I’m David Feuer. I maintain containers, co-maintain unordered-containers and pqueue, and contribute to various other projects. Data structures are fun. I’m a good person to talk to about laziness subtleties, and about whether particular applications of unsafe IO are safe in context.
I think your version is arguably even more confusing, though I certainly wish we had something clearer. If you can suggest something clearer and equally fast available using extant primitives/instructions, I’m all ears and will happily make the change.