You can do:

    let foo = "1234".to_string();
    let mut foo = foo;
    foo = "5678";

This only works if you have ownership of the variable though. And in practice it's a pretty good compromise, because it's pretty hard to do this by mistake.

Note: const in rust is different to const in some other languages: it represents a value that is duplicated inline into each use site at compile time.