Guides concurrency and async decisions in Rust. Use when encountering Send/Sync errors (E0277), choosing between threads vs async, designing shared state, or debugging deadlocks.
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Layer 1: Language Mechanics
Is this CPU-bound or I/O-bound, and what's the sharing model?
Before choosing concurrency primitives:
Before adding concurrency:
What's the workload?
std::thread, rayon)What's the sharing model?
Arc<T>Arc<Mutex<T>> or Arc<RwLock<T>>What are the Send/Sync requirements?
SendSyncspawn_localWhat type of work?
├─ CPU-bound → std::thread or rayon
├─ I/O-bound → async/await
└─ Mixed → hybrid (spawn_blocking)
Need to share data?
├─ No → message passing (channels)
├─ Immutable → Arc<T>
└─ Mutable →
├─ Read-heavy → Arc<RwLock<T>>
└─ Write-heavy → Arc<Mutex<T>>
└─ Simple counter → AtomicUsize
Async context?
├─ Type is Send → tokio::spawn
├─ Type is !Send → spawn_local
└─ Blocking code → spawn_blocking
| 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> |
| 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 |
// Bad: guard held across await
let guard = mutex.lock().await;
do_async().await; // guard still held!
// Good: scope the lock
{
let guard = mutex.lock().await;
// use guard
} // guard dropped
do_async().await;
// Rc is !Send, can't cross await in spawned task
// Option 1: use Arc instead
// Option 2: use spawn_local (single-thread runtime)
// Option 3: ensure Rc is dropped before .await
| 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? |
| 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 |
ownership — data ownership and borrowingperformance — choosing between rayon, threads, and async for throughputanti-patterns — common concurrency mistakesNo reviews yet. Tell others what you thought.
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