I think it's mainly because unlike Java, where a StringBuilder is effectively an optimisation over concatenating Strings, in Rust managing that memory would be

I think it's mainly because unlike Java, where a StringBuilder is effectively an optimisation over concatenating Strings, in Rust managing that memory would be a total pain, so you tend to keep the mutable thing around. Once that happens, Box<str> becomes kinda unnecessary. There are many cases where it would be the correct type, for example reading from a file in a read-only manner, but most of the time you're going to be doing something to that text, so it makes more sense to just load it up as a String already and avoid the unnecessary copy. Either way, it's mostly a naming problem. &str/String sucks :(