The Rust Programming Language

As mentioned at the start of the chapter, testing is a large discipline, and different people use different terminology and organization. The Rust community tends to think about tests in terms of two main categories: unit tests and integration tests. Unit tests are smaller and more focused, testing one module in isolation at a time, and can test private interfaces. Integration tests are entirely external to your library, and use your code in the same way any other external code would, using only the public interface and exercising multiple modules per test.

Writing both kinds of tests is important to ensure that the pieces of your library are doing what you expect them to separately and together.

The purpose of unit tests is to test each unit of code in isolation from the rest of the code, in order to be able to quickly pinpoint where code is and is not working as expected. We put unit tests in the src directory, in each file with the code that they’re testing. The convention is that we create a module named tests in each file to contain the test functions, and we annotate the module with cfg(test).

The #[cfg(test)] annotation on the tests module tells Rust to compile and run the test code only when we run cargo test, and not when we run cargo build. This saves compile time when we only want to build the library, and saves space in the resulting compiled artifact since the tests are not included. We’ll see that since integration tests go in a different directory, they don’t need the #[cfg(test)] annotation. Because unit tests go in the same files as the code, though, we use #[cfg(test)]to specify that they should not be included in the compiled result.

Remember that when we generated the new adder project in the first section of this chapter, Cargo generated this code for us:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
    }
}
#}

This is the automatically generated test module. The attribute cfg stands for configuration, and tells Rust that the following item should only be included given a certain configuration option. In this case, the configuration option is test, provided by Rust for compiling and running tests. By using this attribute, Cargo only compiles our test code if we actively run the tests with cargo test. This includes any helper functions that might be within this module, in addition to the functions annotated with #[test].

There’s debate within the testing community about whether private functions should be tested directly or not, and other languages make it difficult or impossible to test private functions. Regardless of which testing ideology you adhere to, Rust’s privacy rules do allow you to test private functions. Consider the code in Listing 11-12 with the private function internal_adder:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub fn add_two(a: i32) -> i32 {
    internal_adder(a, 2)
}

fn internal_adder(a: i32, b: i32) -> i32 {
    a + b
}

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

    #[test]
    fn internal() {
        assert_eq!(4, internal_adder(2, 2));
    }
}
#}

Listing 11-12: Testing a private function

Note that the internal_adder function is not marked as pub, but because tests are just Rust code and the tests module is just another module, we can import and call internal_adder in a test just fine. If you don’t think private functions should be tested, there’s nothing in Rust that will compel you to do so.

In Rust, integration tests are entirely external to your library. They use your library in the same way any other code would, which means they can only call functions that are part of your library’s public API. Their purpose is to test that many parts of your library work correctly together. Units of code that work correctly by themselves could have problems when integrated, so test coverage of the integrated code is important as well. To create integration tests, you first need a tests directory.

To write integration tests for our code, we need to make a tests directory at the top level of our project directory, next to src. Cargo knows to look for integration test files in this directory. We can then make as many test files as we’d like in this directory, and Cargo will compile each of the files as an individual crate.

Let’s give it a try! Keep the code from Listing 11-12 in src/lib.rs. Make a tests directory, then make a new file named tests/integration_test.rs, and enter the code in Listing 11-13.

Filename: tests/integration_test.rs

extern crate adder;

#[test]
fn it_adds_two() {
    assert_eq!(4, adder::add_two(2));
}

Listing 11-13: An integration test of a function in the adder crate

We’ve added extern crate adder at the top, which we didn’t need in the unit tests. This is because each test in the tests directory is an entirely separate crate, so we need to import our library into each of them. Integration tests use the library like any other consumer of it would, by importing the crate and using only the public API.

We don’t need to annotate any code in tests/integration_test.rs with #[cfg(test)]. Cargo treats the tests directory specially and will only compile files in this directory if we run cargo test. Let’s try running cargo test now:

cargo test
   Compiling adder v0.1.0 (file:///projects/adder)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
     Running target/debug/deps/adder-abcabcabc

running 1 test
test tests::internal ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/integration_test-ce99bcc2479f4607

running 1 test
test it_adds_two ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests adder

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

Now we have three sections of output: the unit tests, the integration test, and the doc tests. The first section for the unit tests is the same as we have been seeing: one line for each unit test (we have one named internal that we added in Listing 11-12), then a summary line for the unit tests.

The integration tests section starts with the line that says Running target/debug/deps/integration-test-ce99bcc2479f4607 (the hash at the end of your output will be different). Then there’s a line for each test function in that integration test, and a summary line for the results of the integration test just before the Doc-tests adder section starts.

Note that adding more unit test functions in any src file will add more test result lines to the unit tests section. Adding more test functions to the integration test file we created will add more lines to the integration test section. Each integration test file gets its own section, so if we add more files in the tests directory, there will be more integration test sections.

We can still run a particular integration test function by specifying the test function’s name as an argument to cargo test. To run all of the tests in a particular integration test file, use the --test argument of cargo test followed by the name of the file:

$ cargo test --test integration_test
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/integration_test-952a27e0126bb565

running 1 test
test it_adds_two ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

This tests only the file that we specified from the tests directory.

As you add more integration tests, you may want to make more than one file in the tests directory to help organize them; for example, to group the test functions by the functionality they’re testing. As we mentioned, each file in the tests directory is compiled as its own separate crate.

Treating each integration test file as its own crate is useful to create separate scopes that are more like the way end users will be using your crate. However, this means files in the tests directory don’t share the same behavior as files in src do that we learned about in Chapter 7 regarding how to separate code into modules and files.

The different behavior of files in the tests directory is usually most noticeable if you have a set of helper functions that would be useful in multiple integration test files, and you try to follow the steps from Chapter 7 to extract them into a common module. For example, if we create tests/common.rs and place this function named setup in it, where we could put some code that we want to be able to call from multiple test functions in multiple test files:

Filename: tests/common.rs

# #![allow(unused_variables)]
#fn main() {
pub fn setup() {
    // setup code specific to your library's tests would go here
}
#}

If we run the tests again, we’ll see a new section in the test output for the common.rs file, even though this file doesn’t contain any test functions, nor are we calling the setup function from anywhere:

running 1 test
test tests::internal ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/common-b8b07b6f1be2db70

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/integration_test-d993c68b431d39df

running 1 test
test it_adds_two ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests adder

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

Having common show up in the test results with running 0 tests displayed for it is not what we wanted; we just wanted to be able to share some code with the other integration test files.

In order to not have common show up in the test output, we need to use the other method of extracting code into a file that we learned about in Chapter 7: instead of creating tests/common.rs, we’ll create tests/common/mod.rs. When we move the setup function code into tests/common/mod.rs and get rid of the tests/common.rs file, the section in the test output will no longer show up. Files in subdirectories of the tests directory do not get compiled as separate crates or have sections in the test output.

Once we have tests/common/mod.rs, we can use it from any of the integration test files as a module. Here’s an example of calling the setup function from the it_adds_two test in tests/integration_test.rs:

Filename: tests/integration_test.rs

extern crate adder;

mod common;

#[test]
fn it_adds_two() {
    common::setup();
    assert_eq!(4, adder::add_two(2));
}

Note the mod common; declaration is the same as the module declarations we did in Chapter 7. Then in the test function, we can call the common::setup() function.

If our project is a binary crate that only contains a src/main.rs and does not have a src/lib.rs, we aren’t able to create integration tests in the tests directory and use extern crate to import functions defined in src/main.rs. Only library crates expose functions that other crates are able to call and use; binary crates are meant to be run on their own.

This is one of the reasons Rust projects that provide a binary have a straightforward src/main.rs that calls logic that lives in src/lib.rs. With that structure, integration tests can test the library crate by using extern crate to cover the important functionality. If the important functionality works, the small amount of code in src/main.rs will work as well, and that small amount of code does not need to be tested.

Rust’s testing features provide a way to specify how code should function to ensure it continues to work as we expect even as we make changes. Unit tests exercise different parts of a library separately and can test private implementation details. Integration tests cover the use of many parts of the library working together, and they use the library’s public API to test the code in the same way external code will use it. Even though Rust’s type system and ownership rules help prevent some kinds of bugs, tests are still important to help reduce logic bugs having to do with how your code is expected to behave.

Let’s put together the knowledge from this chapter and other previous chapters and work on a project in the next chapter!