Small correction: contrary to what the post claims this does not compile:

    fn main() {
        let s = String::from("hello");
        foo(s);
        println!("{}", s);
    }

    fn foo(le_string: String) -> String {
        println!("{}", le_string);
        le_string
    }

This does, which is probably what the author meant:

    fn main() {
        let s = String::from("hello");
        let s = foo(s);
        println!("{}", s);
    }

    fn foo(le_string: String) -> String {
        println!("{}", le_string);
        le_string
    }

Great post!