error[E0277]: `main` has invalid return type `impl Future`
  --> $DIR/issue-68523.rs:3:20
   |
LL | async fn main() -> Result<i32, ()> {
   |                    ^^^^^^^^^^^^^^^ `main` can only return types that implement `Termination`
   |
   = help: consider using `()`, or a `Result`

error[E0752]: `main` function is not allowed to be `async`
  --> $DIR/issue-68523.rs:3:1
   |
LL | async fn main() -> Result<i32, ()> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0277, E0752.
For more information about an error, try `rustc --explain E0277`.