Rust: What is the difference between OnceLock and OnceCell?

OnceLock — thread-safe lazy init. OnceCell — single-threaded lazy init.

OnceLock:

• Thread-safe.
• Multiple threads.
• Requires Send + Sync.

OnceCell:

• Single-threaded.
• One thread.
• No Send + Sync.

1use std::sync::OnceLock;
2
3static DATA: OnceLock<String> = OnceLock::new();
4
5fn get_data() -> &'static str {
6    DATA.get_or_init(|| String::from("hello"))
7}

Key: OnceLock for threads, OnceCell for single thread.