How to test private functions

You test private functions by placing the test code in the same module as the function or by using the #[cfg(test)] attribute to expose the function only during testing. In Rust, tests defined within the same module can access private items, while tests in separate files cannot. Use the #[cfg(test)] attribute to wrap your test module, ensuring it only compiles when running cargo test.

pub fn public_function() {
    private_helper();
}

fn private_helper() {
    // private logic
}

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

    #[test]
    fn test_private_helper() {
        // Directly call the private function here
        private_helper();
        assert!(true);
    }
}

Private functions are like internal tools in a workshop that only the main builder can see. To test them, you simply build a small testing station right inside the workshop where the tools are kept. This way, the tester can use the tools directly without needing to make them public to the outside world.