Crate vec1

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Expand description

This crate provides a Vec wrapper (Vec1) which guarantees to have at least 1 element.

This can be useful if you have a API which accepts one ore more ofe a kind. Instead of accepting a Vec and returning an error if it’s empty a Vec1 can be used assuring there is at least 1 element and through this reducing the number of possible error causes.


extern crate vec1;

use vec1::Vec1;

fn main() {
    // vec1![] makes sure at compiler time
    // there is at least one element
    //let names = vec1! [ ];
    let names = vec1! [ "Liz" ];

fn greet(names: Vec1<&str>) {
    // methods like first/last which return a Option on Vec do
    // directly return the value, we know it's possible
    let first = names.first();
    println!("hallo {}", first);
    for name in names.iter().skip(1) {
        println!("  who is also know as {}", name)


  • std (default): If disabled this crate will only use core and alloc but not std as dependencies. Because of this some traits and method are not available if it is disabled.

  • serde: Implements Serialize and Deserialize for Vec1. Also implements it for SmallVec1 if both serde and smallvec-v1 features are enabled. Note that enabling both serde and smallvec-v1 implements Serialize and Deserialize for SmallVec1 but will not enable smallvec/serde and as such will not implement the serde traits for smallvec::SmallVec.

  • smallvec-v1 : Adds support for a vec1 variation backed by the smallvec crate version 1.x.y. (In the future there will likely be a additional smallvec-v2.). Works with no_std, i.e. if the default features are disabled.

  • smallvec-v1-write: Enables smallvec/write, this requires std. As we can’t tell cargo to automatically enable smallvec/write if and only if smallvec-v1 and std are both enabled this needs to be an extra feature.

  • unstable-nightly-try-from-impl (deprecated) : Was used to enable TryFrom/TryInto implementations before the traits became stable. Doesn’t do anything by now, but still exist for compatibility reasons.


To have all intra-(and inter-) doc links working properly it is recommended to generate the documentation with nightly rustdoc. This is only for the links in the documentation, library code and test should run at least on stable-2 (a two versions old stable) and might work on older versions too.

§Rust Version / Stability

Besides intra-doc links everything else is supposed to work on a two versions old stable release and everything newer, through it might work on older releases.

Features which require nightly/beta will be prefixed with unstable-.

For forwards compatibility the prefixed feature will be kept even if it’s no longer unstable, through the code it feature gated is now also either always available or behind a non-prefixed feature gate which the unstable- prefixed feature gate enables.

While I do try to keep unstable- features API stable this might not always be possible so enabling a unstable- prefixed features does exclude the stability guarantees normally expected from SemVer for code related to that feature. Still no patch version change will be pushed which brakes any code, even if it’s unstable- prefixed!

Updating dependencies follows following rules

SemVer Dep. Update KindPublicly exposed dep?Update of this Crate
patch updateyespatch (or minor)
minor updateyesminor
major updateyeswon’t happen, smallvec gets a second feature for v2
patch updatenopatch (or minor)
minor updatenominor
major updatenominor

If smallvec gets a major update a additional feature will be added supporting both major versions of it without introducing a major update for this crate.

I do my best so that I will never have to release a major version update for this crate as this would lead to API incompatibilities for other crates using this crate in their public API.


  • smallvec_v1smallvec-v1
    A alternative Vec1 implementation backed by an SmallVec1.


  • A macro similar to vec! to create a Vec1.


  • Error returned by operations which would cause Vec1 to have a length of 0.
  • std::vec::Vec wrapper which guarantees to have at least 1 element.