Macro for_each_component 

macro_rules! for_each_component {
    ($cb:ident) => { ... };
    ($cb:ident; $($extra:tt)*) => { ... };
}

The one component list. $cb is a macro that receives the Variant => Type entries and expands to whatever registry it builds from them. Type paths are absolute so the list resolves from any crate that consumes it.

The $cb; $extra form prepends arbitrary tokens to what $cb receives, so a consumer holding a group this crate does not name (the authoring-only vocabulary in concinnity-cook) can hand its own list to the same callback.