Sunbeam Studios

Our open-source projects, here for you!

Portugal

https://sunbeam.pt

Repositories

20

Projects Packages

once_cell (1.21.4)

Published 2026-03-26 11:00:46 +00:00 by siennathesane

Installation

Set up this registry in the Cargo configuration file (for example ~/.cargo/config.toml):

[registry]
default = "gitea"

[registries.gitea]
index = "sparse+" # Sparse index
# index = "" # Git

[net]
git-fetch-with-cli = true

To install the package using Cargo, run the following command:

cargo add once_cell@1.21.4

For more information on the Cargo registry, see the documentation.

About this package

Single assignment cells and lazy values.

Overview

once_cell provides two new cell-like types, unsync::OnceCell and sync::OnceCell. OnceCell might store arbitrary non-Copy types, can be assigned to at most once and provide direct access to the stored contents. In a nutshell, API looks roughly like this:

impl OnceCell<T> {
    fn new() -> OnceCell<T> { ... }
    fn set(&self, value: T) -> Result<(), T> { ... }
    fn get(&self) -> Option<&T> { ... }
}

Note that, like with RefCell and Mutex, the set method requires only a shared reference. Because of the single assignment restriction get can return an &T instead of Ref<T> or MutexGuard<T>.

once_cell also has a Lazy<T> type, build on top of OnceCell which provides the same API as the lazy_static! macro, but without using any macros:

use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;

static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
    let mut m = HashMap::new();
    m.insert(13, "Spica".to_string());
    m.insert(74, "Hoyten".to_string());
    Mutex::new(m)
});

fn main() {
    println!("{:?}", GLOBAL_DATA.lock().unwrap());
}

More patterns and use-cases are in the docs!

Related crates

• double-checked-cell
• lazy-init
• lazycell
• mitochondria
• lazy_static
• async_once_cell
• generic_once_cell (bring your own mutex)

Parts of once_cell API are included into std as of Rust 1.70.0.

Dependencies

ID	Version
critical-section	^1.1.3
parking_lot_core	^0.9.10
portable-atomic	^1.8
critical-section	^1.1.3
regex	^1.10.6

Keywords

lazy static

Details

Cargo

2026-03-26 11:00:46 +00:00

0

Aleksey Kladov <aleksey.kladov@gmail.com>

Repository Site

Documentation Site

MIT OR Apache-2.0

33 KiB

Assets (1)

once_cell-1.21.4.crate 33 KiB

Versions (1) View all

1.21.4 2026-03-26