But the instances (of the objects) don’t actually share the value - it’s actually encapsulated away from the objects themselves and exists only in that let scope. My main worry would be if there was any performance issue with doing it this way, other than that I would feel comfortable with it. I’m pretty sure that I don’t want it as a global variable like const would be.
As for mutable vs immutable I don’t worry too much about it as long as I can decide and that it is clear when I do decide. Sometimes like this I definitely want mutability, at other times I don’t. I also think having something as const and mutating it in what looks like an underhanded way seems worse. It’s sending two conflicting messages to the reader - that something is constant but not really. If something is a constant shouldn’t it really be a constant?