@usableFromInline annotation can only be attached to
internal declarations. Why can't I attach it to
private ones too? Whether it's usable from inline should be completely orthogonal to access control (except in that
public implies usable from inline already).
For context, I've got a property that I want to make
private(set), because I need to preserve certain invariants when modifying it, but I also want to make a public function that modifies it