Using arena for memory allocation per request.

[Jason5Lee]

Recently I found some arena libraries. I really like the idea behind it. I can use an arena through a request. I can alloc a data and shared the reference across many types as long as they are local to the request. I don't need to worry too much about lifetime, as all the data lives through the request.
This method works well for non-streaming body, I just need to write the handler like:

fn handler(request: Request) -> impl Responder {
    let arena = Arena::new();
    let result = do_my_work(&arena);
    HttpResponse::Ok().json(result)
}

The problem is for the streaming body. Since the content of the stream might rely on the arena, the stream only lives no longer than the arena. If arena is created locally, the stream won't live long enough.

So I hope I can write handler like this

fn handler<'a>(arena: &'a Arena, request: Request) -> impl Responder + 'a {
   ...
}

The type and the creation of the arena can be set by user (builtin is OK if it is not convinient), and it will be created before executing handler, dropped after the respond is fully transmitted to the client. In this case responding a stream becomes much easier.