Author of compact_str here, you hit the nail on the head, great explanation!
> ... and another (217) signifies that it's a weird static allocation (I do not know why you'd do this)
In addition to String Rust also has str[1], which is an entirely different type. It's common to represent string literals known at compile time as `&'static str`, but they can't be used in all of the same places that a String can. For example, you can't put a &'static str into a Vec<String> unless you first heap allocate and create a String. We added the additional variant of 217 so users of CompactString could abstract over both string literals and strings created at runtime to solve cases like the example.
> ... and another (217) signifies that it's a weird static allocation (I do not know why you'd do this)
In addition to String Rust also has str[1], which is an entirely different type. It's common to represent string literals known at compile time as `&'static str`, but they can't be used in all of the same places that a String can. For example, you can't put a &'static str into a Vec<String> unless you first heap allocate and create a String. We added the additional variant of 217 so users of CompactString could abstract over both string literals and strings created at runtime to solve cases like the example.
[1]: https://doc.rust-lang.org/std/primitive.str.html