Comment by simonask
3 days ago
You can't really do anything useful with a `&mut str`. For example, you can't change the length of the string or any characters in the string (without unsafe code). The latter is perhaps surprising, but it's because strings in Rust are always UTF-8, so changing a logical character could imply changing the length of the string, and changing a byte could result in invalid UTF-8.
You're right, but just to add a bit more to this, you can call make_ascii_lowercase and make_ascii_uppercase on &mut str, which can sometimes be useful.