Rust: How do you implement thread-safe singletons?

1use std::sync::OnceLock;
2
3static INSTANCE: OnceLock<MyApp> = OnceLock::new();
4
5fn get_instance() -> &'static MyApp {
6    INSTANCE.get_or_init(|| MyApp::new())
7}

1use once_cell::sync::Lazy;
2
3static INSTANCE: Lazy<Mutex<MyApp>> = Lazy::new(|| {
4    Mutex::new(MyApp::new())
5});