How to Use #[cfg] for Conditional Compilation

Use the #[cfg(test)] attribute above a module to include it only when running tests.

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }
}

Run the tests with cargo test to compile and execute the conditional code.

The #[cfg] attribute lets you include specific code blocks only when you are running tests, keeping your production code clean. Think of it like a secret door that only opens when you are in testing mode, allowing you to verify your logic without affecting the final product.