Rust error E0728: await is only allowed inside async functions and blocks

Q: What does Rust error E0728 mean?

.await can only be used inside an async fn or an async { } block.

Q: How do I fix E0728 (await is only allowed inside async functions and blocks)?

Make the function async: And await it from its caller. Async main with a runtime: Use a loop instead of a closure: Or futures::future::join_all for concurrency.

What it means.await can only be used inside an async fn or an async { } block.

What rustc prints

error[E0728]: `await` is only allowed inside `async` functions and blocks
 --> main.rs:6:27
  |
5 | fn foo() {
  | -------- this is not `async`
6 |     wake_and_yield_once().await // `await` is used outside `async` context
  |                           ^^^^^ only allowed inside `async` functions and blocks

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

Why it happens

Calling .await in main or another normal function.
Awaiting inside a closure passed to map or for_each, which is not async.

How to fix it

1. Make the function async

And await it from its caller.

async fn load() -> String {
    fetch().await
}

2. Async main with a runtime

#[tokio::main]
async fn main() {
    let s = fetch().await;
}

3. Use a loop instead of a closure

Or futures::future::join_all for concurrency.

for url in urls {
    let body = get(url).await?;
}

Other wordings of this error

await is only allowed inside async functions and blocks, only allowed inside `async` functions and blocks

E0728Everything else

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

await has been used outside async function or async block.

Erroneous code example:

fn foo() {
    wake_and_yield_once().await // `await` is used outside `async` context
}

await is used to suspend the current computation until the given future is ready to produce a value. So it is legal only within an async context, like an async function or an async block.

async fn foo() {
    wake_and_yield_once().await // `await` is used within `async` function
}

fn bar(x: u8) -> impl Future<Output = u8> {
    async move {
        wake_and_yield_once().await; // `await` is used within `async` block
        x
    }
}

From rustc --explain E0728 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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