Simplify Service Trait with Async Trait in Next Major Version

[cyborg42]

The current Service trait is defined as:

pub trait Service<Req> {
    type Response;
    type Error;
    type Future: Future<Output = Result<Self::Response, Self::Error>>;

    fn poll_ready(&self, ctx: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>>;
    fn call(&self, req: Req) -> Self::Future;
}

In the next major version, we could leverage async traits to simplify the trait definition and improve ergonomics. Proposed change:

pub trait Service<Req> {
    type Response;
    type Error;

    fn poll_ready(&self, ctx: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>>;
    fn call(&self, req: Req) -> impl Future<Output = Result<Self::Response, Self::Error>>;
}

This allows implementations like:

struct MyMiddleware;

impl<Req> Service<Req> for MyMiddleware {
    type Response = ();
    type Error = ();
    
    fn poll_ready(&self, ctx: &mut task::Context<'_>) -> Poll<Result<(), Self::Error>> {
        todo!()
    }
    
    async fn call(&self, req: Req) -> Result<Self::Response, Self::Error> {
        todo!()
    }
}

Benefits:

• Eliminates the need to define a Future associated type.
• Simplifies implementation by using async fn directly.
• Improves readability and maintainability.

We can also use async trait for ServiceFactory and Transform