SystemParam QueryRecursive for safe recursive iteration.

[mintlu8]

Objective

Recursive iterating through the hierarchy while keeping a reference to parent requires unsafe, as seen in propagate_transform in bevy_transform. This PR alleviates this issue by generalizing recursive iteration into a safe interface.

Solution

Added QueryRecursive that contains 2 queries from 5 generic parameters.

root: Query<(Shared, Root), RootFilter>
child: Query<(Shared, Child), ChildFilter>

QueryData Shared gets passed from parent to children as a reference during iteration.

Additionally we support passing evaluation results from parent to children in iter_mut_with.

Example: transform pipeline (naive implementation)

fn propagate_transforms(mut query: QueryRecursive<
    (&Transform, &mut GlobalTransform),
    (),
    (),
    Without<Parent>,
    ()
>) {
    query.for_each_mut(
        |(transform, global_transform), ()|
            **global_transform = (**transform).into(),
        |(_, parent), (transform, global_transform), ()|
            **global_transform = parent.mul_transform(**transform),
    );
}

Testing

Added some tests.

---

Changelog

Added QueryRecursive.

[iiYese]

Aside from the API being IMO a bit rough & needing more design the issue with this is it's a stopgap that can be done via third party. Specifically:

• I already have safe wrappers for this in aery
• Aery also has hierarchy compatibility

So the question then is what additional value would there be to providing this stopgap as a first party? Bare in mind this is already a kind of niche scenario simply due to the fact that hierarchy right now is very limited.

[mintlu8]

Aside from the API being IMO a bit rough & needing more design the issue with this is it's a stopgap that can be done via third party.

I think this PR is very important since

• Third party unsafe code has less scrutiny and therefore less trust worthy, especially if the author failed to account for changes in new bevy releases.

• This feature is used in bevy itself, for instance the transform pipeline and visibility pipeline can be rewritten using this PR if proven safe. In the future if we upstream bevy_mod_billboard or add styling inheritance this can be useful as well.

• This PR is not the full picture. A filter method for DFS and/or parallel iteration can be added to compliment QueryRecursive.

I welcome suggestion to changes in the API.