(macros): improve error message for return type mismatch in #[tokio::main]

[daschinmoy21]

Motivation

Fixes #7784
When using #[tokio::main] with conflicting return statements, the error message was misleading:

#[tokio::main]
async fn main() -> () {
    if true {
        return 1;
    }
    return ();
}

The compiler would report:

error[E0308]: mismatched types
 --> src/main.rs:7:12
  |
7 |     return ();
  |            ^^ expected integer, found `()`

This blames return () (which matches the declared -> ()) instead of return 1.

Solution

Added a check_output identity function that explicitly constrains the async block's output type to match the declared function return type:

fn check_output<F: Future<Output = #output_type>>(f: F) -> F { f }
check_output(body)

This ensures the compiler reports type mismatches against the declared return type rather than an inferred type. The function has zero runtime overhead since it just returns its input unchanged.

After this change, the error is clearer:

error[E0308]: mismatched types
 --> src/main.rs:6:12
  |
6 |     return ();
  |            ^^ expected integer, found `()`
  |
note: return type inferred to be `{integer}` here
 --> src/main.rs:4:16
  |
4 |         return 1;
  |                ^

error[E0271]: expected `{async block@src/main.rs:1:1: 1:15}` to be a future that resolves to `()`, but it resolves to `{integer}`
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^ expected `()`, found integer
  |
note: required by a bound in `check_output`

[taiki-e]

As said in #6306 (comment) (and #3039 (comment) and #5619 (comment) and #6438 (comment)), this needs to handle cases where the impl trait is included in Output (tokio::main can be used for functions other than the main function). Alternatively, for now, you could restrict its use to functions that return a unit.

(I've mentioned this point repeatedly, so we should probably add a test case like the following.)

#[tokio::main]
async fn never() -> ! {
    loop {}
}
#[tokio::main]
async fn impl_trait() -> impl Iterator<Item = impl core::fmt::Debug> {
    [()].into_iter()
}

[daschinmoy21]

Updated the PR as requested by @taiki-e
All checks pass.including test cases added in #7857 ,thanks to @Darksonn

[daschinmoy21]

Added another test case where impl trait can also be placed inside other types, so previous check wasn't sufficient.

#[tokio::main]
async fn impl_trait2() -> Result<(), impl core::fmt::Debug> {
    Err(())
}

updated code and test case to cover this

[taiki-e]

LGTM, thanks @daschinmoy21!

[daschinmoy21]

any update on this @taiki-e ?