Vec & Slices

Vec<T> owns a growable buffer. &[T] borrows a contiguous view into Vec, arrays, or other data.

Recipe

fn sum(slice: &[i32]) -> i32 {
    slice.iter().sum()
}
 
fn main() {
    let v = vec![1, 2, 3];
    println!("{}", sum(&v));
    println!("{:?}", &v[1..]);
}

When to reach for this: Default dynamic array; expose &[T] in read APIs.

Working Example

fn dedup_sorted(v: &mut Vec<i32>) {
    v.sort_unstable();
    v.dedup();
}
 
fn main() {
    let mut data = vec![3, 1, 2, 2, 3];
    dedup_sorted(&mut data);
    println!("{:?}", data);
}

What this demonstrates:

sort_unstable + dedup in-place
Mutable Vec passed by &mut
Slice &v[1..] shares backing buffer

Deep Dive

len, capacity, reserve, shrink_to_fit, split_at, windows, chunks. Vec deref coerces to [T].

Gotchas

Index panic - v[i] out of bounds. Fix: get(i).
Iterating while mutating - Borrow conflict. Fix: Separate passes or retain.
Excess capacity memory - shrink_to_fit after large temp build.
drain semantics - Removes range while iterating.
Zero-sized types - Vec<()> odd capacity behavior - edge case.

Alternatives

Alternative	Use When	Don't Use When
`SmallVec`	Usually small	Always heap ok
`ArrayVec`	Fixed max, stack	Unbounded growth
`Box<[T]>`	Immutable sized slice owned	Need push

FAQs

with_capacity?

When len known approx.

from_raw_parts?

Unsafe - avoid unless expert.

splice?

Replace range with iterator.

truncate?

Drop tail elements.

as_slice?

Explicit &[T] from Vec.

mem::take?

Replace vec with empty quickly.

align?

Vec aligns to T requirements.

serde Vec?

Default seq serialization.

no_std?

alloc::vec::Vec with alloc crate.

sort stable?

sort vs sort_unstable tradeoff.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Working Example

fn dedup_sorted(v: &mut Vec<i32>) {
    v.sort_unstable();
    v.dedup();
}
 
fn main() {
    let mut data = vec![3, 1, 2, 2, 3];
    dedup_sorted(&mut data);
    println!("{:?}", data);
}

What this demonstrates:

sort_unstable + dedup in-place

Mutable Vec passed by &mut

Slice &v[1..] shares backing buffer

Alternative

Use When

Don't Use When

SmallVec

Usually small

Always heap ok

ArrayVec

Fixed max, stack

Unbounded growth

Box<[T]>

Immutable sized slice owned

Need push

FAQs

with_capacity?

When len known approx.

from_raw_parts?

Unsafe - avoid unless expert.

splice?

Replace range with iterator.

truncate?

Drop tail elements.

as_slice?

Explicit &[T] from Vec.

mem::take?

Replace vec with empty quickly.

align?

Vec aligns to T requirements.

serde Vec?

Default seq serialization.

no_std?

alloc::vec::Vec with alloc crate.

sort stable?

sort vs sort_unstable tradeoff.

Vec & Slices

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Vec & Slices