How to Use Test Modules and the #[cfg(test)] Attribute

Use the #[cfg(test)] attribute to wrap a module containing test functions marked with #[test], ensuring they only compile when running tests. Place this module at the bottom of your source file to access private items via use super::*.

pub fn add(left: u64, right: u64) -> u64 {
    left + right
}

#[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.

The #[cfg(test)] attribute lets you write code that only runs when you are checking your software for errors, not when you are using it. Think of it like a practice mode in a video game that is hidden from the final release. You use it to automatically verify that your functions work correctly before sharing them.