That is exactly the same way Haskell does "parametrized enumerants", and in C terms, str is like any pointer to a string, while String is like a heap-allocated string. What's "new" about any of this? C++ has the same thing going on with string and string_view.

If anything, Rust's managed to be a whole lot simpler than other languages. Python, JavaScript and C++ have some of the worst string encoding stories I've ever seen. And most popular languages don't even have sum types, so it's like they have an "and" and no "or".