With dynamically sized types like `str`, Rust allows to separate "what kind of pointer + metadata is this" from "what kind of data does it point at". So, for example, you can have the types `str`, `[T]` or `Path`, and can have them behind the pointer types `&T`, `Box<T>` or `Arc<T>`.

If Rust had defined a special struct `Str` for `&str`, then it would have to define special structs for all the combinations possible: Str, ArcSlice, BoxPath, etc.