macros: fix the clippy::needless_return lint triggered by #[tokio::test] and #[tokio::main]

[Veetaha]

Motivation

I started seeing clippy::needless_return lints in a CI job that runs the nightly cargo clippy against my code (CI link):

error: unneeded `return` statement
Error:   --> bon/tests/integration/builder/mod.rs:53:27
   |
53 |     assert_eq!(actual, 42);
   |                           ^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_return
   = note: `-D clippy::needless-return` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_return)]`
help: remove `return`
   |
53 |     assert_eq!(actual, 42)assert_eq!(actual, 42);
   |                           ~~~~~~~~~~~~~~~~~~~~~~~

That test doesn't do anything fancy:

#[tokio::test]
async fn async_fn() {
    #[builder]
    async fn sut(arg: u32) -> u32 {
        std::future::ready(arg).await
    }

    let actual = sut().arg(42).call().await;
    assert_eq!(actual, 42);
}

I think clippy started covering more cases of "needless return" where the function has a return for an expression that returns a unit type.

Solution

Added an allow for the lint to the generated code.

Questions

I see there are no CI jobs that exercise the clippy on nightly. Would it be fine to add one to catch regressions from nightly early? The job doesn't need to be required to pass. It can be continue-on-error: true and shouldn't block PRs from merging (like I do in my repo).

[Darksonn]

This is a duplicate of #6874.