CRITICAL: Use for concurrency/async. Triggers: E0277 Send Sync, cannot be sent between threads, thread, spawn, channel, mpsc, Mutex, RwLock, Atomic, async, await, Future, tokio, deadlock, race condition, 并发, 线程, 异步, 死锁

Concurrency Layer 1: Language Mechanics Core Question Is this CPU bound or I/O bound, and what's the sharing model? Before choosing concurrency primitives: What's the workload type? What data needs to be shared? What's the thread safety requirement? Error → Design Question Error Don't Just Say Ask Instead E0277 Send "Add Send bound" Should this type cross threads? E0277 Sync "Wrap in Mutex" Is shared access really needed? Future not Send "Use spawn local" Is async the right choice? Deadlock "Reorder locks" Is the locking design correct? Thinking Prompt Before adding concurrency: 1. What's the workload? CPU bound → threads (std::thread, rayon) I/O bound → async (tokio, async std) Mixed → hybrid approach 2. What's the sharing model? No sharing → message passing (channels) Immutable sharing → Arc<T Mutable sharing → Arc<Mutex<T or Arc<RwLock<T 3. What are the Send/Sync requirements? Cross thread ownership → Send Cross thread references → Sync Single thread async → spawn local Trace Up ↑ (MANDATORY) CRITICAL : Don't just fix the error. Trace UP to find domain constraints. Domain Detection Table Context Keywords Load Domain Skill Key Constraint Web API, HTTP, axum, actix, handler domain web Handlers run on any thread 交易, 支付, trading, payment domain fintech Audit + thread safety gRPC, kubernetes, microservice domain cloud native Distributed tracing CLI, terminal, clap domain cli Usually single thread OK Example: Web API + Rc Error Generic Trace Situation Trace To Question Send/Sync in Web domain web What's the state management pattern? Send/Sync in CLI domain cli Is multi thread really needed? Mutex vs channels m09 domain Shared state or message passing? Async vs threads m10 performance What's the workload profile? Trace Down ↓ From design to implementation: Send/Sync Markers Marker Meaning Example Send Can transfer ownership between threads Most types Sync Can share references between threads Arc<T !Send Must stay on one thread Rc<T !Sync No shared refs across threads RefCell<T Quick Reference Pattern Thread Safe Blocking Use When std::thread Yes Yes CPU bound parallelism async/await Yes No I/O bound concurrency Mutex<T Yes Yes Shared mutable state RwLock<T Yes Yes Read heavy shared state mpsc::channel Yes Optional Message passing Arc<Mutex<T Yes Yes Shared mutable across threads Decision Flowchart Common Errors Error Cause Fix E0277 Send not satisfied Non Send in async Use Arc or spawn local E0277 Sync not satisfied Non Sync shared Wrap with Mutex Deadlock Lock ordering Consistent lock order future is not Send Non Send across await Drop before await MutexGuard across await Guard held during suspend Scope guard properly Anti Patterns Anti Pattern Why Bad Better Arc<Mutex<T everywhere Contention, complexity Message passing thread::sleep in async Blocks executor tokio::time::sleep Holding locks across await Blocks other tasks Scope locks tightly Ignoring deadlock risk Hard to debug Lock ordering, try lock Async Specific Patterns Avoid MutexGuard Across Await Non Send Types in Async Related Skills When See Smart pointer choice m02 resource Interior mutability m03 mutability Performance tuning m10 performance Domain concurrency needs domain

m07-concurrency