #60896)

Inserts an owned copy of the given value into the set if it is not present, then returns a reference to the value in the set.

Examples

#![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); // a new "fish" was inserted

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, 

🔬 This is a nightly-only experimental API. (hash_set_entry #60896)

Inserts a value computed from f into the set if the given value is not present, then returns a reference to the value in the set.

Examples

#![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); // a new "fish" was inserted

pub fn is_disjoint(&self, other: &HashSet<T, S>) -> bool

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::from([1, 2, 3]);
let mut b = HashSet::new();

assert_eq!(a.is_disjoint(&b), true);
b.insert(4);
assert_eq!(a.is_disjoint(&b), true);
b.insert(1);
assert_eq!(a.is_disjoint(&b), false);

pub fn is_subset(&self, other: &HashSet<T, S>) -> bool

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::from([1, 2, 3]);
let mut set = HashSet::new();

assert_eq!(set.is_subset(&sup), true);
set.insert(2);
assert_eq!(set.is_subset(&sup), true);
set.insert(4);
assert_eq!(set.is_subset(&sup), false);

pub fn is_superset(&self, other: &HashSet<T, S>) -> bool

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::from([1, 2]);
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);

pub fn insert(&mut self, value: T) -> bool

Adds a value to the set.

Returns whether the value was newly inserted. That is:

• If the set did not previously contain this value, true is returned.
• If the set already contained this value, 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);

pub fn replace(&mut self, value: T) -> Option<T>

Adds a value to the set, replacing the existing value, if any, that is equal to the given one. Returns the replaced value.

Examples

use 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);

pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool where
    T: Borrow<Q>,
    Q: Hash + Eq, 

Removes a value from the set. Returns whether the value was present in the set.

The value may be any borrowed form of the set’s value type, but Hash and Eq on the borrowed form must match those for the value type.

Examples

use std::collections::HashSet;

let mut set = HashSet::new();

set.insert(2);
assert_eq!(set.remove(&2), true);
assert_eq!(set.remove(&2), false);

pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T> where
    T: Borrow<Q>,
    Q: Hash + Eq, 

Removes and returns the value in the set, if any, that is equal to the given one.

The value may be any borrowed form of the set’s value type, but Hash and Eq on the borrowed form must match those for the value type.

Examples

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3]);
assert_eq!(set.take(&2), Some(2));
assert_eq!(set.take(&2), None);

Trait Implementations

impl<T, S> BitAnd<&HashSet<T, S>> for &HashSet<T, S> where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default, 

fn bitand(self, rhs: &HashSet<T, S>) -> HashSet<T, S>

Returns the intersection of self and rhs as a new HashSet<T, S>.

Examples

use std::collections::HashSet;

let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([2, 3, 4]);

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());

type Output = HashSet<T, S>

The resulting type after applying the & operator.

impl<T, S> BitOr<&HashSet<T, S>> for &HashSet<T, S> where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default, 

fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S>

Returns the union of self and rhs as a new HashSet<T, S>.

Examples

use std::collections::HashSet;

let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([3, 4, 5]);

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());

type Output = HashSet<T, S>

The resulting type after applying the | operator.

impl<T, S> BitXor<&HashSet<T, S>> for &HashSet<T, S> where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default, 

fn bitxor(self, rhs: &HashSet<T, S>) -> 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::from([1, 2, 3]);
let b = HashSet::from([3, 4, 5]);

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());

type Output = HashSet<T, S>

The resulting type after applying the ^ operator.

impl<T, S> Clone for HashSet<T, S> where
    T: Clone,
    S: Clone, 

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, other: &Self)

Performs copy-assignment from source.

impl<T, S> Debug for HashSet<T, S> where
    T: Debug, 

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T, S> Default for HashSet<T, S> where
    S: Default, 

fn default() -> HashSet<T, S>

Creates an empty HashSet<T, S> with the Default value for the hasher.

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)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: &'a T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

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)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: T)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

impl<T, const N: usize> From<[T; N]> for HashSet<T, RandomState> where
    T: Eq + Hash, 

fn from(arr: [T; N]) -> Self

Examples

use std::collections::HashSet;

let set1 = HashSet::from([1, 2, 3, 4]);
let set2: HashSet<_> = [1, 2, 3, 4].into();
assert_eq!(set1, set2);

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>

Creates a value from an iterator.

impl<'a, T, S> IntoIterator for &'a HashSet<T, S>

type Item = &'a T

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<T, S> IntoIterator for HashSet<T, S>

fn into_iter(self) -> 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}");
}

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?

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

fn eq(&self, other: &HashSet<T, S>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl<T, S> Sub<&HashSet<T, S>> for &HashSet<T, S> where
    T: Eq + Hash + Clone,
    S: BuildHasher + Default, 

fn sub(self, rhs: &HashSet<T, S>) -> HashSet<T, S>

Returns the difference of self and rhs as a new HashSet<T, S>.

Examples

use std::collections::HashSet;

let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([3, 4, 5]);

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());

type Output = HashSet<T, S>

The resulting type after applying the - operator.

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