Passing An Owned Value Into A Function Can Move It

In Rust, function parameters are variables too, so passing a value into a function follows the same ownership rules as assignment in source

fn print_message(message: String) {
    println!("{message}");
}
 
fn main() {
    let message = String::from("Server started");
 
    print_message(message);
}

Here, calling the print_message(message) moves the String into the function parameter. So, if you’ll try the following program, it won’t work:

fn main() {
    let message = String::from("Server started");
 
    print_message(message);
 
    println!("{message}");
}

The Rust compiler will scream that message was used after being moved.

But as we have seen before, we can return the ownership to its caller as well:

fn print_and_return(message: String) -> String {
    println!("{message}");
    message
}
 
fn main() {
    let message = String::from("Server started");
    let message = print_and_return(message);
 
    println!("{message}");
}

Here, we are returning message from print_and_return() method and then the ownership gets to the message variable that is in the main function source

That’s why, when you’ll print message, this will work completely fine.