Just had a chance to play with @Observable and I noticed that structs cannot be equatable:
@Observable
struct Foo: Equatable {}
// 🛑 Type 'Foo' does not conform to protocol 'Equatable'
The only reason why it isn't equatable is because ObservationRegistrar is not. I suppose ObservationRegistrar could trivially be made equatable by always returning true from ==.
Does that seem reasonable to do? And if not, what is the use case of observable structs? It doesn't seem very useful if they can't be made equatable, hashable, codable, etc…. Should @Observable restrict to only AnyObject?