Pull bevy_ecs::label out into its own separate crate

[ItsDoot]

What problem does this solve or what need does it fill?

The label infrastructure that bevy_ecs provides is not actually dependent on anything ECS-related, and could find use as a separate crate from bevy_ecs.

What solution would you like?

Introduce a bevy_label crate that holds everything currently defined in bevy_ecs::label.

What alternative(s) have you considered?

Leave it as-is.

[ilyvion]

It feels odd to me that this is marked with both C-Code-Quality A section of code that is hard to understand or change , described as "A section of code that is hard to understand or change" and D-Straightforward Simple bug fixes and API improvements, docs, test and examples , described as "Simple bug fixes and API improvements, docs, test and examples."

Is it hard to understand or change; or is it simple?

[hukasu]

i actually uses bevy_ecs::intern

bevy/crates/bevy_ecs/src/label.rs

Lines 139 to 146 in 11db717

	fn intern(&self) -> $crate::intern::Interned<dyn $label_trait_name>
	where Self: Sized {
	$interner_name.intern(self)
	}
	}
	#[diagnostic::do_not_recommend]
	impl $label_trait_name for $crate::intern::Interned<dyn $label_trait_name> {

[ItsDoot]

Yea, we would pull out bevy_ecs::intern into the new crate as well. It's only used by labels presently.

[hukasu]

it is also used on the derive macros

bevy/crates/bevy_ecs/macros/src/lib.rs

Lines 558 to 582 in 2f38464

	pub fn derive_schedule_label(input: TokenStream) -> TokenStream {
	let input = parse_macro_input!(input as DeriveInput);
	let mut trait_path = bevy_ecs_path();
	trait_path.segments.push(format_ident!("schedule").into());
	let mut dyn_eq_path = trait_path.clone();
	trait_path
	.segments
	.push(format_ident!("ScheduleLabel").into());
	dyn_eq_path.segments.push(format_ident!("DynEq").into());
	derive_label(input, "ScheduleLabel", &trait_path, &dyn_eq_path)
	}
	/// Derive macro generating an impl of the trait `SystemSet`.
	///
	/// This does not work for unions.
	#[proc_macro_derive(SystemSet)]
	pub fn derive_system_set(input: TokenStream) -> TokenStream {
	let input = parse_macro_input!(input as DeriveInput);
	let mut trait_path = bevy_ecs_path();
	trait_path.segments.push(format_ident!("schedule").into());
	let mut dyn_eq_path = trait_path.clone();
	trait_path.segments.push(format_ident!("SystemSet").into());
	dyn_eq_path.segments.push(format_ident!("DynEq").into());
	derive_label(input, "SystemSet", &trait_path, &dyn_eq_path)
	}

[hukasu]

i already reached a working solution, just not that happy with it