Given your performance concerns, I'm imagining you're unlikely to run Zig-Fil in practice, due to the GC overhead?

Yeah, probably not. I wouldn't mind fuzzing it with Zig-Fil, but the interpreter (Zicl) is embedded in a larger C project that uses a lot of dynamic linking, so chances of using it in production is pretty much zero. I do have a lot of asserts that only have a small overhead, so I'll probably use ReleaseSafe most of the time, since it does catch a lot of the issues.