## Test Organization

As mentioned at the start of the chapter, testing is a complex discipline, and
different people use different terminology and organization. The Rust community
thinks about tests in terms of two main categories: unit tests and integration
tests. *Unit tests* are small 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 potentially 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.

### Unit Tests

The purpose of unit tests is to test each unit of code in isolation from the
rest of the code to quickly pinpoint where code is and isn’t working as
expected. You’ll put unit tests in the *src* directory in each file with the
code that they’re testing. The convention is to create a module named `tests`
in each file to contain the test functions and to annotate the module with
`cfg(test)`.

#### The Tests Module and `#[cfg(test)]`

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

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

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch11-writing-automated-tests/listing-11-01/src/lib.rs}}
```

This code 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`, which is provided by Rust for compiling and
running tests. By using the `cfg` attribute, Cargo compiles our test code only
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]`.

#### Testing Private Functions

There’s debate within the testing community about whether or not private
functions should be tested directly, 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`.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch11-writing-automated-tests/listing-11-12/src/lib.rs}}
```

<span class="caption">Listing 11-12: Testing a private function</span>

Note that the `internal_adder` function is not marked as `pub`. Tests are just
Rust code, and the `tests` module is just another module. As we discussed in
the [“Paths for Referring to an Item in the Module Tree”][paths]<!-- ignore -->
section, items in child modules can use the items in their ancestor modules. In
this test, we bring all of the `test` module’s parent’s items into scope with
`use super::*`, and then the test can call `internal_adder`. If you don’t think
private functions should be tested, there’s nothing in Rust that will compel
you to do so.

### Integration Tests

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
whether many parts of your library work together correctly. Units of code that
work correctly on their own 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.

#### The *tests* Directory

We create 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 want, and Cargo will compile each of the
files as an individual crate.

Let’s create an integration test. With the code in Listing 11-12 still in the
*src/lib.rs* file, make a *tests* directory, create a new file named
*tests/integration_test.rs*, and enter the code in Listing 11-13.

<span class="filename">Filename: tests/integration_test.rs</span>

```rust,ignore
{{#rustdoc_include ../listings/ch11-writing-automated-tests/listing-11-13/tests/integration_test.rs}}
```

<span class="caption">Listing 11-13: An integration test of a function in the
`adder` crate</span>

Each file in the `tests` directory is a separate crate, so we need to bring our
library into each test crate’s scope. For that reason we add `use adder` at the
top of the code, which we didn’t need in the unit tests.

We don’t need to annotate any code in *tests/integration_test.rs* with
`#[cfg(test)]`. Cargo treats the `tests` directory specially and compiles files
in this directory only when we run `cargo test`. Run `cargo test` now:

```console
{{#include ../listings/ch11-writing-automated-tests/listing-11-13/output.txt}}
```

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

The integration tests section starts with the line `Running
tests/integration_test.rs`. Next, there is 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.

Each integration test file has 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 the tests in a
particular integration test file, use the `--test` argument of `cargo test`
followed by the name of the file:

```console
{{#include ../listings/ch11-writing-automated-tests/output-only-05-single-integration/output.txt}}
```

This command runs only the tests in the *tests/integration_test.rs* file.

#### Submodules in Integration Tests

As you add more integration tests, you might want to make more files in the
*tests* directory to help organize them; for example, you can group the test
functions by the functionality they’re testing. As mentioned earlier, each file
in the *tests* directory is compiled as its own separate crate, which is useful
for creating separate scopes to more closely imitate 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, as you learned in Chapter 7
regarding how to separate code into modules and files.

The different behavior of *tests* directory files is most noticeable when you
have a set of helper functions to use in multiple integration test files and
you try to follow the steps in the [“Separating Modules into Different
Files”][separating-modules-into-files]<!-- ignore --> section of Chapter 7 to
extract them into a common module. For example, if we create *tests/common.rs*
and place a function named `setup` in it, we can add some code to `setup` that
we want to call from multiple test functions in multiple test files:

<span class="filename">Filename: tests/common.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch11-writing-automated-tests/no-listing-12-shared-test-code-problem/tests/common.rs}}
```

When 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
did we call the `setup` function from anywhere:

```console
{{#include ../listings/ch11-writing-automated-tests/no-listing-12-shared-test-code-problem/output.txt}}
```

Having `common` appear in the test results with `running 0 tests` displayed for
it is not what we wanted. We just wanted to share some code with the other
integration test files.

To avoid having `common` appear in the test output, instead of creating
*tests/common.rs*, we’ll create *tests/common/mod.rs*. This is an alternate
naming convention that Rust also understands. Naming the file this way tells
Rust not to treat the `common` module as an integration test file. When we move
the `setup` function code into *tests/common/mod.rs* and delete the
*tests/common.rs* file, the section in the test output will no longer appear.
Files in subdirectories of the *tests* directory don’t get compiled as separate
crates or have sections in the test output.

After we’ve created *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*:

<span class="filename">Filename: tests/integration_test.rs</span>

```rust,ignore
{{#rustdoc_include ../listings/ch11-writing-automated-tests/no-listing-13-fix-shared-test-code-problem/tests/integration_test.rs}}
```

Note that the `mod common;` declaration is the same as the module declaration
we demonstrated in Listing 7-21. Then in the test function, we can call the
`common::setup()` function.

#### Integration Tests for Binary Crates

If our project is a binary crate that only contains a *src/main.rs* file and
doesn’t have a *src/lib.rs* file, we can’t create integration tests in the
*tests* directory and bring functions defined in the *src/main.rs* file into
scope with a `use` statement. Only library crates expose functions that other
crates can 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* file that calls logic that lives in the
*src/lib.rs* file. Using that structure, integration tests *can* test the
library crate with `use` to make the important functionality available.
If the important functionality works, the small amount of code in the
*src/main.rs* file will work as well, and that small amount of code doesn’t
need to be tested.

## Summary

Rust’s testing features provide a way to specify how code should function to
ensure it continues to work as you expect, even as you make changes. Unit tests
exercise different parts of a library separately and can test private
implementation details. Integration tests check that many parts of the library
work together correctly, 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
reduce logic bugs having to do with how your code is expected to behave.

Let’s combine the knowledge you learned in this chapter and in previous
chapters to work on a project!

[paths]: ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html
[separating-modules-into-files]:
ch07-05-separating-modules-into-different-files.html
