Crate

once_cell

Single-assignment cells for lazy and global initialization in Rust.

What it is

once_cell provides OnceCell and Lazy types for values initialized exactly once. Lazy<T> initializes on first access; OnceCell<T> allows explicit one-time setting. The standard library adopted these patterns as std::sync::OnceLock and std::sync::LazyLock in Rust 1.70.

Why it is commonly used

Used to initialize global state lazily without static mut or mutex overhead — connection pools, compiled regex, configuration, and any singleton that is expensive to construct but must be shared across threads.

867

Repositories

88%

Actively maintained

761

Updated ≤ 30 days

3.5M

Total stars

747

Owners

Counted from the Cargo.toml manifests of the 867 indexed repositories that declare once_cell as a dependency — not download counts. Dependency data last verified 2026-08-13.

Browse 867 repositories using once_cell in Repos →

Often used with

Crates that show up unusually often in once_cell projects. The most distinctive pairings rank first — crates these projects use far more than the average indexed Rust project does, not just crates that are popular everywhere. Each percentage is the share of once_cell projects that also use it.

either11.6%num_cpus15.9%tracing-opentelemetry13.5%prometheus11.5%opentelemetry_sdk16.5%strum_macros13.1%opentelemetry17.8%opentelemetry-otlp15.5%tonic16%dashmap27%lru14.2%serde_with16.6%async-stream15.6%crossbeam-channel13.7%proptest11.8%bincode19.7%paste14.5%cfg-if14%jsonwebtoken13.4%ahash10.7%parking_lot33.9%tikv-jemallocator10.1%byteorder14.2%tracing-appender15.5%

Used by these organizations

Cloudflare Mozilla AWS Microsoft Google Oxide Computer Parity Technologies Deno Fly.io SurrealDB Embark Studios Buoyant The Rust Project Fuel Labs Tauri OpenAI Warp Meilisearch Tokio Bytecode Alliance Linebender Polars Quickwit Datadog Actix gfx-rs Helix Zellij PyO3 Wasmer Typst Tracel AI smol async RustCrypto rustls Servo Paradigm Matrix.org

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