I don't know Rust, but I assume `String` is a library type, like presumably many other types from the standard library which aren't listed (maps, vectors, etc.). From [Rust docs](https://doc.rust-lang.org/std/string/struct.String.html):
The String type is the most common string type that has ownership over the contents of the string. It has a close relationship with its borrowed counterpart, the primitive str.