First, thank you for creating this crate. Looks like it will be useful for a project I'm experimenting with (AsciiDoc parsing), so I'm excited to put it to work.

I want to suggest a small design shift in your Span::new function which I think will help readability at the call sites: When I'm reading the call site, I have no idea what true or false means without looking at the Span documentation.

I can think of two alternatives:

1. Split Span::new into two new functions Span::new_for_utf8 and Span::new_for_ascii (naming help welcome)
2. Replace the bool parameter with a enum Encoding which has Utf8 and Ascii variants.

Hope this helps!

(P.S. My thinking roughly aligns with this article from Alex Kondov.)