Sets and HashSet

Rust’s HashSet<T> is the standard hash-based collection of unique values — the direct counterpart to the JavaScript Set. If you have ever reached for new Set() to deduplicate an array or to answer “have I seen this before?”, you already know the use case; this page shows the idiomatic Rust version and the rich set algebra Rust gives you out of the box.

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Quick Overview

A HashSet<T> stores each value at most once, with average O(1) insertion, removal, and membership checks. Unlike a JavaScript Set, a Rust HashSet is homogeneous (every element is the same type T), elements must be hashable and comparable (T: Hash + Eq), and iteration order is unspecified and randomized. Rust also ships first-class set operations — union, intersection, difference, and symmetric_difference — as methods and operators, which JavaScript only standardized very recently.

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TypeScript/JavaScript Example

// Track which user IDs have already seen a feature flag, then reason about roles.
const seen = new Set<number>();

const events = [10, 20, 10, 30, 20, 10];
let firstTime = 0;
for (const id of events) {
  if (!seen.has(id)) {
    seen.add(id);
    firstTime++;
  }
}

console.log("unique users:", seen.size); // 3
console.log("first-time impressions:", firstTime); // 3
console.log("has user 20:", seen.has(20)); // true

// Set algebra. Native Set methods (union/intersection/difference) only landed
// in Node 22 / ES2024; before that everyone hand-rolled these with spreads.
const granted = new Set(["read", "write", "deploy"]);
const required = new Set(["read", "deploy", "admin"]);

const missing = required.difference(granted); // Set { 'admin' }
const hasAll = required.isSubsetOf(granted);   // false
console.log([...missing], hasAll);

// The classic one-liner: deduplicate an array.
const unique = [...new Set(["a", "b", "a", "c", "b"])]; // ['a', 'b', 'c']
console.log(unique);

Key points:

• A Set can hold values of mixed types (new Set([1, "a", {}])).
• has, add, and delete are the core operations.
• Membership uses SameValueZero equality (reference equality for objects — two distinct {x:1} objects are different members).
• Insertion order is preserved and iteration is deterministic.

---

Rust Equivalent

use std::collections::HashSet;

fn main() {
    // Track which user IDs have already been seen.
    let mut seen: HashSet<u64> = HashSet::new();

    let events = [10u64, 20, 10, 30, 20, 10];
    let mut first_time = 0;
    for id in events {
        // `insert` returns `true` if the value was NOT already present.
        if seen.insert(id) {
            first_time += 1;
        }
    }

    println!("unique users: {}", seen.len()); // 3
    println!("first-time impressions: {first_time}"); // 3
    println!("has user 20: {}", seen.contains(&20)); // true

    // Set algebra — built in since Rust 1.0.
    let granted: HashSet<&str> = HashSet::from(["read", "write", "deploy"]);
    let required: HashSet<&str> = HashSet::from(["read", "deploy", "admin"]);

    let missing: Vec<&str> = required.difference(&granted).copied().collect();
    let has_all = required.is_subset(&granted);
    println!("missing: {missing:?}, has_all: {has_all}"); // missing: ["admin"], has_all: false

    // Deduplicate by collecting into a HashSet.
    let unique: HashSet<&str> = ["a", "b", "a", "c", "b"].into_iter().collect();
    println!("unique count: {}", unique.len()); // 3
}

Running this prints (your exact ordering for missing will be stable here because it has one element, but multi-element sets print in random order):

unique users: 3
first-time impressions: 3
has user 20: true
missing: ["admin"], has_all: false
unique count: 3

Key points:

• HashSet<T> is homogeneous — one element type, checked at compile time.
• insert and remove return a bool telling you whether the set changed.
• contains takes a reference (&20), not the value.
• The element type must satisfy Hash + Eq (see Key Differences).

---

Detailed Explanation

Importing and constructing

HashSet lives in std::collections, so it always needs a use line — there is no Set in the prelude the way Vec and String are:

use std::collections::HashSet;

fn main() {
    // Empty, type annotated.
    let mut a: HashSet<String> = HashSet::new();
    a.insert("rust".to_string());

    // Preallocate buckets when you know roughly how many elements you'll store.
    let mut b: HashSet<u32> = HashSet::with_capacity(100);
    b.insert(1);

    // From an array literal (Rust 1.56+). Type inferred from the elements.
    let c = HashSet::from([1, 2, 3, 4]);

    // From any iterator via `collect`.
    let d: HashSet<i32> = (1..=5).collect();

    println!("{} {} {} {}", a.len(), b.len(), c.len(), d.len()); // 1 1 4 5
}

Note: Unlike Vec (which has the vec! macro), there is no set! macro in the standard library. Use HashSet::from([...]) or .collect().

insert and remove return whether the set changed

This is the single most useful difference from JavaScript’s add/delete, which return the set and a boolean respectively but are rarely used for their return value. In Rust the return value is the idiomatic way to do “insert if new”:

use std::collections::HashSet;

fn main() {
    let mut tags: HashSet<String> = HashSet::new();

    let added = tags.insert("wasm".to_string()); // true  — newly inserted
    let again = tags.insert("wasm".to_string()); // false — already present
    println!("added = {added}, again = {again}");

    let removed = tags.remove("wasm"); // true  — it was there
    let missing = tags.remove("nope"); // false — wasn't there
    println!("removed = {removed}, missing = {missing}");
}

Output:

added = true, again = false
removed = true, missing = false

Membership: contains, get, and take

use std::collections::HashSet;

fn main() {
    let words: HashSet<String> = HashSet::from(["rust".to_string()]);

    // `contains` borrows the query. Thanks to the `Borrow` trait you can query a
    // HashSet<String> with a &str — no allocation needed.
    println!("{}", words.contains("rust")); // true

    // `get` returns Option<&T> — useful when the stored value carries more than
    // the part you compare on.
    println!("{:?}", words.get("rust")); // Some("rust")
    println!("{:?}", words.get("go"));   // None

    // `take` removes and hands you back the owned value.
    let mut owned: HashSet<String> = HashSet::from(["a".to_string(), "b".to_string()]);
    let taken = owned.take("a"); // Some("a")
    println!("{:?}, len {}", taken, owned.len()); // Some("a"), len 1
}

Set operations return lazy iterators

union, intersection, difference, and symmetric_difference do not allocate a new set. They return an iterator of references (&T) that you consume — typically with .collect() or .copied()/.cloned() first. This is consistent with Rust’s lazy iterator philosophy.

use std::collections::HashSet;

fn main() {
    let a: HashSet<i32> = HashSet::from([1, 2, 3, 4]);
    let b: HashSet<i32> = HashSet::from([3, 4, 5, 6]);

    // `.copied()` turns the &i32 iterator into an i32 iterator (i32 is Copy).
    // We sort only to get deterministic output for this example.
    let mut union: Vec<i32> = a.union(&b).copied().collect();
    union.sort();
    println!("union = {union:?}"); // [1, 2, 3, 4, 5, 6]

    let mut inter: Vec<i32> = a.intersection(&b).copied().collect();
    inter.sort();
    println!("intersection = {inter:?}"); // [3, 4]

    // difference is directional: a - b (in a but not in b).
    let mut diff: Vec<i32> = a.difference(&b).copied().collect();
    diff.sort();
    println!("difference (a - b) = {diff:?}"); // [1, 2]

    // symmetric_difference: in exactly one of the two.
    let mut sym: Vec<i32> = a.symmetric_difference(&b).copied().collect();
    sym.sort();
    println!("symmetric_difference = {sym:?}"); // [1, 2, 5, 6]
}

Output:

union = [1, 2, 3, 4, 5, 6]
intersection = [3, 4]
difference (a - b) = [1, 2]
symmetric_difference = [1, 2, 5, 6]

Tip: When you want a new HashSet rather than a Vec, collect straight into one: let u: HashSet<i32> = a.union(&b).copied().collect();.

Operator overloads on &HashSet

For sets of Clone elements, the bitwise operators are overloaded to mirror set algebra. They operate on references and produce an owned HashSet:

use std::collections::HashSet;

fn main() {
    let a: HashSet<i32> = HashSet::from([1, 2, 3]);
    let b: HashSet<i32> = HashSet::from([2, 3, 4]);

    let union = &a | &b;                  // union
    let intersection = &a & &b;           // intersection
    let difference = &a - &b;             // a minus b
    let symmetric = &a ^ &b;              // symmetric difference

    let mut u: Vec<i32> = union.into_iter().collect();        u.sort();
    let mut i: Vec<i32> = intersection.into_iter().collect(); i.sort();
    let mut d: Vec<i32> = difference.into_iter().collect();   d.sort();
    let mut s: Vec<i32> = symmetric.into_iter().collect();    s.sort();

    println!("a | b = {u:?}"); // [1, 2, 3, 4]
    println!("a & b = {i:?}"); // [2, 3]
    println!("a - b = {d:?}"); // [1]
    println!("a ^ b = {s:?}"); // [1, 4]
}

Relationship predicates

use std::collections::HashSet;

fn main() {
    let a: HashSet<i32> = HashSet::from([1, 2, 3, 4]);
    let small: HashSet<i32> = HashSet::from([1, 2]);
    let other: HashSet<i32> = HashSet::from([9, 10]);

    println!("{}", small.is_subset(&a));   // true
    println!("{}", a.is_superset(&small)); // true
    println!("{}", a.is_disjoint(&other)); // true  — no shared elements
}

In-place mutation: extend and retain

use std::collections::HashSet;

fn main() {
    // Union into an existing set without allocating a new one.
    let mut acc: HashSet<i32> = HashSet::from([1, 2, 3]);
    acc.extend([3, 4, 5]); // 3 already present, ignored

    let mut v: Vec<i32> = acc.iter().copied().collect();
    v.sort();
    println!("after extend = {v:?}"); // [1, 2, 3, 4, 5]

    // Filter in place — keep only the elements matching the predicate.
    let mut nums: HashSet<i32> = (1..=10).collect();
    nums.retain(|&n| n % 2 == 0);
    let mut evens: Vec<i32> = nums.into_iter().collect();
    evens.sort();
    println!("evens = {evens:?}"); // [2, 4, 6, 8, 10]
}

---

Key Differences

Aspect	JavaScript Set	Rust HashSet<T>
Element types	Mixed allowed (Set<any>)	Homogeneous; one T, enforced at compile time
Element requirements	Any value	T: Hash + Eq
Object equality	Reference identity ({} ≠ {})	Value equality via derived Hash + Eq
Iteration order	Insertion order, deterministic	Unspecified and randomized per run
add/insert return	The set itself	bool (was it newly inserted?)
has/contains	set.has(x)	set.contains(&x) (takes a reference)
Set algebra	union/intersection/difference (ES2024, Node 22+)	Built in since 1.0, plus `
Result of set ops	A new Set	A lazy iterator of &T you .collect()

Why Hash + Eq?

A hash set finds elements by hashing them into buckets and comparing for equality within a bucket. So T must be able to (a) produce a hash (Hash) and (b) be compared for total equality (Eq, which implies PartialEq). Most built-in types — integers, bool, char, String, &str, tuples and Vecs of those — already implement both. For your own structs and enums, derive them:

use std::collections::HashSet;

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct UserId(u64);

fn main() {
    let mut ids: HashSet<UserId> = HashSet::new();
    ids.insert(UserId(1));
    ids.insert(UserId(1)); // value-equal, ignored
    println!("{}", ids.len()); // 1
}

Note: Two structs are “the same element” when their derived Hash and Eq say so — i.e. all fields are equal. This is value equality, the opposite of JavaScript, where two distinct objects are always different Set members even if their fields match.

Why is iteration order random?

Rust’s default hasher (SipHash 1-3) is seeded with a per-process random key. This makes the collection resistant to HashDoS attacks, where an attacker crafts colliding keys to degrade performance. The trade-off is that you must never rely on iteration order. If you need ordering, use a BTreeSet (sorted) or collect into a Vec and sort().

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Common Pitfalls

Pitfall 1: Using a type that isn’t Hash + Eq

Forgetting to derive the required traits produces a confusing error: it says insert “exists but its trait bounds were not satisfied.”

use std::collections::HashSet;

#[derive(Debug)]
struct Point { x: i32, y: i32 } // no Eq/Hash

fn main() {
    let mut seen: HashSet<Point> = HashSet::new();
    seen.insert(Point { x: 1, y: 2 }); // does not compile (error[E0599])
}

Real compiler output:

error[E0599]: the method `insert` exists for struct `HashSet<Point>`, but its trait bounds were not satisfied
 --> src/main.rs:8:10
  |
4 | struct Point { x: i32, y: i32 }
  | ------------ doesn't satisfy `Point: Eq` or `Point: Hash`
...
8 |     seen.insert(Point { x: 1, y: 2 });
  |          ^^^^^^
  |
  = note: the following trait bounds were not satisfied:
          `Point: Eq`
          `Point: Hash`
help: consider annotating `Point` with `#[derive(Eq, Hash, PartialEq)]`

The fix is exactly what the compiler suggests: #[derive(PartialEq, Eq, Hash)].

Pitfall 2: Floating-point elements

f64/f32 implement PartialEq but not Eq (because NaN != NaN), and they have no Hash. So HashSet<f64> does not work:

use std::collections::HashSet;

fn main() {
    let mut s: HashSet<f64> = HashSet::new();
    s.insert(3.14); // does not compile (error[E0599])
}

Real compiler output:

error[E0599]: the method `insert` exists for struct `HashSet<f64>`, but its trait bounds were not satisfied
 --> src/main.rs:5:7
  |
5 |     s.insert(3.14);
  |       ^^^^^^
  |
  = note: the following trait bounds were not satisfied:
          `f64: Eq`
          `f64: Hash`

Coming from JavaScript — where new Set([3.14]) just works — this surprises people. Store an integer key, a rounded/quantized representation, or use the ordered-float crate’s OrderedFloat wrapper if you genuinely need float set membership.

Pitfall 3: Expecting deterministic iteration order

// JavaScript: order is the insertion order, every time.
const s = new Set(["c", "a", "b"]);
console.log([...s]); // ['c', 'a', 'b'] — always

In Rust, for x in &set visits elements in a random order that changes between program runs. Do not write tests or output that assume an order. To compare a set to an expected sequence, sort first:

use std::collections::HashSet;

fn main() {
    let s: HashSet<&str> = HashSet::from(["c", "a", "b"]);
    let mut v: Vec<&str> = s.into_iter().collect();
    v.sort();
    assert_eq!(v, ["a", "b", "c"]); // deterministic
}

Pitfall 4: Passing the value instead of a reference to contains

contains (and remove, get, take) take a reference. set.contains(20) for a HashSet<i32> fails to compile; write set.contains(&20). For a HashSet<String>, you can pass a &str directly (set.contains("hi")) thanks to the Borrow trait — no String allocation required.

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Best Practices

• Reach for HashSet to deduplicate. iter.collect::<HashSet<_>>() is the idiomatic dedup; if you also need to preserve first-seen order, keep a Vec and a HashSet together and consult the set before pushing.
• Use the insert return value to express “insert if new” in one branch instead of a separate contains + insert. The two-call version also double-hashes.
• Derive Hash, Eq, PartialEq (plus Clone/Debug) on any struct or enum you intend to put in a set.
• Preallocate with with_capacity when the size is roughly known to avoid repeated rehashing — see collection performance.
• Choose BTreeSet when you need order (sorted iteration or range queries); choose HashSet for the fastest membership tests. See BTreeMap and BTreeSet.
• Collect set operations directly into the type you want. Need a set back? a.union(&b).copied().collect::<HashSet<_>>(). Need a list? collect into a Vec and sort().
• Swap the hasher for non-adversarial hot paths. SipHash is DoS-resistant but not the fastest; for trusted internal data, a crate like ahash or rustc-hash (FxHashSet) is materially faster. Keep the default for anything touching untrusted input.

---

Real-World Example

A small access-control check: given the permissions a user has been granted and the permissions an action requires, compute exactly which permissions are missing, and deduplicate an incoming audit log of accessed resources.

use std::collections::HashSet;

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Permission(String);

/// Returns the permissions that `required` needs but `granted` lacks.
fn missing_permissions(granted: &HashSet<Permission>, required: &HashSet<Permission>) -> Vec<Permission> {
    let mut missing: Vec<Permission> = required.difference(granted).cloned().collect();
    missing.sort_by(|a, b| a.0.cmp(&b.0)); // stable output for logging
    missing
}

fn perm(name: &str) -> Permission {
    Permission(name.to_string())
}

fn main() {
    let granted: HashSet<Permission> = ["read", "write", "deploy"].into_iter().map(perm).collect();
    let required: HashSet<Permission> = ["read", "deploy", "admin"].into_iter().map(perm).collect();

    if required.is_subset(&granted) {
        println!("access granted");
    } else {
        let missing = missing_permissions(&granted, &required);
        println!("access denied; missing: {missing:?}");
    }

    // The action also accessed several resources, some repeatedly. How many
    // distinct resources were touched?
    let accessed = ["db", "cache", "db", "queue", "cache", "db"];
    let distinct: HashSet<&str> = accessed.into_iter().collect();
    println!("distinct resources touched: {}", distinct.len());

    // Which resources are "hot" (touched) but not in our known set?
    let known: HashSet<&str> = HashSet::from(["db", "cache"]);
    let mut unknown: Vec<&str> = distinct.difference(&known).copied().collect();
    unknown.sort();
    println!("unknown resources: {unknown:?}");
}

Output:

access denied; missing: [Permission("admin")]
distinct resources touched: 3
unknown resources: ["queue"]

This is the kind of code where Rust’s compile-time guarantees pay off: Permission is a distinct type (not a bare string you might mistype), the set bounds are enforced, and the set algebra reads almost like the prose specification.

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Further Reading

• std::collections::HashSet API docs
• std::collections module overview — when to use which collection
• The Rust Book, Ch. 8: Common Collections
• Sibling pages in this section:
  • Vectors (Vec<T>) — the growable array
  • HashMaps (HashMap<K, V>) — the key/value sibling of HashSet; a set is essentially a map with no values
  • BTreeMap and BTreeSet — the sorted set when you need ordering or range queries
  • Iterators and Iterator consumers — how union/difference results are consumed
  • Collection performance — Big-O and choosing the right collection
• Foundational background:
  • Ownership — why contains borrows and set ops yield &T
  • Data structures — deriving Hash, Eq, PartialEq
  • Basic types — why f64 can’t be a set element
• Next: Error Handling

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Exercises

Exercise 1: Count unique words

Difficulty: Beginner

Objective: Use a HashSet to deduplicate, ignoring case.

Instructions: Implement count_unique_words(text: &str) -> usize that returns the number of distinct words (split on whitespace), treating "The" and "the" as the same word.

use std::collections::HashSet;

fn count_unique_words(text: &str) -> usize {
    // TODO: lowercase each word and collect into a HashSet
    todo!()
}

fn main() {
    assert_eq!(count_unique_words("the cat the dog The CAT"), 3);
    println!("ok");
}

Solution

use std::collections::HashSet;

fn count_unique_words(text: &str) -> usize {
    text.split_whitespace()
        .map(|w| w.to_lowercase())
        .collect::<HashSet<String>>()
        .len()
}

fn main() {
    assert_eq!(count_unique_words("the cat the dog The CAT"), 3);
    println!("ok");
}

Exercise 2: Common elements of two slices

Difficulty: Intermediate

Objective: Use set intersection to find shared elements.

Instructions: Implement common(a: &[i32], b: &[i32]) -> Vec<i32> that returns the values present in both slices, sorted ascending with no duplicates.

use std::collections::HashSet;

fn common(a: &[i32], b: &[i32]) -> Vec<i32> {
    // TODO: build two HashSets and intersect them
    todo!()
}

fn main() {
    assert_eq!(common(&[1, 2, 3, 4], &[3, 4, 5, 6]), vec![3, 4]);
    println!("ok");
}

Solution

use std::collections::HashSet;

fn common(a: &[i32], b: &[i32]) -> Vec<i32> {
    let set_a: HashSet<i32> = a.iter().copied().collect();
    let set_b: HashSet<i32> = b.iter().copied().collect();
    let mut out: Vec<i32> = set_a.intersection(&set_b).copied().collect();
    out.sort();
    out
}

fn main() {
    assert_eq!(common(&[1, 2, 3, 4], &[3, 4, 5, 6]), vec![3, 4]);
    println!("ok");
}

Exercise 3: First duplicate in a stream

Difficulty: Advanced

Objective: Exploit the boolean return value of insert to detect repeats in a single pass.

Instructions: Implement first_duplicate(items: &[&str]) -> Option<String> returning the first item that appears a second time (in input order), or None if all items are distinct. Iterate only once.

use std::collections::HashSet;

fn first_duplicate(items: &[&str]) -> Option<String> {
    // TODO: insert each item; the first time `insert` returns false, you found it
    todo!()
}

fn main() {
    assert_eq!(first_duplicate(&["a", "b", "c", "b", "a"]), Some("b".to_string()));
    assert_eq!(first_duplicate(&["a", "b", "c"]), None);
    println!("ok");
}

Solution

use std::collections::HashSet;

fn first_duplicate(items: &[&str]) -> Option<String> {
    let mut seen: HashSet<&str> = HashSet::new();
    for &item in items {
        // `insert` returns false when the value was already present.
        if !seen.insert(item) {
            return Some(item.to_string());
        }
    }
    None
}

fn main() {
    assert_eq!(first_duplicate(&["a", "b", "c", "b", "a"]), Some("b".to_string()));
    assert_eq!(first_duplicate(&["a", "b", "c"]), None);
    println!("ok");
}