Sendable

What can go to another thread, which the compiler decides: numbers, Bools, Strings, Enums, Callbacks, Channels, Mutexes, Atomics, and a Shared Class's objects and Weaks of them, and the Structs, tuples, and fixed arrays whose fields and elements all are, with a T? of any of them. A type can list it, or an Interface that extends it, to have that checked.