Rust Essentials - Second Edition

Testing with cargo

An executable project, or crate as it is called in Rust, needs to have a startup function main(), but a library crate, to be used in other crates, does not need a function main(). Create a new library crate mylib with cargo as follows:

cargo new mylib

This creates a subfolder src with a source file lib.rs which contains the following:

#[cfg(test)] 
mod tests { 
    #[test] 
    fn it_works() { 
    } 
} 

So, a library crate is created with no code of its own, but it does contain a test template annotated with a cfg(test) attribute. This attribute indicates that the code that follows will only be compiled in test mode. To differentiate with normal library code, use a prefix not in the attribute like this:

#[cfg(not(test))] fn main() { println!("Normal ...