State arguments cannot be Query-, CommandBuffer-, SubWorld, or Entity-typed

[tel]

Consider this example

#[system]
fn do_a_thing(#[state] target: &Entity) { ... }

this may be a system which is focused on working around a single entity; its starting entity is fed in during system init do_a_thing_system(ent). Today, this fails with the error "error: simple systems cannot contain component references, consider using #[system(for_each)]" which is somewhat confusing. I am not intending to reference a component.

Of course, Entity is a magic component as far as #[system] is concerned and ends up being referenced specially when it shows up in a type. The workaround is to produce a new, distinct type struct Target(Entity), take that as the state type fn do_a_thing(#[state] target: &Target) { ... } and then build it with that type do_a_thing_system(Target(ent)).

This workaround isn't too bad, and maybe is even the "ideal" way to handle this situation all together.

Expected behavior

Either (a) the #[state] attribute overrides type-based dispatch or (b) at the very least the error is more clear noting how &Entity refs are special-cased.