Why not return Pending struct instead of impl Future<Output = Result<Response, Error>>

[JadKHaddad]

Why does Client::execute(request: Request) not simply return the Pending struct?:

pin_project! {
    pub struct Pending {
        #[pin]
        inner: PendingInner,
    }
}

it returns impl Future<Output = Result<Response, Error>>

Background

I am trying to pin the outgoing request in a struct that implements Future. With tokio::sleep() I can pin the Sleep struct returned by the function without using Pin<Box<dyn Future>> . Is it a design decision to return an impl Future and hide the Pending struct? Wouldn't it be helpful to return a concrete type instead?

Thanks!

[seanmonstar]

It's for a similar reason why in Java or C# you might return an interface instead of a class, or similarly in any other language. Sure you can conceive of reasons you as a user might like the concrete type, there are also benefits to the library for only promising specific behavior of the return type.

By returning an impl Future, we can change what the exact type is. We can refactor it to be an async fn or similar.