m07-concurrency

This skill is the concurrency and async reference (module m07, "Layer 1: Language Mechanics") from the actionbook "rust-skills" collection. It addresses a recurring source of confusion in Rust: Send/Sync trait errors, deadlocks, and the choice between threads and async. Its core framing is to ask whether a workload is CPU-bound or I/O-bound and what the data-sharing model is, so that concurrency primitives are chosen deliberately rather than by trial-and-error against compiler errors like E0277.

The SKILL.md supplies error-to-design-question tables, a mandatory "trace up" step that detects the surrounding domain (web, fintech, cloud-native, CLI) to surface domain constraints, and "trace down" guidance mapping needs to primitives: `std::thread`/`rayon` for CPU work, `tokio`/async for I/O, `Arc<T>` for shared immutable data, `Arc<Mutex<T>>` or `Arc<RwLock<T>>` for shared mutable state, and channels for message passing. Supporting files include async patterns (task spawning, select, mpsc/oneshot/broadcast/watch channels, structured concurrency with JoinSet, cancellation tokens, graceful shutdown, and backpressure with bounded channels and semaphores), common concurrency errors, thread patterns, and a comparison document.

The audience is intermediate-to-advanced Rust developers building concurrent or async systems who want to resolve Send/Sync problems at the design level and pick correct synchronization primitives. Use cases include diagnosing "cannot be sent between threads" errors, choosing between mutexes and channels, structuring tokio task groups, and implementing cancellation and backpressure. Like its sibling modules it carries bilingual (English/Chinese) trigger phrases.