How to Write Tests
Tests are Rust functions that verify that the non-test code is functioning in the expected manner. The bodies of test functions typically perform these three actions:
- Set up any needed data or state.
- Run the code you want to test.
- Assert the results are what you expect.
Let’s look at the features Rust provides specifically for writing tests that
take these actions, which include 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; one example is
the derive attribute we used with structs in Chapter 5. To change a function
into a test function, add #[test] on the line before fn. When you run your
tests with the cargo test command, Rust builds a test runner binary that runs
the functions annotated with the test attribute and reports on whether each
test function passes or fails.
In Chapter 7, we saw that when we make a new library project with Cargo, a test module with a test function in it is automatically generated for us. This module helps you start writing your tests so you don’t have to look up the exact structure and syntax of test functions every time you start a new project. You can add as many additional test functions and as many test modules as you want!
We’ll 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 --lib
Created library `adder` project
$ cd adder
The contents of the src/lib.rs file in your adder library should look like
Listing 11-1.
Filename: src/lib.rs
# fn main() {} #[cfg(test)] mod tests { #[test] fn it_works() { assert_eq!(2 + 2, 4); } }
Listing 11-1: The test module and function generated
automatically 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 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 by using the #[test] attribute.
The function body uses the assert_eq! macro to assert that 2 + 2 equals 4.
This assertion serves as an example of the format for a typical test. Let’s run
it to see that this test passes.
The cargo test command runs all tests in our project, as shown in Listing
11-2.
$ cargo test
Compiling adder v0.1.0 (file:///projects/adder)
Finished dev [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; 0 filtered out
Doc-tests adder
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Listing 11-2: The output from running the automatically generated test
Cargo compiled and ran the test. After the Compiling, Finished, and
Running lines is 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. The overall summary of running the tests appears next. The
text test result: ok. means that all the tests passed, and the portion that
reads 1 passed; 0 failed totals the number of tests that passed or failed.
Because we don’t have any tests we’ve marked as ignored, the summary shows 0 ignored. We also haven’t filtered the tests being run, so the end of the
summary shows 0 filtered out. We’ll talk about ignoring and filtering out
tests in the next section, “Controlling How Tests Are Run.”
The 0 measured statistic is for benchmark tests that measure performance.
Benchmark tests are, as of this writing, only available in nightly Rust. See
the documentation about benchmark tests to learn more.
The next part of the test output, which 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
discuss how to write documentation tests in the “Documentation Comments as
Tests” section of Chapter 14. For now, we’ll ignore the Doc-tests output.
Let’s change the name of our test to see how that changes the test output.
Change the it_works function to a different name, such as exploration, like
so:
Filename: src/lib.rs
# fn main() {} #[cfg(test)] mod tests { #[test] fn exploration() { assert_eq!(2 + 2, 4); } }
Then run cargo test again. The output now shows exploration instead of
it_works:
running 1 test
test tests::exploration ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Let’s add another test, but this time we’ll make a test that fails! Tests fail
when something in the test function panics. Each test is run in a new thread,
and when the main thread sees that a test thread has died, the test is marked
as failed. We talked about the simplest way to cause a panic in Chapter 9,
which is to call the panic! macro. Enter the new test, another, so your
src/lib.rs file looks like Listing 11-3.
Filename: src/lib.rs
# fn main() {} #[cfg(test)] mod tests { #[test] fn exploration() { assert_eq!(2 + 2, 4); } #[test] fn another() { panic!("Make this test fail"); } }
Listing 11-3: Adding a second test that will fail because
we call the panic! macro
Run the tests again using 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:10:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
tests::another
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
error: test failed
Listing 11-4: Test results when one test passes and one test fails
Instead of ok, the line test tests::another shows FAILED. Two new
sections appear between the individual results and the summary: the first
section displays the detailed reason for each test failure. In this case,
another failed because it panicked at 'Make this test fail', which happened
on line 10 in the src/lib.rs file. 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 to more easily debug it; we’ll talk more about ways to run tests in
the “Controlling How Tests Are Run” section.
The summary line displays at the end: overall, our test result is FAILED.
We had one test pass and one test fail.
Now that you’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,
the assert! macro calls the panic! macro, which causes the test to fail.
Using the assert! macro helps us check that our code is functioning in the
way we intend.
In Chapter 5, Listing 5-15, we used a Rectangle struct and a can_hold
method, which are repeated here in Listing 11-5. Let’s put this code in the
src/lib.rs file and write some tests for it using the assert! macro.
Filename: src/lib.rs
# 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: Using 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, we 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
# 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 whether a
larger rectangle can indeed hold 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 in the “Privacy Rules” section. Because the tests
module is an inner module, we need to bring the code under test in the outer
module into the scope of the inner module. We use a glob here so 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; 0 filtered out
It does pass! Let’s add another test, this time asserting that a smaller rectangle cannot hold a larger rectangle:
Filename: src/lib.rs
# fn main() {} #[cfg(test)] mod tests { use super::*; #[test] fn larger_can_hold_smaller() { // --snip-- } #[test] fn smaller_cannot_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. As a
result, our test will pass if can_hold returns false:
running 2 tests
test tests::smaller_cannot_hold_larger ... ok
test tests::larger_can_hold_smaller ... ok
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Two tests that pass! Now let’s see what happens to our test results when we
introduce a bug in our code. Let’s change the implementation of the can_hold
method by replacing the greater than sign with a less than sign when it
compares the lengths:
# fn main() {} # #[derive(Debug)] # pub struct Rectangle { # length: u32, # width: u32, # } // --snip-- impl Rectangle { pub fn can_hold(&self, other: &Rectangle) -> bool { self.length < other.length && self.width > other.width } }
Running the tests now produces the following:
running 2 tests
test tests::smaller_cannot_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:22:8
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; 0 filtered out
Our tests caught the bug! Because larger.length is 8 and smaller.length is
5, the comparison of the lengths in can_hold now returns false: 8 is not
less than 5.
Testing Equality with the assert_eq! and assert_ne! Macros
A common way to test functionality is to compare the result of the code under
test to the value you expect the code to return to make sure they’re equal. You
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—assert_eq! and assert_ne!—to perform this test
more conveniently. These macros compare two arguments for equality or
inequality, respectively. They’ll also print the two values if the assertion
fails, which makes it easier to see why the test failed; conversely, the
assert! macro only indicates that it got a false value for the ==
expression, not the values that lead to the false value.
In Listing 11-7, we write a function named add_two that adds 2 to its
parameter and returns the result. Then we test this function using the
assert_eq! macro.
Filename: src/lib.rs
# 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; 0 filtered out
The first argument we gave to the assert_eq! macro, 4, is equal to the
result of calling add_two(2). The line for this test is 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:
# fn main() {} pub fn add_two(a: i32) -> i32 { a + 3 }
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:11:8
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; 0 filtered out
Our test caught the bug! The it_adds_two test failed, displaying the message
assertion failed: `(left == right)` and showing that left was 4 and
right was 5. This message is useful and helps us start debugging: it means
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, 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 write the assertion in this test as assert_eq!(add_two(2), 4), which
would result in a failure message that displays assertion failed: `(left == right)` and that left was 5 and right was 4.
The assert_ne! macro will pass if the two values we give it are not equal and
fail if they’re equal. This macro is most useful for cases when we’re not sure
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’re testing 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 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 to assert that values of
those types are equal or not equal. You’ll need to implement Debug to print
the values when the assertion fails. Because both traits are derivable traits,
as mentioned in Listing 5-12 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.
Adding Custom Failure Messages
You can also add a custom message to be printed with the failure message as
optional arguments to the assert!, assert_eq!, and assert_ne! macros. 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 (discussed in Chapter 8 in the “Concatenation with the +
Operator or the format! Macro” section), so you can pass a format string that
contains {} placeholders and values to go in those placeholders. Custom
messages are useful to document what an assertion means; when a test fails,
you’ll 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
# 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 when the requirements change,
so instead of checking for exact equality to the value returned from the
greeting function, we’ll just assert that the output contains the text of the
input parameter.
Let’s introduce a bug into this code by changing greeting to not include
name to see what this test failure looks like:
# fn main() {} pub fn greeting(name: &str) -> String { String::from("Hello!") }
Running this test produces the following:
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:12:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
tests::greeting_contains_name
This result just indicates that the assertion failed and which line the
assertion is on. A more useful failure message in this case would print the
value we got from the greeting function. Let’s change the test function,
giving it 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 when we run the test, we’ll get a more informative error message:
---- tests::greeting_contains_name stdout ----
thread 'tests::greeting_contains_name' panicked at 'Greeting did not
contain name, value was `Hello!`', src/lib.rs:12:8
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,
Listing 9-9. Other code that uses Guess depends on the guarantee that Guess
instances will contain only 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 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; the
test will fail if the code inside the function doesn’t panic.
Listing 11-8 shows a test that checks that the error conditions of Guess::new
happen when we expect them to.
Filename: src/lib.rs
# fn main() {} pub 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 } } } #[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!
We place the #[should_panic] attribute after the #[test] attribute and
before the test function it applies to. Let’s look at the result when this test
passes:
running 1 test
test tests::greater_than_100 ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
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:
# fn main() {} # pub struct Guess { # value: u32, # } # // --snip-- impl Guess { pub fn new(value: u32) -> Guess { if value < 1 { panic!("Guess value must be between 1 and 100, got {}.", value); } Guess { value } } }
When we run the test in 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; 0 filtered out
We don’t get a very helpful message in this case, but when we look at the test
function, we see that it’s annotated with #[should_panic]. The failure we got
means that the code in the test function did not cause a panic.
Tests that use should_panic can be imprecise because they only indicate 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 is too small or too large.
Filename: src/lib.rs
# fn main() {} # pub struct Guess { # value: u32, # } # // --snip-- 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 } } } #[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 should_panic attribute’s
expected parameter is a substring of the message that the Guess::new
function panics with. We could have specified the entire panic message that we
expect, which in this case would be Guess value must be less than or equal to 100, got 200. What you choose to specify in the expected parameter for
should_panic 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 test function executes
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:11:12
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; 0 filtered out
The failure message indicates that this test did indeed panic as we expected,
but the panic message did not include the expected string 'Guess value must be less than or equal to 100'. The panic message that we did get in this case was
Guess value must be greater than or equal to 1, got 200. Now we can start
figuring out where our bug is!
Now that you know several ways to write tests, let’s look at what is happening
when we run our tests and explore the different options we can use with cargo test.