`EitherBody` helper

[damooo]

It will help greatly with middleware implementations, when one middleware may override responses.

parallel to actix_http::body::EitherBody

[indietyp]

Here's a (probably incomplete) version of EitherBody. Happy to clean it up and create a PR if wanted: https://gist.github.com/indietyp/0f7a707ada980f69cdab957e5d2933a6

[davidpdrsn]

Either is a little annoying since it's an enum with tuple variants, which pin-project-lite doesn't support. So requires bringing in pin-project or writing the unsafe code.

But I agree it would be good to have.

[indietyp]

We could also extend this by just saying

struct Either<L, R> {
   Left {
     #[pin]
     left: L
   },
   Right {
     #[pin]
     right: R
   }
}

which would be supported by pin-project-lite. I am still contemplating if it might be better to restrict the type Data generic in my implementation to a single Buf, instead of Either.

[davidpdrsn]

I don't like that. You don't really see that anywhere else. I'd rather copy the unsafe code that pin-project generates.

[indietyp]

Okay then I'll go ahead and create a PR.