You're exactly right on the union: I tried to edit out my error on that before someone noticed (my principal point is about the mallocs), but your comment appeared right as I saved. Shame be upon me.

So this holds (on a 64-bit machine) 8-bytes for the pointer, 8-bytes for the length (string not null terminated), and 8-bytes for the capacity. Alternately, via a union, it stores 24 bytes of string (null terminated). It knows whether it is a or b via a separate flag that it holds separately in RBasic.

I retract my jab about memory loss, but it still sounds rather terrible. Every bit of code dealing with strings needs to validate flags on every use to determine what it is dealing with, alternate between length specified or null terminated, etc. Ugh.