## 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 resultant 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}}
```

On the automatically generated `tests` 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`.

<Listing number="11-12" file-name="src/lib.rs" caption="Testing a private function">

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

</Listing>

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 `tests` 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, and create a new file named
_tests/integration_test.rs_. Your directory structure should look like this:

```text
adder
├── Cargo.lock
├── Cargo.toml
├── src
│   └── lib.rs
└── tests
    └── integration_test.rs
```

Enter the code in Listing 11-13 into the _tests/integration_test.rs_ file.

<Listing number="11-13" file-name="tests/integration_test.rs" caption="An integration test of a function in the `adder` crate">

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

</Listing>

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::add_two;` 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. Note that if any test in a section fails, the following sections
will not be run. For example, if a unit test fails, there won’t be any output
for integration and doc tests because those tests will only be run if all unit
tests are passing.

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_. The
project directory now looks like this:

```text
├── Cargo.lock
├── Cargo.toml
├── src
│   └── lib.rs
└── tests
    ├── common
    │   └── mod.rs
    └── integration_test.rs
```

This is the older naming convention that Rust also understands that we
mentioned in the [“Alternate File Paths”][alt-paths]<!-- ignore --> section of
Chapter 7. 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
[alt-paths]: ch07-05-separating-modules-into-different-files.html#alternate-file-paths
