Better account for invalid async fn main() {}

The following

async fn main() {
}

produces a lot of output that is not quite good:

error[E0277]: `main` has invalid return type `impl Future`
 --> src/main.rs:1:17
  |
1 | async fn main() {
  |                 ^ `main` can only return types that implement `Termination`
  |
  = help: consider using `()`, or a `Result`

error[E0752]: `main` function is not allowed to be `async`
 --> src/main.rs:1:1
  |
1 | async fn main() {
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

The two errors (E0277 and E0752) should be only a single error.

(Taken from a subset of https://users.rust-lang.org/t/how-can-we-return-a-anonymous-type-in-result-trait/51258)

Output on rustc 1.80.0:

error[E0752]: `main` function is not allowed to be `async`
 --> projects/rust/test.rs:5:1
  |
5 | async fn main() {
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0752`.

The two errors (E0277 and E0752) should be only a single error.

Output on rustc 1.80.0:

Triage: Since the expected behavior is now in place, we can close this issue now, right?

Triage: Looks like we don't have a test for this, so we just need to add a test.

@rustbot

@rustbot claim

Rollup merge of rust-lang#162616 - malezjaa:add-async-main-regression…

Unrolled build for #162616 in rollup 162622

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Better account for invalid `async fn main() {}` #78905

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Better account for invalid async fn main() {} #78905

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veera-sivarajan commented on Jul 26, 2024

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Better account for invalid `async fn main() {}` #78905