The Rust docs are amazing and you can see they bring up indexing in the "UTF-8" section, but it requires me reading a section of the doc which I may not have realized was the reason for my frustration with a compiler error regarding indexing.

I would say this is a bug on the compiler error: it should be making it clear not only that you can't index on the string, but also why. If the explanation is too long, it should be linking to the right section of the book.