fn main() {
    let mut s1 = String::from("hello");

    // Mutation by reference requires &mut on both
    // the function declaration and the call site
    change(&mut s1);

    let len = calculate_length(&s1);

    println!("The length of '{}' is {}.", s1, len);
}


fn calculate_length(s: &String) -> usize { //  s is a reference to a String
    s.len()
} // Here, s goes out of scope. But because it does not have ownership of what
// it refers to, nothing happens.

// Change takes a mutable reference
// only one mutable reference can exist at a time
fn change(some_string: &mut String) {
    some_string.push_str(", world");
}