I agree I've misunderstood things. I thought that the general class of "some bytes that are guaranteed to only contain certain bit patterns" was addressed by a

I agree I've misunderstood things. I thought that the general class of "some bytes that are guaranteed to only contain certain bit patterns" was addressed by a single feature, but actually there's a single feature that addresses this for only primitives and types similar to primitives in some sense. The builtin string type is doing some unrelated thing where it has a thing and it assumes that the thing will contain only a strict subset of the possible bit patterns of the thing or you get nasal demons, and user code is free to do that, but doing that is unrelated to niche optimizations. Thanks.