Looking at both the docs and the code, it appears that str is a primitive type, while String is a struct { Vec<u8> }. Now as str is to a [u8] what String is to a Vec<u8>, couldn't str have been defined as
struct str { slice: [u8]; }
similar to how AsciiStr is defined? Why was/is it (still?) defined as primitive?
Once dynamically sized types came along, there no longer remained any good reason for str to be a primitive type; it could entirely reasonably have become a structure as you indicate, with a lang item for the benefit of string literals. But there didn’t seem any especially good reason to change it either (though the possibility was discussed a few times), and so the status quo remained.