Concurrency & Async

Thread Safety

Error	Cause	Fix
`Rc<T>` cannot be sent between threads	`Rc` is not `Send`	Use `Arc`
`RefCell<T>` is not `Sync`	Not thread-safe	Use `Mutex` or `RwLock`
Cannot mutate through `Arc`	No interior mutability	`Arc<Mutex<T>>` or `Arc<AtomicT>`

Lock ordering: Always acquire multiple locks in the same order across all threads
Self-deadlock: Never lock the same std::Mutex twice (use parking_lot::ReentrantMutex if needed)
Scope locks tightly: Drop guards as soon as possible
Atomics for primitives: Use AtomicBool/AtomicUsize instead of Mutex<bool>
Choose memory order carefully: Relaxed / Acquire / Release / SeqCst