## How to Write Tests

Tests are Rust functions that verify non-test code is functioning in the program in the expected manner. The bodies of test functions typically contain some setup, running the code we want to test, then asserting that the results are what we expect. Let's look at the features Rust provides specifically for writing tests: the `test` attribute, a few macros, and the `should_panic` attribute.

### The Anatomy of a Test Function

At its simplest, a test in Rust is a function that's annotated with the `test` attribute. Attributes are metadata about pieces of Rust code: the `derive` attribute that we used with structs in Chapter 5 is one example. To make a function into a test function, we add `#[test]` on the line before `fn`. When we run our tests with the `cargo test` command, Rust will build a test runner binary that runs the functions annotated with the `test` attribute and reports on whether each test function passes or fails.

We saw in Chapter 7 that when you make a new library project with Cargo, a test module with a test function in it is automatically generated for us. This is to help us get started writing our tests, since we don't have to go look up the exact structure and syntax of test functions every time we start a new project. We can add as many additional test functions and as many test modules as we want, though!

We're going to explore some aspects of how tests work by experimenting with the template test generated for us, without actually testing any code. Then we'll write some real-world tests that call some code that we've written and assert that its behavior is correct.

Let's create a new library project called `adder`:

    $ cargo new adder
         Created library `adder` project
    $ cd adder

The contents of the `src/lib.rs` file in your adder library should be as follows:

Filename: src/lib.rs

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

    #}

Listing 11-1: The test module and function generated automatically for us by `cargo new`

For now, let's ignore the top two lines and focus on the function to see how it works. Note the `#[test]` annotation before the `fn` line: this attribute indicates this is a test function, so that the test runner knows to treat this function as a test. We could also have non-test functions in the `tests` module to help set up common scenarios or perform common operations, so we need to indicate which functions are tests with the `#[test]` attribute.

The function currently has no body, which means there is no code to fail the test; an empty test is a passing test! Let's run it and see that this test passes.

The `cargo test` command runs all tests we have in our project, as shown in Listing 11-2:

    $ cargo test
       Compiling adder v0.1.0 (file:///projects/adder)
        Finished debug [unoptimized + debuginfo] target(s) in 0.22 secs
         Running target/debug/deps/adder-ce99bcc2479f4607

    running 1 test
    test tests::it_works ... 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

Listing 11-2: The output from running the one automatically generated test

Cargo compiled and ran our test. After the `Compiling`, `Finished`, and `Running` lines, we see the line `running 1 test`. The next line shows the name of the generated test function, called `it_works`, and the result of running that test, `ok`. Then we see the overall summary of running the tests: `test result: ok.` means all the tests passed. `1 passed; 0 failed` adds up the number of tests that passed or failed.

We don't have any tests we've marked as ignored, so the summary says `0 ignored`. We're going to talk about ignoring tests in the next section on different ways to run tests. The `0 measured` statistic is for benchmark tests that measure performance. Benchmark tests are, as of this writing, only available in nightly Rust. See Appendix D for more information about nightly Rust.

The next part of the test output that starts with `Doc-tests adder` is for the results of any documentation tests. We don't have any documentation tests yet, but Rust can compile any code examples that appear in our API documentation. This feature helps us keep our docs and our code in sync! We'll be talking about how to write documentation tests in the "Documentation Comments" section of Chapter 14. We're going to ignore the `Doc-tests` output for now.

Let's change the name of our test and see how that changes the test output. Give the `it_works` function a different name, such as `exploration`, like so:

Filename: src/lib.rs

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

    #}

And run `cargo test` again. In the output, we'll now see `exploration` instead of `it_works`:

    running 1 test
    test tests::exploration ... ok

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

Let's add another test, but this time we'll make a test that fails! Tests fail when something in the test function panics. We talked about the simplest way to cause a panic in Chapter 9: call the `panic!` macro! Type in the new test so that your `src/lib.rs` now looks like Listing 11-3:

Filename: src/lib.rs

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

        #[test]
        fn another() {
            panic!("Make this test fail");
        }
    }

    #}

Listing 11-3: Adding a second test; one that will fail since we call the `panic!` macro

And run the tests again with `cargo test`. The output should look like Listing 11-4, which shows that our `exploration` test passed and `another` failed:

    running 2 tests
    test tests::exploration ... ok
    test tests::another ... FAILED

    failures:

    ---- tests::another stdout ----
        thread 'tests::another' panicked at 'Make this test fail', src/lib.rs:9
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

    failures:
        tests::another

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

    error: test failed

Listing 11-4: Test results when one test passes and one test fails

Instead of `ok`, the line `test tests::another` says `FAILED`. We have two new sections between the individual results and the summary: the first section displays the detailed reason for the test failures. In this case, `another` failed because it `panicked at 'Make this test fail'`, which happened on src/lib.rs line 9. The next section lists just the names of all the failing tests, which is useful when there are lots of tests and lots of detailed failing test output. We can use the name of a failing test to run just that test in order to more easily debug it; we'll talk more about ways to run tests in the next section.

Finally, we have the summary line: overall, our test result is `FAILED`. We had 1 test pass and 1 test fail.

Now that we've seen what the test results look like in different scenarios, let's look at some macros other than `panic!` that are useful in tests.

### Checking Results with the `assert!` Macro

The `assert!` macro, provided by the standard library, is useful when you want to ensure that some condition in a test evaluates to `true`. We give the `assert!` macro an argument that evaluates to a boolean. If the value is `true`, `assert!` does nothing and the test passes. If the value is `false`, `assert!` calls the `panic!` macro, which causes the test to fail. This is one macro that helps us check that our code is functioning in the way we intend.

Remember all the way back in Chapter 5, Listing 5-9, where we had a `Rectangle` struct and a `can_hold` method, repeated here in Listing 11-5. Let's put this code in src/lib.rs instead of src/main.rs and write some tests for it using the `assert!` macro.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[derive(Debug)]
    pub struct Rectangle {
        length: u32,
        width: u32,
    }

    impl Rectangle {
        pub fn can_hold(&self, other: &Rectangle) -> bool {
            self.length > other.length && self.width > other.width
        }
    }

    #}

Listing 11-5: The `Rectangle` struct and its `can_hold` method from Chapter 5

The `can_hold` method returns a boolean, which means it's a perfect use case for the `assert!` macro. In Listing 11-6, let's write a test that exercises the `can_hold` method by creating a `Rectangle` instance that has a length of 8 and a width of 7, and asserting that it can hold another `Rectangle` instance that has a length of 5 and a width of 1:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn larger_can_hold_smaller() {
            let larger = Rectangle { length: 8, width: 7 };
            let smaller = Rectangle { length: 5, width: 1 };

            assert!(larger.can_hold(&smaller));
        }
    }

    #}

Listing 11-6: A test for `can_hold` that checks that a larger rectangle indeed holds a smaller rectangle

Note that we've added a new line inside the `tests` module: `use super::*;`. The `tests` module is a regular module that follows the usual visibility rules we covered in Chapter 7. Because we're in an inner module, we need to bring the code under test in the outer module into the scope of the inner module. We've chosen to use a glob here so that anything we define in the outer module is available to this `tests` module.

We've named our test `larger_can_hold_smaller`, and we've created the two `Rectangle` instances that we need. Then we called the `assert!` macro and passed it the result of calling `larger.can_hold(&smaller)`. This expression is supposed to return `true`, so our test should pass. Let's find out!

    running 1 test
    test tests::larger_can_hold_smaller ... ok

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

It does pass! Let's add another test, this time asserting that a smaller rectangle cannot hold a larger rectangle:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn larger_can_hold_smaller() {
            let larger = Rectangle { length: 8, width: 7 };
            let smaller = Rectangle { length: 5, width: 1 };

            assert!(larger.can_hold(&smaller));
        }

        #[test]
        fn smaller_can_hold_larger() {
            let larger = Rectangle { length: 8, width: 7 };
            let smaller = Rectangle { length: 5, width: 1 };

            assert!(!smaller.can_hold(&larger));
        }
    }

    #}

Because the correct result of the `can_hold` function in this case is `false`, we need to negate that result before we pass it to the `assert!` macro. This way, our test will pass if `can_hold` returns `false`:

    running 2 tests
    test tests::smaller_can_hold_larger ... ok
    test tests::larger_can_hold_smaller ... ok

    test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

Two passing tests! Now let's see what happens to our test results if we introduce a bug in our code. Let's change the implementation of the `can_hold` method to have a less-than sign when it compares the lengths where it's supposed to have a greater-than sign:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #[derive(Debug)]
    pub struct Rectangle {
        length: u32,
        width: u32,
    }

    impl Rectangle {
        pub fn can_hold(&self, other: &Rectangle) -> bool {
            self.length < other.length && self.width > other.width
        }
    }

    #}

Running the tests now produces:

    running 2 tests
    test tests::smaller_can_hold_larger ... ok
    test tests::larger_can_hold_smaller ... FAILED

    failures:

    ---- tests::larger_can_hold_smaller stdout ----
        thread 'tests::larger_can_hold_smaller' panicked at 'assertion failed:
        larger.can_hold(&smaller)', src/lib.rs:22
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

    failures:
        tests::larger_can_hold_smaller

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

Our tests caught the bug! Since `larger.length` is 8 and `smaller.length` is 5, the comparison of the lengths in `can_hold` now returns `false` since 8 is not less than 5.

### Testing Equality with the `assert_eq!` and `assert_ne!` Macros

A common way to test functionality is to take the result of the code under test and the value we expect the code to return and check that they're equal. We could do this using the `assert!` macro and passing it an expression using the `==` operator. However, this is such a common test that the standard library provides a pair of macros to perform this test more conveniently: `assert_eq!` and `assert_ne!`. These macros compare two arguments for equality or inequality, respectively. They'll also print out the two values if the assertion fails, so that it's easier to see why the test failed, while the `assert!` macro only tells us that it got a `false` value for the `==` expression, not the values that lead to the `false` value.

In Listing 11-7, let's write a function named `add_two` that adds two to its parameter and returns the result. Then let's test this function using the `assert_eq!` macro:

Filename: src/lib.rs

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

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

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

    #}

Listing 11-7: Testing the function `add_two` using the `assert_eq!` macro

Let's check that it passes!

    running 1 test
    test tests::it_adds_two ... ok

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

The first argument we gave to the `assert_eq!` macro, 4, is equal to the result of calling `add_two(2)`. We see a line for this test that says `test tests::it_adds_two ... ok`, and the `ok` text indicates that our test passed!

Let's introduce a bug into our code to see what it looks like when a test that uses `assert_eq!` fails. Change the implementation of the `add_two` function to instead add 3:

    # #![allow(unused_variables)]
    #
    #fn main() {
    pub fn add_two(a: i32) -> i32 {
        a + 3
    }

    #}

And run the tests again:

    running 1 test
    test tests::it_adds_two ... FAILED

    failures:

    ---- tests::it_adds_two stdout ----
        thread 'tests::it_adds_two' panicked at 'assertion failed: `(left ==
        right)` (left: `4`, right: `5`)', src/lib.rs:11
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

    failures:
        tests::it_adds_two

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

Our test caught the bug! The `it_adds_two` test failed with the message `assertion failed: `(left == right)` (left: `4`, right: `5`)`. This message is useful and helps us get started debugging: it says the `left` argument to `assert_eq!` was 4, but the `right` argument, where we had `add_two(2)`, was 5.

Note that in some languages and test frameworks, the parameters to the functions that assert two values are equal are called `expected` and `actual` and the order in which we specify the arguments matters. However, in Rust, they're called `left` and `right` instead, and the order in which we specify the value we expect and the value that the code under test produces doesn't matter. We could have written the assertion in this test as `assert_eq!(add_two(2), 4)`, which would result in a failure message that says `assertion failed: `(left == right)` (left: `5`, right: `4`)`.

The `assert_ne!` macro will pass if the two values we give to it are not equal and fail if they are equal. This macro is most useful for cases when we're not sure exactly what a value will be, but we know what the value definitely won't be, if our code is functioning as we intend. For example, if we have a function that is guaranteed to change its input in some way, but the way in which the input is changed depends on the day of the week that we run our tests, the best thing to assert might be that the output of the function is not equal to the input.

Under the surface, the `assert_eq!` and `assert_ne!` macros use the operators `==` and `!=`, respectively. When the assertions fail, these macros print their arguments using debug formatting, which means the values being compared must implement the `PartialEq` and `Debug` traits. All of the primitive types and most of the standard library types implement these traits. For structs and enums that you define, you'll need to implement `PartialEq` in order to be able to assert that values of those types are equal or not equal. You'll need to implement `Debug` in order to be able to print out the values in the case that the assertion fails. Because both of these traits are derivable traits, as we mentioned in Chapter 5, this is usually as straightforward as adding the `#[derive(PartialEq, Debug)]` annotation to your struct or enum definition. See Appendix C for more details about these and other derivable traits.

### Custom Failure Messages

We can also add a custom message to be printed with the failure message as optional arguments to `assert!`, `assert_eq!`, and `assert_ne!`. Any arguments specified after the one required argument to `assert!` or the two required arguments to `assert_eq!` and `assert_ne!` are passed along to the `format!` macro that we talked about in Chapter 8, so you can pass a format string that contains `{}` placeholders and values to go in the placeholders. Custom messages are useful in order to document what an assertion means, so that when the test fails, we have a better idea of what the problem is with the code.

For example, let's say we have a function that greets people by name, and we want to test that the name we pass into the function appears in the output:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    pub fn greeting(name: &str) -> String {
        format!("Hello {}!", name)
    }

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

        #[test]
        fn greeting_contains_name() {
            let result = greeting("Carol");
            assert!(result.contains("Carol"));
        }
    }

    #}

The requirements for this program haven't been agreed upon yet, and we're pretty sure the `Hello` text at the beginning of the greeting will change. We decided we don't want to have to update the test for the name when that happens, so instead of checking for exact equality to the value returned from the `greeting` function, we're just going to assert that the output contains the text of the input parameter.

Let's introduce a bug into this code to see what this test failure looks like, by changing `greeting` to not include `name`:

    # #![allow(unused_variables)]
    #
    #fn main() {
    pub fn greeting(name: &str) -> String {
        String::from("Hello!")
    }

    #}

Running this test produces:

    running 1 test
    test tests::greeting_contains_name ... FAILED

    failures:

    ---- tests::greeting_contains_name stdout ----
        thread 'tests::greeting_contains_name' panicked at 'assertion failed:
        result.contains("Carol")', src/lib.rs:12
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

    failures:
        tests::greeting_contains_name

This just tells us that the assertion failed and which line the assertion is on. A more useful failure message in this case would print the value we did get from the `greeting` function. Let's change the test function to have a custom failure message made from a format string with a placeholder filled in with the actual value we got from the `greeting` function:

    #[test]
    fn greeting_contains_name() {
        let result = greeting("Carol");
        assert!(
            result.contains("Carol"),
            "Greeting did not contain name, value was `{}`", result
        );
    }

Now if we run the test again, we'll get a much more informative error message:

    ---- tests::greeting_contains_name stdout ----
        thread 'tests::greeting_contains_name' panicked at 'Result did not contain
        name, value was `Hello`', src/lib.rs:12
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

We can see the value we actually got in the test output, which would help us debug what happened instead of what we were expecting to happen.

### Checking for Panics with `should_panic`

In addition to checking that our code returns the correct values we expect, it's also important to check that our code handles error conditions as we expect. For example, consider the `Guess` type that we created in Chapter 9 in Listing 9-8. Other code that uses `Guess` is depending on the guarantee that `Guess` instances will only contain values between 1 and 100. We can write a test that ensures that attempting to create a `Guess` instance with a value outside that range panics.

We can do this by adding another attribute, `should_panic`, to our test function. This attribute makes a test pass if the code inside the function panics, and the test will fail if the code inside the function does non panic.

Listing 11-8 shows how we'd write a test that checks the error conditions of `Guess::new` happen when we expect:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    struct Guess {
        value: u32,
    }

    impl Guess {
        pub fn new(value: u32) -> Guess {
            if value < 1 || value > 100 {
                panic!("Guess value must be between 1 and 100, got {}.", value);
            }

            Guess {
                value: value,
            }
        }
    }

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

        #[test]
        #[should_panic]
        fn greater_than_100() {
            Guess::new(200);
        }
    }

    #}

Listing 11-8: Testing that a condition will cause a `panic!`

The `#[should_panic]` attribute goes after the `#[test]` attribute and before the test function it applies to. Let's see what it looks like when this test passes:

    running 1 test
    test tests::greater_than_100 ... ok

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

Looks good! Now let's introduce a bug in our code, by removing the condition that the `new` function will panic if the value is greater than 100:

    # #![allow(unused_variables)]
    #
    #fn main() {
    # struct Guess {
    #     value: u32,
    # }
    #
    impl Guess {
        pub fn new(value: u32) -> Guess {
            if value < 1  {
                panic!("Guess value must be between 1 and 100, got {}.", value);
            }

            Guess {
                value: value,
            }
        }
    }

    #}

If we run the test from Listing 11-8, it will fail:

    running 1 test
    test tests::greater_than_100 ... FAILED

    failures:

    failures:
        tests::greater_than_100

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

We don't get a very helpful message in this case, but once we look at the test function, we can see that it's annotated with `#[should_panic]`. The failure we got means that the code in the function, `Guess::new(200)`, did not cause a panic.

`should_panic` tests can be imprecise, however, because they only tell us that the code has caused some panic. A `should_panic` test would pass even if the test panics for a different reason than the one we were expecting to happen. To make `should_panic` tests more precise, we can add an optional `expected` parameter to the `should_panic` attribute. The test harness will make sure that the failure message contains the provided text. For example, consider the modified code for `Guess` in Listing 11-9 where the `new` function panics with different messages depending on whether the value was too small or too large:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #
    #fn main() {
    struct Guess {
        value: u32,
    }

    impl Guess {
        pub fn new(value: u32) -> Guess {
            if value < 1 {
                panic!("Guess value must be greater than or equal to 1, got {}.",
                       value);
            } else if value > 100 {
                panic!("Guess value must be less than or equal to 100, got {}.",
                       value);
            }

            Guess {
                value: value,
            }
        }
    }

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

        #[test]
        #[should_panic(expected = "Guess value must be less than or equal to 100")]
        fn greater_than_100() {
            Guess::new(200);
        }
    }

    #}

Listing 11-9: Testing that a condition will cause a `panic!` with a particular panic message

This test will pass, because the value we put in the `expected` parameter of the `should_panic` attribute is a substring of the message that the `Guess::new` function panics with. We could have specified the whole panic message that we expect, which in this case would be `Guess value must be less than or equal to 100, got 200.` It depends on how much of the panic message is unique or dynamic and how precise you want your test to be. In this case, a substring of the panic message is enough to ensure that the code in the function that gets run is the `else if value > 100` case.

To see what happens when a `should_panic` test with an `expected` message fails, let's again introduce a bug into our code by swapping the bodies of the `if value < 1` and the `else if value > 100` blocks:

    if value < 1 {
        panic!("Guess value must be less than or equal to 100, got {}.", value);
    } else if value > 100 {
        panic!("Guess value must be greater than or equal to 1, got {}.", value);
    }

This time when we run the `should_panic` test, it will fail:

    running 1 test
    test tests::greater_than_100 ... FAILED

    failures:

    ---- tests::greater_than_100 stdout ----
        thread 'tests::greater_than_100' panicked at 'Guess value must be greater
        than or equal to 1, got 200.', src/lib.rs:10
    note: Run with `RUST_BACKTRACE=1` for a backtrace.
    note: Panic did not include expected string 'Guess value must be less than or
    equal to 100'

    failures:
        tests::greater_than_100

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

The failure message indicates that this test did indeed panic as we expected, but the panic message `did not include expected string 'Guess value must be less than or equal to 100'`. We can see the panic message that we did get, which in this case was `Guess value must be greater than or equal to 1, got` We could then start figuring out where our bug was!

Now that we've gone over ways to write tests, let's look at what is happening when we run our tests and talk about the different options we can use with `cargo test`.