once_cell for lazy static initialization

once_cell provides Lazy and OnceCell for safe lazy initialization. Unlike lazy_static!, it doesn't require macros. Lazy<T> is initialized on first access via a closure. I use it for config, regex patterns, or expensive-to-create globals. OnceCell is similar but initialized explicitly, useful for runtime-determined values. Both are thread-safe and lock-free after initialization. The standard library added OnceLock in Rust 1.70, but once_cell still has a richer API and works on older Rust versions. Lazy initialization defers costs until needed and can improve startup time. It's especially useful in libraries where you don't control initialization order.