New lint: unused-async

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What it does

Checks for functions that are declared async but have no .awaits inside of them.

Categories (optional)

• Kind: pedantic

Reduces code smell and overhead by encouraging async functions to be refactored to synchronous functions.

Drawbacks

There are valid cases where a programmer may want to wrap synchronous code in an asynchronous future: for example, spawning a synchronous task on an asynchronous scheduler, or implementing an async function for a trait where the implementation is synchronous. These cases are the reason this lint is pedantic. However, there are a few additional steps I want to take with this lint to avoid causing hassle or confusion for programmers:

• This lint should avoid trait functions, as there are valid reasons why an async trait function might have a non-async implementation.
• This lint should recommend to the programmer the proper way to make an synchronous task into a future: using the async {} block.

Example

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");
    }
}

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I'd like to implement this myself, if possible. Please let me know if this lint is a bad idea, or if you have any suggestions for improvement.

@rustbot claim