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.

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