Derive Macro for MapEntites (#17611)

[Wuketuke]

Objective

instead of writing boilerplate like this

impl MapEntities for Foo {
    fn map_entities<M: EntityMapper>(&mut self, entity_mapper: &mut M) {
        self.a.map_entities(entity_mapper);
        self.b.map_entities(entity_mapper);
    }
}

users can now use the derive macro MapEntities.
If any fields dont implement that trait, they can be skipped with a #[skip_mapping] attribute.

#[derive(MapEntities)]
struct Foo {
    a: A,
    b: B,
    #[skip_mapping]
    c: C, // C doesnt implement MapEntities, so we need to skip it
}

Fixes #17611

Solution

My implementation is a heavily simplified version of @Liam19's version, as I didnt want to add too much complexity at once, and I am not familiar with that trait. So feedback is appreciated.
Since its possible that people want to derive enums as well, I have put the derive function in its own file, so that it has space to grow

Testing

Theres now an example in the documentation

[waynemwashuma]

An extra # here

[Wuketuke]

to eliminate that double newline?

[Bleachfuel]

just continue without checking! This is unnessesary code

[Bleachfuel]

Suggested change

	'fields: for (i, field) in fields.iter().enumerate() {
	for (i, field) in fields.iter().enumerate() {

[Wuketuke]

the label is needed here, because later, we iterate over all attributes to find a "skip_mapping".
we could use an attr.iter().find(|attr| attr.path().is_ident("skip_mapping") to get rid of the label, but that hasnt made the code any easier to follow

[Bleachfuel]

Suggested change

	match &attr.meta {
	syn::Meta::Path(_) => continue 'fields,
	_ => panic!("just use the bare `#[skip_mapping]`"),
	}
	continue

[Wuketuke]

here i check that the user has really put a #[skip_mapping] and not #[skip_mapping(something random)]
it might not be necessary, but i prefer this tool to tell the user exactly how it wants to be used

[Bleachfuel]

Suggested change

	let map_field = if let Some(field_name) = &field.ident {
	// Named field (struct)
	quote! {
	<#ty as bevy_ecs::entity::MapEntities>::map_entities(&mut self.#field_name, entity_mapper);
	}
	} else {
	// Unnamed field (tuple-like struct)
	let idx = syn::Index::from(i);
	quote! {
	<#ty as bevy_ecs::entity::MapEntities>::map_entities(&mut self.#idx, entity_mapper);
	}
	};
	map_entities.push(map_field);
	let member = as_member(&field.ident, i);
	map_entities.push(quote! {
	<#ty as bevy_ecs::entity::MapEntities>::map_entities(&mut self.#member, entity_mapper);
	});

[Wuketuke]

i revisited this pr, and when merging from main, i noticed that this pr has already been made obsolete by #18432