On dependencies… I generally think it would be nice if a target could declare that the values in any particular attribute (say, dependencies) must be a subtype of some list of types. So

jvm_binary

could say “If anything listed in dependencies is not a

JvmTarget

, error”). (Also, it would be nice if all attributes were optional, so if a target type shouldn’t have dependencies, writing dependencies on it at all should be an error)