An async runtime schedules asynchronous work and manages Future polling. When a Future is not ready, polling returns Poll::Pending; when it completes, polling returns Poll::Ready(value). The .await keyword asks the runtime to perform this process for you. section 19.3.2
A runtime becomes necessary when asynchronous code reaches the program boundary:
- Place .await inside an async function or async block; ordinary functions cannot use it directly. section 19.3.2
- Use a runtime to decide when suspended Futures should be polled again. section 19.3.2
- In common Rust programs, add #[tokio::main] so Tokio manages the asynchronous main body. section 19.3.3
Running an async function
use tokio;\n\nasync fn async_give_8() -> u8 {\n 8\n}\n\n#[tokio::main]\nasync fn main() {\n let number = async_give_8().await;\n println!("{number}");\n}\n\nThe macro keeps main a regular program entry point while creating a runtime that blocks until the async body finishes. For network calls such as reqwest requests, each asynchronous operation is followed by .await. section 19.3.3
use tokio; async fn async_give_8() -> u8 { 8 } fn main() { #1 let body = async { #2 let some_number = async_give_8().await; let second_number = async_give_8().await; { ::std::io::_print(format_args!("{0}, {1}\n", some_number, second_number)); }; }; { return tokio::runtime::Builder::new_multi_thread() #3 .enable_all() .build() .expect("Failed building the Runtime") .block_on(body); #4 } } #1 Look here—async fn is a lie! It’s actually just a regular fn main(). As far as Rust is concerned, the main() function is not async at all. #2 First, everything gets enclosed inside a big async block called body. The .await keyword can be used inside here. #3 Now the Tokio run time starts. It uses the builder pattern to set some configuration. #4 And, finally, the part that matters: a method called block_on(). Tokio is actually just blocking until everything has been resolved!
Inspect the expanded form to see the async block, Tokio runtime builder, and block_on call hidden behind #[tokio::main]. section 19.3.3
