In general, a unit test is a function that instantiates a small portion of an application and verifies its behavior independently from other parts of the code base. In Rust, unit tests are usually written within a module. Ideally, they should only aim to cover the module's functionality and its interfaces.
The following is our very first unit test:
// first_unit_test.rs
#[test]
fn basic_test() {
assert!(true);
}A unit test is written as a function and is marked with a #[test] attribute. There's nothing complex in the preceding basic_test function. We have a basic assert! call passing in true. For better organization, you may also create a child module called tests (by convention) and put all related test code inside it.