| Rust is a programming language that cares a lot about correctness and it includes support for writing software tests within the language itself. Testing comes in three styles: - Unit testing.
- Doc testing.
- Integration testing.
Also Rust has support for specifying additional dependencies for tests:
Unit testingTests are Rust functions that verify that the non-test code is functioning in the expected manner. The bodies of test functions typically perform some setup, run the code we want to test, then assert whether the results are what we expect. Most unit tests go into a tests mod with the #[cfg(test)] attribute. Test functions are marked with the #[test] attribute. Tests fail when something in the test function panics. There are some helper macros: assert!(expression) - panics if expression evaluates to false.assert_eq!(left, right) and assert_ne!(left, right) - testing left and right expressions for equality and inequality respectively.
pub fn add(a: i32, b: i32) -> i32 {
a + b
}
// This is a really bad adding function, its purpose is to fail in this
// example.
#[allow(dead_code)]
fn bad_add(a: i32, b: i32) -> i32 {
a - b
}
#[cfg(test)]
mod tests {
// Note this useful idiom: importing names from outer (for mod tests) scope.
use super::*;
#[test]
fn test_add() {
assert_eq!(add(1, 2), 3);
}
#[test]
fn test_bad_add() {
// This assert would fire and test will fail.
// Please note, that private functions can be tested too!
assert_eq!(bad_add(1, 2), 3);
}
}
Tests can be run with cargo test. $ cargo test
running 2 tests
test tests::test_bad_add ... FAILED
test tests::test_add ... ok
failures:
---- tests::test_bad_add stdout ----
thread 'tests::test_bad_add' panicked at 'assertion failed: `(left == right)`
left: `-1`,
right: `3`', src/lib.rs:21:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
tests::test_bad_add
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
Tests and ?None of the previous unit test examples had a return type. But in Rust 2018, your unit tests can return Result<()>, which lets you use ? in them! This can make them much more concise. fn sqrt(number: f64) -> Result<f64, String> {
if number >= 0.0 {
Ok(number.powf(0.5))
} else {
Err("negative floats don't have square roots".to_owned())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_sqrt() -> Result<(), String> {
let x = 4.0;
assert_eq!(sqrt(x)?.powf(2.0), x);
Ok(())
}
}
Testing panicsTo check functions that should panic under certain circumstances, use attribute #[should_panic]. This attribute accepts optional parameter expected = with the text of the panic message. If your function can panic in multiple ways, it helps make sure your test is testing the correct panic.
pub fn divide_non_zero_result(a: u32, b: u32) -> u32 {
if b == 0 {
panic!("Divide-by-zero error");
} else if a < b {
panic!("Divide result is zero");
}
a / b
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_divide() {
assert_eq!(divide_non_zero_result(10, 2), 5);
}
#[test]
#[should_panic]
fn test_any_panic() {
divide_non_zero_result(1, 0);
}
#[test]
#[should_panic(expected = "Divide result is zero")]
fn test_specific_panic() {
divide_non_zero_result(1, 10);
}
}
Running these tests gives us: $ cargo test
running 3 tests
test tests::test_any_panic ... ok
test tests::test_divide ... ok
test tests::test_specific_panic ... ok
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Doc-tests tmp-test-should-panic
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running specific testsTo run specific tests one may specify the test name to cargo test command. $ cargo test test_any_panic
running 1 test
test tests::test_any_panic ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 2 filtered out
Doc-tests tmp-test-should-panic
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
To run multiple tests one may specify part of a test name that matches all the tests that should be run. $ cargo test panic
running 2 tests
test tests::test_any_panic ... ok
test tests::test_specific_panic ... ok
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out
Doc-tests tmp-test-should-panic
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Ignoring testsTests can be marked with the #[ignore] attribute to exclude some tests. Or to run them with command cargo test -- --ignored pub fn add(a: i32, b: i32) -> i32 {
a + b
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_add() {
assert_eq!(add(2, 2), 4);
}
#[test]
fn test_add_hundred() {
assert_eq!(add(100, 2), 102);
assert_eq!(add(2, 100), 102);
}
#[test]
#[ignore]
fn ignored_test() {
assert_eq!(add(0, 0), 0);
}
}
$ cargo test
running 3 tests
test tests::ignored_test ... ignored
test tests::test_add ... ok
test tests::test_add_hundred ... ok
test result: ok. 2 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out
Doc-tests tmp-ignore
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
$ cargo test -- --ignored
running 1 test
test tests::ignored_test ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Doc-tests tmp-ignore
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Documentation testingThe primary way of documenting a Rust project is through annotating the source code. Documentation comments are written in CommonMark Markdown specification and support code blocks in them. Rust takes care about correctness, so these code blocks are compiled and used as documentation tests. /// First line is a short summary describing function.
///
/// The next lines present detailed documentation. Code blocks start with
/// triple backquotes and have implicit `fn main()` inside
/// and `extern crate <cratename>`. Assume we're testing `doccomments` crate:
///
/// ```
/// let result = doccomments::add(2, 3);
/// assert_eq!(result, 5);
/// ```
pub fn add(a: i32, b: i32) -> i32 {
a + b
}
/// Usually doc comments may include sections "Examples", "Panics" and "Failures".
///
/// The next function divides two numbers.
///
/// # Examples
///
/// ```
/// let result = doccomments::div(10, 2);
/// assert_eq!(result, 5);
/// ```
///
/// # Panics
///
/// The function panics if the second argument is zero.
///
/// ```rust,should_panic
/// // panics on division by zero
/// doccomments::div(10, 0);
/// ```
pub fn div(a: i32, b: i32) -> i32 {
if b == 0 {
panic!("Divide-by-zero error");
}
a / b
}
Code blocks in documentation are automatically tested when running the regular cargo test command: $ cargo test
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Doc-tests doccomments
running 3 tests
test src/lib.rs - add (line 7) ... ok
test src/lib.rs - div (line 21) ... ok
test src/lib.rs - div (line 31) ... ok
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Motivation behind documentation testsThe main purpose of documentation tests is to serve as examples that exercise the functionality, which is one of the most important guidelines. It allows using examples from docs as complete code snippets. But using ? makes compilation fail since main returns unit. The ability to hide some source lines from documentation comes to the rescue: one may write fn try_main() -> Result<(), ErrorType>, hide it and unwrap it in hidden main. Sounds complicated? Here's an example: /// Using hidden `try_main` in doc tests.
///
/// ```
/// # // hidden lines start with `#` symbol, but they're still compilable!
/// # fn try_main() -> Result<(), String> { // line that wraps the body shown in doc
/// let res = doccomments::try_div(10, 2)?;
/// # Ok(()) // returning from try_main
/// # }
/// # fn main() { // starting main that'll unwrap()
/// # try_main().unwrap(); // calling try_main and unwrapping
/// # // so that test will panic in case of error
/// # }
/// ```
pub fn try_div(a: i32, b: i32) -> Result<i32, String> {
if b == 0 {
Err(String::from("Divide-by-zero"))
} else {
Ok(a / b)
}
}
Integration testingUnit tests are testing one module in isolation at a time: they're small and can test private code. Integration tests are external to your crate and use only its public interface in the same way any other code would. Their purpose is to test that many parts of your library work correctly together. Cargo looks for integration tests in tests directory next to src. File src/lib.rs: // Define this in a crate called `adder`.
pub fn add(a: i32, b: i32) -> i32 {
a + b
}
File with test: tests/integration_test.rs: #[test]
fn test_add() {
assert_eq!(adder::add(3, 2), 5);
}
Running tests with cargo test command: $ cargo test
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running target/debug/deps/integration_test-bcd60824f5fbfe19
running 1 test
test test_add ... 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
Each Rust source file in the tests directory is compiled as a separate crate. In order to share some code between integration tests we can make a module with public functions, importing and using it within tests. File tests/common/mod.rs: pub fn setup() {
// some setup code, like creating required files/directories, starting
// servers, etc.
}
File with test: tests/integration_test.rs // importing common module.
mod common;
#[test]
fn test_add() {
// using common code.
common::setup();
assert_eq!(adder::add(3, 2), 5);
}
Creating the module as tests/common.rs also works, but is not recommended because the test runner will treat the file as a test crate and try to run tests inside it.
Development dependenciesSometimes there is a need to have dependencies for tests (or examples, or benchmarks) only. Such dependencies are added to Cargo.toml in the [dev-dependencies] section. These dependencies are not propagated to other packages which depend on this package. One such example is pretty_assertions, which extends standard assert_eq! and assert_ne! macros, to provide colorful diff.
File Cargo.toml: # standard crate data is left out
[dev-dependencies]
pretty_assertions = "1"
File src/lib.rs: pub fn add(a: i32, b: i32) -> i32 {
a + b
}
#[cfg(test)]
mod tests {
use super::*;
use pretty_assertions::assert_eq; // crate for test-only use. Cannot be used in non-test code.
#[test]
fn test_add() {
assert_eq!(add(2, 3), 5);
}
}
| 
