Add<&str> for String in a loop isn't as problematic as adding two, for example, Java Strings because of the capacity growth curve and buffer reuse, but it is still worse than not having a `String::with_capacity` call before the loop. I am sad to see that clippy doesn't have any lints for this, but I can see how it would be hard to make a general good suggestion for it.

Keep in mind that the potential expense of string concatenation was only _one_ example.

There's a reason why I explicitly mentioned immutability: Rust's `String` type has more in common with C#'s `StringBuilder` class than it does with strings in most languages. Meanwhile, the behavour of str/&str is very different.