Hey, author here! Happy to answer any questions people have, and happy to hear any feedback.
"unicode bytes" aren't a thing; bytes implies encoding (and subscripting yielding <=0xff), otherwise it's "codepoints" (and subscripting yielding an int somewhere on the unicode planes).
Further down:
String and str are guaranteed to be valid UTF-8 encoded Unicode strings. If you’re wondering why UTF-8 is the standard encoding for Rust strings, check out the Rust FAQ’s answer to that question.
That's spot on. Please add this to the first part, too; "... buffer of UTF-8 encoded unicode bytes". or even just "encoded unicode string." It will be clear what is (and is not) meant.
Otherwise nice article! Even understandable for someone with no Rust experience.
As a kinda funny aside, I also wrote the linked-to FAQ answer. Took a number of drafts to get all the fiddly Unicode terminology right.
When I see 'sort' collocated with 'string' my mind immediately jumps to the verb meaning of 'sort', not the noun meaning of 'sort', so it was a bit confusing.
Also the type-theoretic meaning of "kind" is accurate here, no? Owned vs slice really are two different kinds - it's just that Rust can't represent kindedness in general (it has specialized support for tracking ownedness, but you can't reuse that system to track other properties).