Collections

Updating the largest element in a BinaryHeap shouldn’t take two heap operations. peek_mut lets you mutate it in place and re-heapifies on drop — one sift instead of two.

The pop-and-push dance

Say you keep tasks in a max-heap by priority and you need to bump the top one down. The naive recipe is to pop, change it, and push it back:

1
2
3
4
5
6
7
8
9

use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from([3, 1, 5, 2, 4]);

// Pull the max out, edit it, push it back.
let top = heap.pop().unwrap();
heap.push(top - 10);

assert_eq!(heap.peek(), Some(&4));

That’s two O(log n) heap operations — a sift-down for pop, a sift-up for push — plus an awkward two-step where the heap briefly forgets it ever had a max.

peek_mut does it in one shot

peek_mut returns a PeekMut guard that derefs to the top element. You mutate it in place, and the heap fixes itself with a single sift-down when the guard is dropped:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

use std::collections::BinaryHeap;

let mut heap = BinaryHeap::from([3, 1, 5, 2, 4]);

if let Some(mut top) = heap.peek_mut() {
    *top -= 10; // 5 becomes -5
}
// PeekMut dropped here — heap re-heapifies once.

assert_eq!(heap.peek(), Some(&4));

One traversal, no temporary owned value, and the heap invariant is restored before the next line of code runs.

Conditionally pop without a re-peek

PeekMut also has an associated pop function. Look at the top, decide whether to keep or remove it, all without peeking twice:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

use std::collections::{BinaryHeap, binary_heap::PeekMut};

let mut heap = BinaryHeap::from([10, 7, 4, 1]);

if let Some(top) = heap.peek_mut() {
    if *top > 5 {
        // Pop only when the top passes a check.
        PeekMut::pop(top);
    }
}

assert_eq!(heap.peek(), Some(&7));

Same shape as Vec::pop_if, but for the heap’s max — pull the top out only when it actually meets your condition.

When to reach for it

Any time you’d write pop().push() to edit the max, peek_mut is the better tool: one heap fixup instead of two, and the borrow stays inside the heap so you don’t shuffle ownership for nothing. Great fit for priority queues that adjust the top entry — schedulers, event loops, top-k trackers.

Need to swap a chunk of a Vec for a different number of items? Most people reach for drain plus extend and a bunch of bookkeeping. Vec::splice does the whole thing in one call — and even hands back what it removed.

The clunky way

You want to replace v[1..4] with two different values. Without splice, you end up juggling indices:

1
2
3
4
5
6
7
8
9

let mut v = vec![1, 2, 3, 4, 5];

// Remove the middle, save the tail, then stitch it back together.
let tail: Vec<_> = v.drain(4..).collect();
v.truncate(1);
v.extend([10, 20]);
v.extend(tail);

assert_eq!(v, [1, 10, 20, 5]);

Three steps, one temporary Vec, and you didn’t even capture what was removed. Easy to off‑by‑one, easy to get wrong.

Enter splice

splice takes a range and an iterator and swaps them in place — the replacement can be any length:

1
2
3
4
5

let mut v = vec![1, 2, 3, 4, 5];

v.splice(1..4, [10, 20]);

assert_eq!(v, [1, 10, 20, 5]);

One call. No tail buffer. Works whether the replacement is shorter, longer, or the same size as the range.

It returns the removed items too

splice is a draining iterator — collect it and you get the elements that were taken out:

1
2
3
4
5
6

let mut v = vec![10, 20, 30, 40, 50];

let removed: Vec<_> = v.splice(1..4, [99]).collect();

assert_eq!(removed, [20, 30, 40]);
assert_eq!(v, [10, 99, 50]);

Perfect for “swap this section, but undo it later” patterns.

Insert without removing

Pass an empty range and splice becomes a multi‑item insert:

1
2
3
4
5

let mut v = vec![1, 5];

v.splice(1..1, [2, 3, 4]);

assert_eq!(v, [1, 2, 3, 4, 5]);

That’s far nicer than calling insert in a loop and watching every push shift the tail again.

When to reach for it

Any time you’d write drain(..) followed by extend(..) or a tower of insert calls, try splice first. One method, one allocation pattern, and the removed items come along for free.

Want to sort a Vec in descending order? The old trick is v.sort_by(|a, b| b.cmp(a)), and every time you write it you flip a and b and pray you got it right. std::cmp::Reverse is a one-word replacement.

The closure swap trick

To sort descending, you reverse the comparison. The closure is short but easy to mess up:

1
2
3
4

let mut scores = vec![30, 10, 50, 20, 40];
scores.sort_by(|a, b| b.cmp(a));

assert_eq!(scores, [50, 40, 30, 20, 10]);

Swap the a and b and you silently get ascending order again. Not a bug the compiler can help with.

Enter Reverse

std::cmp::Reverse<T> is a tuple wrapper whose Ord impl is the reverse of T’s. Hand it to sort_by_key and you’re done:

1
2
3
4
5
6

use std::cmp::Reverse;

let mut scores = vec![30, 10, 50, 20, 40];
scores.sort_by_key(|&s| Reverse(s));

assert_eq!(scores, [50, 40, 30, 20, 10]);

No chance of flipping the comparison the wrong way — the intent is right there in the name.

Sorting by a field, descending

It really shines when you’re sorting structs by a specific field:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19

use std::cmp::Reverse;

#[derive(Debug)]
struct Player {
    name: &'static str,
    score: u32,
}

let mut roster = vec![
    Player { name: "Ferris", score: 42 },
    Player { name: "Corro",  score: 88 },
    Player { name: "Rusty",  score: 65 },
];

roster.sort_by_key(|p| Reverse(p.score));

assert_eq!(roster[0].name, "Corro");
assert_eq!(roster[1].name, "Rusty");
assert_eq!(roster[2].name, "Ferris");

Mixing ascending and descending

Sort by one field ascending and another descending? Pack them in a tuple — Reverse composes cleanly:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18

use std::cmp::Reverse;

let mut items = vec![
    ("apple",  3),
    ("banana", 1),
    ("apple",  1),
    ("banana", 3),
];

// Name ascending, then count descending.
items.sort_by_key(|&(name, count)| (name, Reverse(count)));

assert_eq!(items, [
    ("apple",  3),
    ("apple",  1),
    ("banana", 3),
    ("banana", 1),
]);

It works anywhere Ord does

Because Reverse<T> is an Ord type, you can use it with BinaryHeap to turn a max-heap into a min-heap, or with BTreeSet to iterate in reverse order — no extra wrapper types needed.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

use std::cmp::Reverse;
use std::collections::BinaryHeap;

// BinaryHeap is max-heap by default. Wrap in Reverse for min-heap behaviour.
let mut heap = BinaryHeap::new();
heap.push(Reverse(3));
heap.push(Reverse(1));
heap.push(Reverse(2));

assert_eq!(heap.pop(), Some(Reverse(1)));
assert_eq!(heap.pop(), Some(Reverse(2)));
assert_eq!(heap.pop(), Some(Reverse(3)));

When to reach for it

Any time you’d write b.cmp(a) — reach for Reverse instead. The code reads top-to-bottom, the intent is obvious, and there’s no comparator to accidentally flip.

Tired of pushing a default into a Vec and then grabbing a mutable reference to fill it in? Rust 1.95 stabilises push_mut, which returns &mut T to the element it just inserted.

The old dance

A common pattern is to push a placeholder value and then immediately mutate it. Before push_mut, you had to do an awkward two-step:

1
2
3
4
5
6
7
8
9

let mut names = Vec::new();

// Push, then index back in to mutate
names.push(String::new());
let last = names.last_mut().unwrap();
last.push_str("hello");
last.push_str(", world");

assert_eq!(names[0], "hello, world");

That last_mut().unwrap() is boilerplate — you know the element is there because you just pushed it. Worse, in more complex code the compiler can’t always prove the reference is safe, forcing you into index gymnastics.

Enter push_mut

push_mut pushes the value and hands you back a mutable reference in one shot:

1
2
3
4
5
6
7

let mut scores = Vec::new();

let entry = scores.push_mut(0);
*entry += 10;
*entry += 25;

assert_eq!(scores[0], 35);

No unwraps, no indexing, no second lookup. The borrow checker is happy because there’s a clear chain of ownership.

Building structs in-place

This really shines when you’re constructing complex values piece by piece:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17

#[derive(Debug, Default)]
struct Player {
    name: String,
    score: u32,
    active: bool,
}

let mut roster = Vec::new();

let p = roster.push_mut(Player::default());
p.name = String::from("Ferris");
p.score = 100;
p.active = true;

assert_eq!(roster[0].name, "Ferris");
assert_eq!(roster[0].score, 100);
assert!(roster[0].active);

Instead of building the struct fully before pushing, you push a default and fill it in. This can be handy when the final field values depend on context you only have after insertion.

Also works for insert_mut

Need to insert at a specific index? insert_mut follows the same pattern:

1
2
3
4
5

let mut v = vec![1, 3, 4];
let inserted = v.insert_mut(1, 2);
*inserted *= 10;

assert_eq!(v, [1, 20, 3, 4]);

Both methods are also available on VecDeque (push_front_mut, push_back_mut, insert_mut) and LinkedList (push_front_mut, push_back_mut).

When to reach for it

Use push_mut whenever you’d otherwise write push followed by last_mut().unwrap(). It’s one less unwrap, one fewer line, and clearer intent: push this, then let me tweak it.

Stabilised in Rust 1.95 (April 2026) — update your toolchain and give it a spin.

Vec::pop_if got a deque-shaped sibling. As of Rust 1.93, VecDeque has pop_front_if and pop_back_if — conditional pops on either end without the peek-then-pop dance.

The problem

You want to remove an element from a VecDeque only when it matches a predicate. Before 1.93, you’d peek, match, then pop:

1
2
3
4
5
6
7

use std::collections::VecDeque;

let mut queue: VecDeque<i32> = VecDeque::from([1, 2, 3, 4]);

if queue.front().is_some_and(|&x| x == 1) {
    queue.pop_front();
}

Two lookups, two branches, one opportunity to desynchronize the check from the pop if you refactor the predicate later.

The fix

pop_front_if takes a closure, checks the front element against it, and pops it only if it matches. pop_back_if does the same on the other end.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

use std::collections::VecDeque;

let mut queue: VecDeque<i32> = VecDeque::from([1, 2, 3, 4]);

let popped = queue.pop_front_if(|x| *x == 1);
assert_eq!(popped, Some(1));
assert_eq!(queue, VecDeque::from([2, 3, 4]));

// Predicate doesn't match — nothing is popped.
let not_popped = queue.pop_front_if(|x| *x > 100);
assert_eq!(not_popped, None);
assert_eq!(queue, VecDeque::from([2, 3, 4]));

The return type is Option<T>: Some(value) if the predicate matched and the element was removed, None otherwise (including when the deque is empty).

One subtle detail worth noting — the closure receives &mut T, not &T. That means |&x| won’t type-check; use |x| *x == ... or destructure with |&mut x|. The extra flexibility lets you mutate the element in-place before deciding to pop it.

Draining a prefix

The pattern clicks when you pair it with a while let loop. Drain everything at the front that matches a condition, stop the moment it doesn’t:

1
2
3
4
5
6
7
8

use std::collections::VecDeque;

let mut events: VecDeque<i32> = VecDeque::from([1, 2, 3, 10, 11, 12]);

// Drain the "small" prefix only.
while let Some(_) = events.pop_front_if(|x| *x < 10) {}

assert_eq!(events, VecDeque::from([10, 11, 12]));

No index tracking, no split_off, no collecting into a new deque.

Why both ends?

VecDeque is a double-ended ring buffer, so it’s natural to support the same idiom on both sides. Processing a priority queue from the back, trimming expired entries from the front, popping a sentinel only when it’s still there — all one method call each.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

use std::collections::VecDeque;

let mut log: VecDeque<&str> = VecDeque::from(["START", "a", "b", "c", "END"]);

let end = log.pop_back_if(|s| *s == "END");
let start = log.pop_front_if(|s| *s == "START");

assert_eq!(start, Some("START"));
assert_eq!(end, Some("END"));
assert_eq!(log, VecDeque::from(["a", "b", "c"]));

When to reach for it

Whenever the shape of your code is “peek, compare, pop.” That’s the tell. pop_front_if / pop_back_if collapse three steps into one atomic operation, and the Option<T> return makes it composable with while let, ?, and the rest of the Option toolbox.

Stabilized in Rust 1.93 — if your MSRV is recent enough, this is a free readability win.

Got an iterator of tuples and need two separate collections? Stop looping and pushing manually — unzip splits pairs into two collections in a single pass.

The problem

You have an iterator that yields pairs — maybe key-value tuples from a computation, or results from enumerate. You need two separate Vecs. The manual approach works, but it’s noisy:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

let pairs = vec![("Alice", 95), ("Bob", 87), ("Carol", 92)];

let mut names = Vec::new();
let mut scores = Vec::new();
for (name, score) in &pairs {
    names.push(*name);
    scores.push(*score);
}

assert_eq!(names, vec!["Alice", "Bob", "Carol"]);
assert_eq!(scores, vec![95, 87, 92]);

Two mutable Vecs, a loop, manual pushes — all to split pairs apart.

The fix

Iterator::unzip does exactly this in one line:

1
2
3
4
5
6

let pairs = vec![("Alice", 95), ("Bob", 87), ("Carol", 92)];

let (names, scores): (Vec<&str>, Vec<i32>) = pairs.into_iter().unzip();

assert_eq!(names, vec!["Alice", "Bob", "Carol"]);
assert_eq!(scores, vec![95, 87, 92]);

The type annotation on the left tells Rust which collections to build. It works with any types that implement Default + Extend — so Vec, String, HashSet, and more.

Works great with enumerate

Need indices and values in separate collections?

1
2
3
4
5
6

let fruits = vec!["apple", "banana", "cherry"];

let (indices, items): (Vec<usize>, Vec<&&str>) = fruits.iter().enumerate().unzip();

assert_eq!(indices, vec![0, 1, 2]);
assert_eq!(items, vec![&"apple", &"banana", &"cherry"]);

Combine with map for transforms

Chain map before unzip to transform on the fly:

1
2
3
4
5
6
7
8
9

let data = vec![("temp_c", 20.0), ("temp_c", 35.0), ("temp_c", 0.0)];

let (labels, fahrenheit): (Vec<&str>, Vec<f64>) = data
    .into_iter()
    .map(|(label, c)| (label, c * 9.0 / 5.0 + 32.0))
    .unzip();

assert_eq!(labels, vec!["temp_c", "temp_c", "temp_c"]);
assert_eq!(fahrenheit, vec![68.0, 95.0, 32.0]);

Unzip into different collection types

Since unzip works with any Default + Extend types, you can collect into mixed collections:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

use std::collections::HashSet;

let entries = vec![("admin", "read"), ("admin", "write"), ("user", "read")];

let (roles, perms): (Vec<&str>, HashSet<&str>) = entries.into_iter().unzip();

assert_eq!(roles, vec!["admin", "admin", "user"]);
assert_eq!(perms.len(), 2); // "read" and "write", deduplicated
assert!(perms.contains("read"));
assert!(perms.contains("write"));

One side is a Vec, the other is a HashSet — unzip doesn’t care, as long as both sides can extend themselves.

Whenever you’re about to write a loop that pushes into two collections, reach for unzip instead.

Need to split a collection into two groups based on a condition? Skip the manual loop — Iterator::partition does it in one call.

The manual way

Without partition, you’d loop and push into two separate vectors:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15

let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8];

let mut evens = Vec::new();
let mut odds = Vec::new();

for n in numbers {
    if n % 2 == 0 {
        evens.push(n);
    } else {
        odds.push(n);
    }
}

assert_eq!(evens, vec![2, 4, 6, 8]);
assert_eq!(odds, vec![1, 3, 5, 7]);

It works, but it’s a lot of ceremony for a simple split.

Enter partition

Iterator::partition collects into two collections in a single pass. Items where the predicate returns true go left, false goes right:

1
2
3
4
5
6
7
8

let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8];

let (evens, odds): (Vec<_>, Vec<_>) = numbers
    .iter()
    .partition(|&&n| n % 2 == 0);

assert_eq!(evens, vec![&2, &4, &6, &8]);
assert_eq!(odds, vec![&1, &3, &5, &7]);

The type annotation (Vec<_>, Vec<_>) is required — Rust needs to know what collections to build. You can partition into any type that implements Default + Extend, not just Vec.

Owned values with into_iter

Use into_iter() when you want owned values instead of references:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

let files = vec![
    "main.rs", "lib.rs", "test_utils.rs",
    "README.md", "CHANGELOG.md",
];

let (rust_files, other_files): (Vec<_>, Vec<_>) = files
    .into_iter()
    .partition(|f| f.ends_with(".rs"));

assert_eq!(rust_files, vec!["main.rs", "lib.rs", "test_utils.rs"]);
assert_eq!(other_files, vec!["README.md", "CHANGELOG.md"]);

A practical use: triaging results

partition pairs beautifully with Result to separate successes from failures:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

let inputs = vec!["42", "not_a_number", "7", "oops", "13"];

let (oks, errs): (Vec<_>, Vec<_>) = inputs
    .iter()
    .map(|s| s.parse::<i32>())
    .partition(Result::is_ok);

let values: Vec<i32> = oks.into_iter().map(Result::unwrap).collect();
let failures: Vec<_> = errs.into_iter().map(Result::unwrap_err).collect();

assert_eq!(values, vec![42, 7, 13]);
assert_eq!(failures.len(), 2);

partition has been stable since Rust 1.0 — one of those hidden gems that’s been there all along. Anytime you reach for a loop to split items into two buckets, reach for partition instead.

Need to remove the last element of a Vec only when it meets a condition? Vec::pop_if does exactly that — no index juggling, no separate check-then-pop.

The old way

Before pop_if, you’d write something like this:

1
2
3
4
5
6

let mut stack = vec![1, 2, 3, 4];

if stack.last().is_some_and(|x| *x > 3) {
    let val = stack.pop().unwrap();
    println!("Popped: {val}");
}

Two separate calls, and a subtle TOCTOU gap if you’re not careful — last() checks one thing, then pop() acts on an assumption.

Enter pop_if

Stabilized in Rust 1.86, pop_if combines the check and the removal into one atomic operation:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

let mut stack = vec![1, 2, 3, 4];

// Pop the last element only if it's greater than 3
let popped = stack.pop_if(|x| *x > 3);
assert_eq!(popped, Some(4));
assert_eq!(stack, [1, 2, 3]);

// Now the last element is 3 — doesn't match, so nothing happens
let stayed = stack.pop_if(|x| *x > 3);
assert_eq!(stayed, None);
assert_eq!(stack, [1, 2, 3]);

The closure receives a &mut T reference to the last element. If it returns true, the element is removed and returned as Some(T). If false (or the vec is empty), you get None.

Mutable access inside the predicate

Because the closure gets &mut T, you can even modify the element before deciding whether to pop it:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16

let mut tasks = vec![
    String::from("buy milk"),
    String::from("URGENT: deploy fix"),
];

let urgent = tasks.pop_if(|task| {
    if task.starts_with("URGENT:") {
        *task = task.replacen("URGENT: ", "", 1);
        true
    } else {
        false
    }
});

assert_eq!(urgent.as_deref(), Some("deploy fix"));
assert_eq!(tasks, vec!["buy milk"]);

A practical use: draining from the back

pop_if is handy for processing a sorted vec from the tail. Think of a priority queue backed by a sorted vec where you only want to process items above a threshold:

1
2
3
4
5
6
7
8
9

let mut scores = vec![10, 25, 50, 75, 90];

let mut high_scores = Vec::new();
while let Some(score) = scores.pop_if(|s| *s >= 70) {
    high_scores.push(score);
}

assert_eq!(high_scores, vec![90, 75]);
assert_eq!(scores, vec![10, 25, 50]);

Clean, expressive, and no off-by-one errors. Another small addition to Vec that makes everyday Rust just a bit nicer.

The borrow checker won’t let you hold two &mut refs into the same collection — even when you know they don’t overlap. get_disjoint_mut fixes that without unsafe.

The problem

You want to update two elements of the same Vec together, but the compiler won’t allow two mutable borrows at once:

1
2
3
4

let mut scores = vec![10u32, 20, 30, 40];
let a = &mut scores[0];
let b = &mut scores[2]; // ❌ cannot borrow `scores` as mutable more than once
*a += *b;

The borrow checker doesn’t know indices 0 and 2 are different slots — it just sees two &mut to the same Vec. The classic escape hatches (split_at_mut, unsafe, RefCell) all feel like workarounds for something that should just work.

get_disjoint_mut to the rescue

Stabilized in Rust 1.86, get_disjoint_mut accepts an array of indices and returns multiple mutable references — verified at runtime to be non-overlapping:

1
2
3
4
5
6
7

let mut scores = vec![10u32, 20, 30, 40];

if let Ok([a, b]) = scores.get_disjoint_mut([0, 2]) {
    *a += *b; // 10 + 30 = 40
}

assert_eq!(scores, [40, 20, 30, 40]); // ✅

The Result is Err only if an index is out of bounds or indices overlap. Duplicate indices are caught at runtime and return Err — no silent aliasing bugs.

Works on HashMap as well

HashMap gets the same treatment. The return type is [Option<&mut V>; N] — one Option per key, since keys can be missing:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16

use std::collections::HashMap;

let mut accounts: HashMap<&str, u32> = HashMap::from([
    ("alice", 100),
    ("bob", 200),
]);

// Transfer 50 from bob to alice
let [alice, bob] = accounts.get_disjoint_mut(["alice", "bob"]);
if let (Some(a), Some(b)) = (alice, bob) {
    *a += 50;
    *b -= 50;
}

assert_eq!(accounts["alice"], 150);
assert_eq!(accounts["bob"], 150); // ✅

Passing duplicate keys to the HashMap version panics — the right tradeoff for a bug that would otherwise silently produce undefined behavior.

When to reach for it

• Swapping or combining two elements in a Vec without split_at_mut gymnastics
• Updating multiple HashMap entries in one pass
• Any place you’d have used unsafe or RefCell just to hold two &mut into the same container

If your indices or keys are known not to overlap, get_disjoint_mut is the clean, safe answer.

Ever needed to split a Vec into two groups — the ones you keep and the ones you remove? retain discards the removed items. Now there’s a better way.

Vec::extract_if (stable since Rust 1.87) removes elements matching a predicate and hands them back as an iterator — in a single pass.

The old way — two passes, logic must stay in sync

1
2
3
4
5
6
7
8
9

let mut numbers = vec![1, 2, 3, 4, 5, 6];

// Collect the evens first…
let evens: Vec<i32> = numbers.iter().filter(|&&x| x % 2 == 0).copied().collect();
// …then remove them (predicate must match exactly)
numbers.retain(|&x| x % 2 != 0);

assert_eq!(numbers, [1, 3, 5]);
assert_eq!(evens,   [2, 4, 6]);

The filter and the retain predicates must be inverses of each other — easy to mistype, and you touch the data twice.

The new way — one pass, one predicate

1
2
3
4
5
6

let mut numbers = vec![1, 2, 3, 4, 5, 6];

let evens: Vec<i32> = numbers.extract_if(.., |&mut x| x % 2 == 0).collect();

assert_eq!(numbers, [1, 3, 5]);
assert_eq!(evens,   [2, 4, 6]);

extract_if walks the Vec, removes every element where the closure returns true, and yields it. The .. is a range — you can narrow it to only consider a slice of the vector.

Real-world example: draining a work queue

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15

#[derive(Debug)]
struct Job { id: u32, priority: u8 }

let mut queue = vec![
    Job { id: 1, priority: 3 },
    Job { id: 2, priority: 9 },
    Job { id: 3, priority: 1 },
    Job { id: 4, priority: 8 },
];

// Pull out all high-priority jobs for immediate processing
let urgent: Vec<Job> = queue.extract_if(.., |j| j.priority >= 8).collect();

assert_eq!(urgent.len(), 2);  // jobs 2 and 4
assert_eq!(queue.len(),  2);  // jobs 1 and 3 remain

HashMap and HashSet also gained extract_if in Rust 1.88.

Note: The closure takes &mut T, so you can even mutate elements mid-extraction before deciding whether to remove them.

Want to insert a value into a HashMap only if the key doesn’t exist yet? Skip the double lookup — use the entry API.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14

use std::collections::HashMap;

let mut scores: HashMap<&str, Vec<u32>> = HashMap::new();

// Instead of checking .contains_key() then inserting:
scores.entry("alice")
    .or_insert_with(Vec::new)
    .push(100);

scores.entry("alice")
    .or_insert_with(Vec::new)
    .push(200);

assert_eq!(scores["alice"], vec![100, 200]);

The entry API returns an Entry enum — either Occupied or Vacant. The convenience methods make common patterns a one-liner:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10

use std::collections::HashMap;

let mut word_count: HashMap<&str, usize> = HashMap::new();
let words = ["hello", "world", "hello", "rust", "hello"];

for word in words {
    *word_count.entry(word).or_insert(0) += 1;
}

// hello => 3, world => 1, rust => 1

or_insert(val) inserts a default, or_insert_with(|| val) lazily computes it, and or_default() uses the type’s Default. All three return a mutable reference to the value, so you can update in place.