3 > Program testing can be a very effective way to show the presence of bugs, but
4 > it is hopelessly inadequate for showing their absence.
6 > Edsger W. Dijkstra, "The Humble Programmer" (1972)
8 Let's talk about how to test Rust code. What we will not be talking about is
9 the right way to test Rust code. There are many schools of thought regarding
10 the right and wrong way to write tests. All of these approaches use the same
11 basic tools, and so we'll show you the syntax for using them.
13 # The `test` attribute
15 At its simplest, a test in Rust is a function that's annotated with the `test`
16 attribute. Let's make a new project with Cargo called `adder`:
23 Cargo will automatically generate a simple test when you make a new project.
24 Here's the contents of `src/lib.rs`:
32 Note the `#[test]`. This attribute indicates that this is a test function. It
33 currently has no body. That's good enough to pass! We can run the tests with
38 Compiling adder v0.0.1 (file:///home/you/projects/adder)
39 Running target/adder-91b3e234d4ed382a
44 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
50 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
53 Cargo compiled and ran our tests. There are two sets of output here: one
54 for the test we wrote, and another for documentation tests. We'll talk about
55 those later. For now, see this line:
61 Note the `it_works`. This comes from the name of our function:
68 We also get a summary line:
71 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
74 So why does our do-nothing test pass? Any test which doesn't `panic!` passes,
75 and any test that does `panic!` fails. Let's make our test fail:
84 `assert!` is a macro provided by Rust which takes one argument: if the argument
85 is `true`, nothing happens. If the argument is false, it `panic!`s. Let's run
90 Compiling adder v0.0.1 (file:///home/you/projects/adder)
91 Running target/adder-91b3e234d4ed382a
94 test it_works ... FAILED
98 ---- it_works stdout ----
99 thread 'it_works' panicked at 'assertion failed: false', /home/steve/tmp/adder/src/lib.rs:3
106 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
108 thread '<main>' panicked at 'Some tests failed', /home/steve/src/rust/src/libtest/lib.rs:247
111 Rust indicates that our test failed:
114 test it_works ... FAILED
117 And that's reflected in the summary line:
120 test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
123 We also get a non-zero status code:
130 This is useful if you want to integrate `cargo test` into other tooling.
132 We can invert our test's failure with another attribute: `should_panic`:
142 This test will now succeed if we `panic!` and fail if we complete. Let's try it:
146 Compiling adder v0.0.1 (file:///home/you/projects/adder)
147 Running target/adder-91b3e234d4ed382a
152 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
158 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
161 Rust provides another macro, `assert_eq!`, that compares two arguments for
168 assert_eq!("Hello", "world");
172 Does this test pass or fail? Because of the `should_panic` attribute, it
177 Compiling adder v0.0.1 (file:///home/you/projects/adder)
178 Running target/adder-91b3e234d4ed382a
183 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
189 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
192 `should_panic` tests can be fragile, as it's hard to guarantee that the test
193 didn't fail for an unexpected reason. To help with this, an optional `expected`
194 parameter can be added to the `should_panic` attribute. The test harness will
195 make sure that the failure message contains the provided text. A safer version
196 of the example above would be:
200 #[should_panic(expected = "assertion failed")]
202 assert_eq!("Hello", "world");
206 That's all there is to the basics! Let's write one 'real' test:
209 pub fn add_two(a: i32) -> i32 {
215 assert_eq!(4, add_two(2));
219 This is a very common use of `assert_eq!`: call some function with
220 some known arguments and compare it to the expected output.
224 There is one way in which our existing example is not idiomatic: it's
225 missing the `tests` module. The idiomatic way of writing our example
229 pub fn add_two(a: i32) -> i32 {
239 assert_eq!(4, add_two(2));
244 There's a few changes here. The first is the introduction of a `mod tests` with
245 a `cfg` attribute. The module allows us to group all of our tests together, and
246 to also define helper functions if needed, that don't become a part of the rest
247 of our crate. The `cfg` attribute only compiles our test code if we're
248 currently trying to run the tests. This can save compile time, and also ensures
249 that our tests are entirely left out of a normal build.
251 The second change is the `use` declaration. Because we're in an inner module,
252 we need to bring our test function into scope. This can be annoying if you have
253 a large module, and so this is a common use of globs. Let's change our
254 `src/lib.rs` to make use of it:
257 pub fn add_two(a: i32) -> i32 {
267 assert_eq!(4, add_two(2));
272 Note the different `use` line. Now we run our tests:
276 Updating registry `https://github.com/rust-lang/crates.io-index`
277 Compiling adder v0.0.1 (file:///home/you/projects/adder)
278 Running target/adder-91b3e234d4ed382a
281 test tests::it_works ... ok
283 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
289 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
294 The current convention is to use the `tests` module to hold your "unit-style"
295 tests. Anything that just tests one small bit of functionality makes sense to
296 go here. But what about "integration-style" tests instead? For that, we have
297 the `tests` directory
299 # The `tests` directory
301 To write an integration test, let's make a `tests` directory, and
302 put a `tests/lib.rs` file inside, with this as its contents:
309 assert_eq!(4, adder::add_two(2));
313 This looks similar to our previous tests, but slightly different. We now have
314 an `extern crate adder` at the top. This is because the tests in the `tests`
315 directory are an entirely separate crate, and so we need to import our library.
316 This is also why `tests` is a suitable place to write integration-style tests:
317 they use the library like any other consumer of it would.
323 Compiling adder v0.0.1 (file:///home/you/projects/adder)
324 Running target/adder-91b3e234d4ed382a
327 test tests::it_works ... ok
329 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
331 Running target/lib-c18e7d3494509e74
336 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
342 test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured
345 Now we have three sections: our previous test is also run, as well as our new
348 That's all there is to the `tests` directory. The `tests` module isn't needed
349 here, since the whole thing is focused on tests.
351 Let's finally check out that third section: documentation tests.
353 # Documentation tests
355 Nothing is better than documentation with examples. Nothing is worse than
356 examples that don't actually work, because the code has changed since the
357 documentation has been written. To this end, Rust supports automatically
358 running examples in your documentation. Here's a fleshed-out `src/lib.rs`
362 //! The `adder` crate provides functions that add numbers to other numbers.
367 //! assert_eq!(4, adder::add_two(2));
370 /// This function adds two to its argument.
375 /// use adder::add_two;
377 /// assert_eq!(4, add_two(2));
379 pub fn add_two(a: i32) -> i32 {
389 assert_eq!(4, add_two(2));
394 Note the module-level documentation with `//!` and the function-level
395 documentation with `///`. Rust's documentation supports Markdown in comments,
396 and so triple graves mark code blocks. It is conventional to include the
397 `# Examples` section, exactly like that, with examples following.
399 Let's run the tests again:
403 Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
404 Running target/adder-91b3e234d4ed382a
407 test tests::it_works ... ok
409 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
411 Running target/lib-c18e7d3494509e74
416 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
421 test add_two_0 ... ok
424 test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
427 Now we have all three kinds of tests running! Note the names of the
428 documentation tests: the `_0` is generated for the module test, and `add_two_0`
429 for the function test. These will auto increment with names like `add_two_1` as
430 you add more examples.