Conditional Routing

Is your feature request related to a problem? Please describe. I'm always frustrated when trying to write a conditional chain of filters using or.

Describe the solution you'd like from the examples, we have this...

pub fn assets_filter() -> BoxedFilter<(impl Reply,)> {
    warp::path("assets")
        .and(warp::fs::dir("./assets"))
        .boxed()
}

I'd like the ability to do something like this...

pub fn assets_filter() -> BoxedFilter<(impl Reply,)> {
    if let Some(optional_filter) = optional_filter {
        warp::path("assets")
            .and(warp::fs::dir("./assets"))
            .or(optional_filter)
            .boxed()
    } else {
        warp::path("assets").and(warp::fs::dir("./assets")).boxed()
    }
}

Describe alternatives you've considered I've tried just about everything to make types work out. While it's ok to return BoxedFilter<(impl Reply,)>, it's not OK to return two different variants of this from the fn body.

Additional context This was raised and closed in https://github.com/seanmonstar/warp/issues/699, but does not seem like a solution for the rest of us.