The definitive non-empty slice/array/vec library for Rust.

Nonempty-by-construction API

let mut my_vec = NonEmpty::<Vec<_>>::of("hello"); // construct once
my_vec.push("world");                             // continue using your normal APIs
let hello: &str = my_vec.first();                 // preserve the guarantee that there is at least one element

#[repr(transparent)] allows advanced usecases and guarantees optimum performance¹:

let src = &mut ["hello", "world"];
let ne = NonEmpty::<[_]>::new_mut(src).unwrap();
//  ^ uses the same backing memory
let world: &str = ne.last();

Total API coverage. For every impl of [From], [TryFrom], [PartialEq] and [PartialOrd] in [std]², there is a corresponding impl in this library for [Slice], [Array] and [Vec]. This includes more exotic types:

let nun: Box<NonEmpty<[_]>> = vec![0xDEAD, 0xBEEF].into();
let cow: Cow<NonEmpty<[_]>> = (&*nun).into();
let arc: Arc<NonEmpty<[_]>> = cow.into_owned().into();

const-friendly API. Where possible, all methods are const.

const TWO: &NonEmpty<[&str]> = slice!["together", "forever"];
const FIRST: &str = TWO.first();
const ONE: &NonEmpty<[&str]> = NonEmpty::<[_]>::of(&"lonely");

Extensive feature gating supporting:

• no-std environments with no allocator.
• alloc-enabled environments.
• full-std-enabled environments.
• interaction with crates like serde and arbitrary.

Iterator support: Specialized [Iterator] methods remove branches to handle empty iterators, and preserve invariants even when chaining combinators.

let v = vec![1, 2, 3];
let _: Option<&u8> = v.iter().last();
    // ^ normally you have to handle the empty case
let _: &u8 = v.iter_ne().last();
    // ^ but we know there is at least one element
let _: u8 = v.iter_ne().copied().last();
                     // ^ using this combinator preserves the invariant

Thoughtful design:

• [NonZeroUsize] is inserted where appropriate.
• Everything Deref/DerefMuts down to a [NonEmpty<Slice<T>>], which in turn deref/muts down to a [T].
• Liberal applications of cmp, borrow, convert traits. If there's a missing API that you'd like, please raise an issue!