Additional suggestion for returning async closure Future is wrong

async fn f(c: impl Future<Output = ()>) {
    c.await
}
#[tokio::main]
async fn main() {
    f(async || {}()).await
}

Cargo clippy said:

error[E0618]: expected function, found `()`
 --> src/main.rs:6:16
  |
6 |     f(async || {}()).await
  |                ^^--                                   |                |
  |                call expression requires function
  |                                                     help: if you meant to create this closure and immediately call it, surround the closure with parentheses          |
6 |     f((async || {})()).await
  |       +           +

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:7
  |
6 |     f(async || {}()).await
  |     - ^^^^^^^^^^^^^ `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |     |
  |     required by a bound introduced by this call
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`
help: use parentheses to call this closure
  |
6 |     f(async || {}()()).await
  |                    ++

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:5
  |
6 |     f(async || {}()).await
  |     ^^--------^^^^^^
  |     | |
  |     | consider calling this closure
  |     `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:22
  |
6 |     f(async || {}()).await
  |       --------       ^^^^^ `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |       |
  |       consider calling this closure
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`

The 2nd suggestion is wrong, according to the 2nd suggestion to add parentheses after async closure can't fix the error. And the 1st suggestion will change to

error[E0618]: expected function, found `()`
 --> src/main.rs:6:16
  |
6 |     f(async || {}()()).await                          |                ^^--
  |                |
  |                call expression requires function

Only surrounding async closure

async fn f(c: impl Future<Output = ()>) {
    c.await
}
#[tokio::main]
async fn main() {
    f((async || {})()).await
}

fixes the error. But after fixing the error, clippy suggests

warning: try not to call a closure in the expression where it is declared                                        --> src/main.rs:6:7
  |                                                     6 |     f((async || {})()).await
  |       ^^^^^^^^^^^^^^^ help: try doing something like: `async {}`

If just insert async closure without parentheses after it

async fn f(c: impl Future<Output = ()>) {
    c.await
}
#[tokio::main]
async fn main() {
    f(async || {}).await
}

Clippy said:

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:7                                      |
6 |     f(async || {}).await
  |     - ^^^^^^^^^^^ `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |     |                                                 |     required by a bound introduced by this call
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`
help: use parentheses to call this closure
  |
6 |     f(async || {}()).await
  |                  ++

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:5
  |
6 |     f(async || {}).await
  |     ^^--------^^^^
  |     | |
  |     | consider calling this closure
  |     `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:20
  |
6 |     f(async || {}).await
  |       --------     ^^^^^ `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |       |
  |       consider calling this closure
  |
  = help: the trait `std::future::Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:1:20
  |
1 | async fn f(c: impl Future<Output = ()>) {
  |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`

For more information about this error, try `rustc --explain E0277`.

Which only suggests adding parentheses after async closure, no suggestion about surrounding async closure.

[reddevilmidzy]

reduce:

#![allow(unused)]
fn f(c: impl Future) {
}

fn main() {
    f(async || {}());
}

output

error[E0618]: expected function, found `()`
 --> src/main.rs:6:16
  |
6 |     f(async || {}());
  |                ^^--
  |                |
  |                call expression requires function
  |
help: if you meant to create this closure and immediately call it, surround the closure with parentheses
  |
6 |     f((async || {})());
  |       +           +

error[E0277]: `{async closure@src/main.rs:6:7: 6:15}` is not a future
 --> src/main.rs:6:7
  |
6 |     f(async || {}());
  |     - ^^^^^^^^^^^^^ `{async closure@src/main.rs:6:7: 6:15}` is not a future
  |     |
  |     required by a bound introduced by this call
  |
  = help: the trait `Future` is not implemented for `{async closure@src/main.rs:6:7: 6:15}`
note: required by a bound in `f`
 --> src/main.rs:2:14
  |
2 | fn f(c: impl Future) {
  |              ^^^^^^ required by this bound in `f`
help: use parentheses to call this closure
  |
6 |     f(async || {}()());
  |                    ++

Some errors have detailed explanations: E0277, E0618.
For more information about an error, try `rustc --explain E0277`.