Test Modules

Unit tests are often put in a nested module (run tests on the Playground):

fn helper(a: &str, b: &str) -> String {
    format!("{a} {b}")
}
pub fn main() {
    println!("{}", helper("Hello", "World"));
}
#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_helper() {
        assert_eq!(helper("foo", "bar"), "foo bar");
    }
}

• This lets you unit test private helpers.
• The #[cfg(test)] attribute is only active when you run cargo test.