Rust: What is the difference between Once and lazy_static?

Once — one-time initialization. lazy_static — lazy static initialization.

Once:

• Initialize once.
• Thread-safe.
• Manual management.

lazy_static:

• Lazy initialization.
• Convenience macro.
• First access initializes.

1use std::sync::Once;
2
3static ONCE: Once = Once::new();
4
5ONCE.call_once(|| {
6    // Initialize once
7});
8
9use lazy_static::lazy_static;
10lazy_static! {
11    static ref DATA: Vec<i32> = vec![1, 2, 3];
12}

Key: Once for manual one-time init, lazy_static for convenience.