Commits on Oct 24, 2020

1. Improve diagnostics for mismatched type in async main …

   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:
   
   ```rust
   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<(), _>`
   ```

   

   Aaron1011 committed Oct 24, 2020
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