Unless we make some special immutable types as mentioned above; then we could share those. Maybe I'm just prematurely optimizing, and should just give up on trying to find ways to avoid the copy. Life is much simpler just copying. :)

How about just starting with always-copy, and later when we have performance data, and experience using it, we can code exceptions? I don't know how easy it would be to code such exceptions, but it would be less likely to break if we didn't make these decisions without empirical evidence.

>twice the overhead... I didn't think it would create any extra mechanisms, just a short circuit rule when deciding to copy or not. Rather than checking the ref counts each time the item is mutated or sent, if it is smaller than a certain amount, we know it was copied.

Now, maybe ref counts are faster than total_size(), but I don't know.