Unit Test in Rust

Lets get started very quick an dirty with a new library.

$ cargo new example_test
Created library `example_test` project

That should give you a new folder in your current directory with a structure like.

$ tree
├── Cargo.lock
├── Cargo.toml
├── src
│   └── lib.rs

Now that we have a project let’s start testing. Take a look at “lib.rs”


mod tests {
    fn it_works() {

Well what do you know right from the start of creating a new project is a sample test module. Though there is now a simple test it really doesn’t do anything useful. However if you are new to Rust and want to see something run go ahead and watch everything pass.

cargo test


As it was just mentioned this test really does nothing useful at all. All it does is run a test function and never asserts anything. Let’s change that by demonstrating 3 different assert macros.

  • assert! - Asserts based on a boolean.
  • assert_eq! - Asserts based on equality of 2 objects.
  • assert_ne! - Asserts based on no equality of 2 objects.

In the “it_works” functions try out some examples and run them to see how things work.

fn it_works() {
    //Passing Assertion
    assert_eq!(1, 1);
    assert_ne!(1, 2);
$ cargo test 

Compiling example_test v0.1.0 (file:///working_dir/example_test)
      Finished dev [unoptimized + debuginfo] target(s) in 1.29 secs
       Running target/debug/deps/example_test-938c230a543dbf89
  running 1 test
  test tests::it_works ... ok
  test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
     Doc-tests example_test
  running 0 tests
  test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

Testing your library.

Now that you know how to do some basic assertions you can now expand the knowledge. Let’s write a test that will add 2 numbers and return a result.

pub fn add(a: i32, b:i32) -> i32 {
    return a + b

mod tests {
    use super::*;

    fn add_test() {
        assert_eq!(add(2,2), 4);