How to use once_cell crate in Rust lazy initialization

Crates/Once Cell

2026-04-17T12:05:42+00:00

Use `once_cell::sync::Lazy` to define static variables that compute their value only on first access for efficient lazy initialization.

Use once_cell::sync::Lazy for thread-safe lazy initialization or once_cell::unsync::Lazy for single-threaded contexts. Define a static variable with the Lazy type, providing the value to compute and the type to store.

use once_cell::sync::Lazy;

static COUNT: Lazy<usize> = Lazy::new(|| {
    println!("Initializing count...");
    42
});

fn main() {
    println!("Count is {}", *COUNT);
}