Allow lens derive to add consts for custom lenses

[cmyr]

It would be nice if, when writing a custom lens, you could annotate the type so
that the derive macro would include the custom lens along with the derived
lenses. This would look something like (forgive my contrived example):

#[derive(Lens)]
#[lens(custom(speed_kmph = "lenses::Kmph"))]
struct Arrow {
    // a unit vector
    heading: Vec2,
    // in mph
    speed: f64,
}

mod lenses {
    struct Kmph;

    const KM_PER_MILE: f64 = 1.609_344;

    impl Lens<Arrow, f64> for MilesToKm {
        fn with<R, F: FnOnce(&f64) -> R>(&self, data: &Arrow, f: F) -> R {
            let kms = data.speed * KM_PER_MILE;
            f(&kms)
        }

        fn with_mut<R, F: FnOnce(&mut f64) -> R>(&self, data: &mut Arrow, f: F) -> R {
            let mut kms = data.speed * KM_PER_MILE;
            let kms_2 = kms;
            let result = f(&mut kms);
            // avoid doing the conversion if unchanged, it might be lossy?
            if !kms.same(&kms_2) {
                let miles = kms * KM_PER_MILE.recip();
                data.speed = miles;
            }
            result
        }
    }
}

and then I would be able to access this lenes as Arrow::speed_kmph.

[luleyleo]

May I ask what the benefit is? That should be all that's needed, right?:

impl Arrow {
    pub const speed_kmph: lenses::Kmph = lenses::Kmph;
}

Or do I miss something?

[cmyr]

yea, it's just a convenience, and encourages people to actually hang their lenses off of a given type, instead of having them floating around (which is the current situation in runebender)