Change ChildOf to Childof { parent: Entity} and support deriving Relationship and RelationshipTarget with named structs

[Bleachfuel]

Objective

fixes #17896

Solution

Change ChildOf ( Entity ) to ChildOf { parent: Entity }

by doing this we also allow users to use named structs for relationship derives, When you have more than 1 field in a struct with named fields the macro will look for a field with the attribute #[relationship] and all of the other fields should implement the Default trait. Unnamed fields are still supported.

When u have a unnamed struct with more than one field the macro will fail.
Do we want to support something like this ?

 #[derive(Component)]
 #[relationship_target(relationship = ChildOf)]
 pub struct Children (#[relationship] Entity, u8);

I could add this, it but doesn't seem nice.

Testing

crates/bevy_ecs - cargo test

Showcase

use bevy_ecs::component::Component;
use bevy_ecs::entity::Entity;

 #[derive(Component)]
 #[relationship(relationship_target = Children)]
 pub struct ChildOf {
     #[relationship]
     pub parent: Entity,
     internal: u8,
 };

 #[derive(Component)]
 #[relationship_target(relationship = ChildOf)]
 pub struct Children {
     children: Vec<Entity>
 };

[alice-i-cecile]

When u have a unnamed struct with more than one field the macro will fail.
Do we want to support something like this ?

No, that's unpleasant, and trivially worked around.

[alice-i-cecile]

Nice work :) Thanks a ton for finishing this up; you're much more adept with the macros than I am!

[cart]

Well done! Thanks for putting this together!