Make Option<F> a filter when F is a filter

[futursolo]

Is your feature request related to a problem? Please describe.
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I wish to make a route / filter that is enabled based on command line argument.
However, I find it is kind of cumbersome to have a filter optionally enabled.

I can currently find 3 solutions:

1. boxing filter and assigning to variable

let mut f = base_filter().boxed();

// invoked with --with-extra-feature
if args.with_extra_feature {
    f = f.or(extra_filter()).unify().boxed();
}

f = f.or(other_filters());

However, this requires coercing filter to the same exact type, which means calling .unify().boxed() after every or() call and all filters needs to call into_response() manually to convert to a Response.

2. use reject() when this filter is not needed.

base_filter()
    .or(
        if args.with_extra_feature {
            extra_filter().boxed()
        } else {
            reject_as_response().boxed()
        }
    )

However, this still requires me to make a custom reject filter that returns a Extract = Response.

3. check with_extra_feature at each request

I would like to avoid this option as it is only determined by a factor that does not change after the program starts.

Describe the solution you'd like
A clear and concise description of what you want to happen.

If we make Option<F> where F: Filter implement Filter, where None always reject.

Then the above can be simplified to:

let f = base_filter()
    .or(args.with_extra_feature.then(extra_filter))
    .or(other_filters());

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

1. or_maybe

Implements a method like or, but accepts an Option<F> where None ignores the filter (rejects?).

let f = base_filter()
    .or_maybe(args.with_extra_feature.then(extra_filter))
    .or(other_filters());

2. Either<L, R>

Similar to futures::future::Either, but accepts 2 filters instead.

let f = base_filter();

let f = if args.with_extra_feature { f.or(extra_filter()).left() } else { f.right() };

Additional context
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N/A