Understanding Iterable and Span's Escapable requirement

I've been experimenting with the new Iterable protocol in combination with some other explorations around ~Escapable types. I ran into something that surprised me. I suspect this is just because I'm very new to these concepts, but I'd really appreciate guidance. Iterable requires its Element to be Escapable, and (I assume) it picks up this constraint from its use of Span. But Span is itself ~Escapable. Why is it that a Span cannot hold ~Escapable elements?