Async/Await

Rust's async model lets you write concurrent code that waits for I/O or other work without blocking a thread. An async fn returns a Future — a value that represents work still in progress. The .await keyword pauses until that work finishes.

Futures don't run on their own: an async runtime (such as Tokio) polls them to completion. Rustlings uses Tokio (via the trpl crate) for these exercises.