async functions return a Future

async-await is lazy in Rust / Rust futures are state machines

async functions do not run an operation immediately when called but rather they return a Future, which is a value representing the operation.

The operation runs when .await is called.

async fn say_world() {
  println! ("world");
}
 
#[tokio::main]
async fn main() {
  let op = say_world();
 
  println!("hello");
 
  op.await;
}

Prints:

hello
world

Reference: Hello Tokio

Unlike how futures are implemented in other languages, a Rust future does not represent a computation happening in the background, rather the Rust future is the computation itself.

Reference: Async in depth