Improve diagnostics for mismatched type in async main

[Aaron1011]

Previously, we wrapped the body of the async main function in an
async block, which we passed to block_on. However, block_on is
generic, so an incorrect return type ends up creating a diagnostic
pointing a block_on, not the user's code. Since the call to block_on
is generated by the #[tokio::main] macro, it ended up with a span of
the #[tokio::main] attribute, producing a confusing diagnostic.

We now wrap the body of the async main function in a new
async main_inner function. This asserts a return type of () earlier
on, producing a diagnostic.

Given this code:

#[tokio::main]
async fn main() {
    Ok(())
}

We currently produce the error:

error[E0308]: mismatched types
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^- help: try adding a semicolon: `;`
  | |
  | expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<(), _>`
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

With this PR, we produce:

error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
3 |     Ok(())
  |     ^^^^^^- help: try adding a semicolon: `;`
  |     |
  |     expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<(), _>`

Tokio doesn't appear to have any kind of UI testing setup, so I'm not sure how to test the error message produced here.

[Aaron1011]

I didn't realize that tokio::main could be used on non-main fuctions, which may have generics. I'l have to see if there's a better way to get the desired diagnostic.

[taiki-e]

Thanks for the PR. Unfortunately, this approach (split an async fn to two async or normal fns) causes a breaking change because #[tokio::main] accepts arguments. The self argument can be handled in the same way as async-trait, but we know that some regressions occur. (see dtolnay/async-trait#122 (comment))

[taiki-e]

If macro generate code like the following, it should work.

    block_on(async {
        let res: #return_ty = #body;
        return res;
    })

(Note: If the return type contains impl trait, macro should omit the generation of : #return_ty)

[Darksonn]

@Aaron1011 Are you interested in continuing to work on this PR?