Async Rust

Async Rust fails in three places: an executor thread that gets blocked, a guard held across an await point, and a future dropped mid-operation. This skill governs correctness under concurrency and cancellation — it does not decide whether the code should be async in the first place.

Async is not free concurrency

An async fn yields a future that does nothing until polled. Concurrency comes from the runtime and from combining futures — join!, select!, spawn — not from the async keyword:

// Serial: the second fetch starts only after the first completes.
let a = fetch_user(id).await;
let b = fetch_orders(id).await;