New lint: unused_async

[snowsignal]

changelog: Adds a lint, unused_async, which checks for async functions with no await statements

unused_async is a lint that reduces code smell and overhead by encouraging async functions to be refactored into synchronous functions.

Fixes #7176

Examples

async fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}

Could be written as:

fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}

Something like this, however, should not be caught by clippy:

#[async_trait]
trait AsyncTrait {
    async fn foo();
}

struct Bar;

#[async_trait]
impl AsyncTrait for Bar {
    async fn foo() {
        println!("bar");
    }
}

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[llogiq]

Thank you for this lint. I think there might be other forms of false positives besides being a trait functions. One could have a pointer to an async function and the type of that pointer might change if the function is no longer async. But as this is a pedantic lint, that's no need for concern.

@bors r+

[bors]

📌 Commit 75ef9dc has been approved by llogiq

[bors]

⌛ Testing commit 75ef9dc with merge acdf43f...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing acdf43f to master...