Or<T> should be a new type of PhantomData<T>

[orzogc]

Objective

Or<T> should be a new type of PhantomData<T> instead of T.

Solution

Make Or<T> a new type of PhantomData<T>.

Migration Guide

Or<T> is just used as a type annotation and shouldn't be constructed.

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

Tricky. I thought this could be used to fetch data as well, much like AnyOf.

In that case, I would expect the stored type to be a collection of Option<T::Item>, not PhantomData<T>. But that's incorrect: the following code compiles.

use bevy::prelude::*;

fn my_system(query: Query<Or<(&Transform, &BackgroundColor)>>) {
    let query_item: bool = query.single();
}

TL;DR: this is the right fix. #7680 strikes again.

[alice-i-cecile]

(I have no idea what to put in the migration guide, but this is technically a breaking change)