---

name: rust-concurrency-primitives description: Design and review thread-based Rust concurrency with explicit ownership, sharing, and synchronization choices. Use when writing, refactoring, or reviewing threads, scoped threads, channels, Arc, Mutex, RwLock, Condvar, atomics, OnceLock, LazyLock, Send, Sync, deadlocks, or shared state. license: MIT compatibility: Codex, Claude Code, and other Agent Skills-compatible clients. metadata: version: "0.1.0" displayName: Rust Concurrency Primitives category: Rust tags: rust,concurrency,threads,synchronization,atomics

Rust Concurrency Primitives

Use this skill to design thread-based Rust concurrency with explicit ownership, sharing, and synchronization. Prefer the simplest primitive that matches the coordination requirement before adding shared mutable state.

Core Workflow

1. Classify the work: independent fork-join, producer/consumer pipeline, shared state, one-time initialization, or low-level atomic coordination.
2. Prefer owned data per thread. Add shared ownership only when data must be observed or mutated by multiple threads.
3. Use std::thread::scope when child threads can borrow stack data and must finish before the function returns.
4. Use channels for ownership transfer and pipelines. Use Arc<Mutex<T>> or Arc<RwLock<T>> only when shared state is the clearer model.
5. Keep lock scopes short and never call user-controlled or blocking code while holding a lock unless that is the invariant being protected.
6. Use OnceLock or LazyLock for thread-safe one-time initialization instead of ad hoc global mutable state.
7. Treat atomics as a specialized tool. Use SeqCst by default until a weaker ordering is justified and documented.

Primitive Selection

Read references/threading-shared-state.md before introducing a new shared state primitive or reviewing deadlock-prone code.

Need	Primitive
Borrow local data into short-lived threads	std::thread::scope
Transfer work or results	std::sync::mpsc or project channel crate
Shared read/write state	Arc<Mutex<T>>
Many readers, rare writers	Arc<RwLock<T>>
Wait for condition changes	Condvar with Mutex
One-time global initialization	LazyLock or OnceLock
Counters, flags, lock-free coordination	std::sync::atomic

Safety And Review Rules

• Require Send for values crossing thread boundaries and Sync for shared references used from multiple threads.
• Decide whether poisoning should propagate panic or recover with PoisonError::into_inner.
• Establish a lock ordering when more than one lock can be acquired.
• Prefer Arc::clone(&value) over value.clone() when the cloned value is an ownership handle and readability matters.
• Use Rayon for data parallel iteration when the problem is pure CPU data parallelism and the project already accepts that dependency.

Tests

• Add deterministic tests around final state, message counts, and shutdown.
• Use barriers, channels, or scoped threads to coordinate tests; avoid sleeps.
• Add at least one test for panic, dropped sender/receiver, or cancellation behavior when the code depends on it.