Rust Async Internals

Decision table

You must	Use	Do not use
Run N futures and keep only the first result	`tokio::select!`	`join!`
Run N futures to completion	`tokio::join!` or `try_join!`	`select!`
Own a set of spawned tasks and collect their results	`JoinSet` + `join_next`	a bare `Vec<JoinHandle>`
Track a long-lived, unbounded task set (an accept loop) for shutdown	`tokio_util::task::TaskTracker` + `close()` + `wait()`	a `JoinSet` that nobody drains; it keeps every result
Abort a task when its owner drops the handle	`tokio_util::task::AbortOnDropHandle`	a bare `JoinHandle`; its drop detaches the task
Bound the concurrency of a work stream	`stream::iter(..).buffer_unordered(K)`	`spawn` in an unbounded loop
Shut down a task tree	`CancellationToken` + `child_token()`	`Notify`
Cancel on any early return or panic	`token.drop_guard()`	manual cleanup at each `?`
Race work against shutdown and keep the value	`token.run_until_cancelled(fut)`	a hand-written `select!`
Do bounded blocking or CPU work	`spawn_blocking`	`block_in_place`
Run an indefinite blocking loop	`std::thread::spawn`	`spawn_blocking`
Deliver every message without loss	`mpsc`	`broadcast`
Keep a critical `.await` sequence atomic	`tokio::spawn` + join the handle	`select!` around it