You want to change the signature of join_words as follows:

fn strcat(prefix: &mut String, suffix: &String) {

Now prefix is a mutable reference to some String on the caller’s side. We need a mutable reference so we can push onto the string.

suffix is a shared reference. A shared reference suffices because we only need to read from suffix.

Note that the return value has also changed: since we are going to be mutating prefix in place, we no longer need to return anything.

Back to the mutable borrowing tutorial.