`SystemLabel` proc-macro no longer allows for generic enums

[SarthakSingh31]

Bevy version

Since c43295a

The Problem

I was trying to update bevy_mod_raycast to bevy main but ran into a problem with this

https://github.com/bevyengine/bevy/pull/4957/files#diff-b131f1941aa2edc166d44e3411ad3e04c125bd541288357846e6c2e018b46b80R133-R142

change to the SystemLabel proc-macro.

https://github.com/aevyrie/bevy_mod_raycast/blob/0a5d1e3334285eaf85e97adc28b14cbb50ab1b48/src/lib.rs#L82-L89

needs a variant with a data field to store the PhantomData but this change makes that impossible.

Possible solutions

1. Make the SystemLabel proc-macro detect if the variant is unreachable by insuring that it has a field with type ::std::convert::Infallible.
2. Add a ignore attribute to the SystemLabel proc-macro.

[alice-i-cecile]

Broken by #4957. @JoJoJet, any opinions?

[joseph-gio]

@alice-i-cecile the attribute idea seems more promising. I'll claim this if that's ok.

[joseph-gio]

I don't want to magically change behavior based on type names since that can be potentially surprising. Instead I think the second suggestion or something similar to it would be best.

#[derive(Label)]
enum MyEnum {
    One,
    #[label(ignore_fields)]
    Two(/* Any data, potentially non-phantom */),
}

This would simply tell the derive macro to ignore the fields of that variant when comparing labels. Though I suspect this may have implications for equality which I'll look into more.