Rust Arc (Atomically Reference Counted) smart pointer for thread-safe shared ownership. Covers construction, cloning, mutation strategies (Mutex, RwLock, atomics, make_mut), Weak references for cycle breaking, reference counting, unwrapping, raw pointer conversion, and trait implementations. Use when writing, reviewing, or refactoring Rust code involving shared ownership across threads: (1) Sharing data between threads with Arc, (2) Choosing between Arc and Rc, (3) Combining Arc with Mutex or RwLock for interior mutability, (4) Using Weak references to break reference cycles, (5) Managing Arc reference counts, (6) Converting Arc to/from raw pointers, (7) Using Arc with tokio::spawn or thread::spawn.