Bad error message with async main

[Nokel81]

If someone tries to do the following async fn main() -> Result<(), Box<dyn std::error::Error> the error message is not helpful (though it is technically correct).

error[E0277]: `main` has invalid return type `impl futures::Future`
  --> rs/agent-updater/src/main.rs:46:20
   |
46 | async fn main() -> Result<(), Box<dyn std::error::Error>> {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` can only return types that implement `std::process::Termination`
   |
   = help: consider using `()`, or a `Result`

I think that it should point to the async keyword and say that standard rust does not support async main.

This issue has been assigned to @Nokel81 via this comment.

[Centril]

Implementation notes:

• This should probably be checked wherever we type check fn main.
• We need to propagate the Spanned from AST (https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.FnHeader.html#structfield.asyncness) to HIR (https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.FnHeader.html#structfield.asyncness)

cc @estebank who has dealt with fn main diagnostics recently.

[estebank]

This is a good first-contribution ticket I can help with.

[Nokel81]

I will take you up on that offer Wednesday evening if that is good with you.

[Centril]

@rustbot assign @Nokel81

[Nokel81]

@rustbot claim