When creating an executable program, cargo adds a file called main.rs to our project as it is created. For a newly created library, it instead adds lib.rs. In either case, that file is the entry point for the whole project.

Let's take a look at the boilerplate main.rs file:

     fn main() {
         println!("Hello, world!");
     }

Simple enough, right? Cargo's default program is a Rust version of the classic hello world program, which has been re-implemented countless times by new programmers in every conceivable programming language.

If we look at a new library's lib.rs file, things are a little more interesting:

     #[cfg(test)]
     mod tests {
         #[test]
         fn it_works() {
             assert_eq!(2 + 2, 4);
         }
     }

Instead of having a main function, which all executable programs need because they need a place to start, the library boilerplate includes a framework for automated tests and a single test that confirms that 2 + 2 = 4.