What is await with question mark (await?) in Rust?

November 9, 2022

I read this portion of the book on Async programming in Rust https://rust-lang.github.io/async-book/03_async_await/01_chapter.html

I see the mention of the .await syntax,

and then I later saw a blog where reqwest is being used to fetch a url. The code looks like this:

let resp200 = client.get("https://httpbin.org/ip")
    .header(CONTENT_TYPE, "application/json")
    .send()
    .await?
    .json::<GETAPIResponse>()
    .await?;

I do not understand the .await?. I know ? is a shorthand for extracting the Ok case or returning from the function with an Error. But as far as I am aware, .await does not return Result so how is it possible to do .await with ?

>Solution :

.await simply turns a impl Future<Output = T> into a T.

? (mostly) turns a Result<T, E> into a T.

When put together, .await? turns a impl Future<Output = Result<T, E>> into a T.

The following code is equivalent:

let future = client.get("https://httpbin.org/ip")
    .header(CONTENT_TYPE, "application/json")
    .send();
let result: Result<_, _> = future.await;
let response: Response = result?;
let json_future = response.json();
let json_response: Result<_, _> = json_future.await;
let json = json_response?;

But, .await? is not a special operator. It’s literally just .await followed by ?