created_at2022-12-09 16:10:09.868956
updated_at2024-01-12 11:04:34.777378
descriptionA rich assertion and matcher library inspired by GoogleTest for C++
Bastien Jacot-Guillarmod (bjacotg)



GoogleTest Rust Apache licensed Build Status

This library brings the rich assertion types of Google's C++ testing library GoogleTest to Rust. It provides:

  • A framework for writing matchers which can be combined to make a wide range of assertions on data,
  • A rich set of matchers providing similar functionality to those included in GoogleTest, and
  • A new set of assertion macros offering similar functionality to those of GoogleTest.

The minimum supported Rust version is 1.66.

:warning: The API is not fully stable and may still be changed until we publish version 1.0.

Moreover, any items or modules starting with __ (double underscores) must not be used directly. Those items or modules are only for internal uses and their API may change without a major version update.

Assertions and matchers

The core of GoogleTest is its matchers. Matchers indicate what aspect of an actual value one is asserting: (in-)equality, containment, regular expression matching, and so on.

To make an assertion using a matcher, GoogleTest offers three macros:

  • assert_that! panics if the assertion fails, aborting the test.
  • expect_that! logs an assertion failure, marking the test as having failed, but allows the test to continue running (called a non-fatal assertion). It requires the use of the googletest::test attribute macro on the test itself.
  • verify_that! has no side effects and evaluates to a Result<()> whose Err variant describes the assertion failure, if there is one. In combination with the ? operator, this can be used to abort the test on assertion failure without panicking. It is also the building block for the other two macros above.

For example:

use googletest::prelude::*;

fn fails_and_panics() {
    let value = 2;
    assert_that!(value, eq(4));

fn two_logged_failures() {
    let value = 2;
    expect_that!(value, eq(4)); // Test now failed, but continues executing.
    expect_that!(value, eq(5)); // Second failure is also logged.

fn fails_immediately_without_panic() -> Result<()> {
    let value = 2;
    verify_that!(value, eq(4))?; // Test fails and aborts.
    verify_that!(value, eq(2))?; // Never executes.

fn simple_assertion() -> Result<()> {
    let value = 2;
    verify_that!(value, eq(4)) // One can also just return the last assertion.

This library includes a rich set of matchers, covering:

  • Equality, numeric inequality, and approximate equality;
  • Strings and regular expressions;
  • Containers and set-theoretic matching.

Matchers are composable:

use googletest::prelude::*;

fn contains_at_least_one_item_at_least_3() {
    let value = vec![1, 2, 3];
    expect_that!(value, contains(ge(3)));

They can also be logically combined:

use googletest::prelude::*;

fn strictly_between_9_and_11() {
    let value = 10;
    expect_that!(value, gt(9).and(not(ge(11))));


One can use the macro matches_pattern! to create a composite matcher for a struct or enum that matches fields with other matchers:

use googletest::prelude::*;

struct AStruct {
    a_field: i32,
    another_field: i32,
    a_third_field: &'static str,

fn struct_has_expected_values() {
    let value = AStruct {
        a_field: 10,
        another_field: 100,
        a_third_field: "A correct value",
    expect_that!(value, matches_pattern!(AStruct {
        a_field: eq(10),
        another_field: gt(50),
        a_third_field: contains_substring("correct"),

Writing matchers

One can extend the library by writing additional matchers. To do so, create a struct holding the matcher's data and have it implement the trait Matcher:

struct MyEqMatcher<T> {
    expected: T,

impl<T: PartialEq + Debug> Matcher for MyEqMatcher<T> {
    type ActualT = T;

    fn matches(&self, actual: &Self::ActualT) -> MatcherResult {
         (self.expected == *actual).into()

    fn describe(&self, matcher_result: MatcherResult) -> String {
        match matcher_result {
            MatcherResult::Match => {
                format!("is equal to {:?} the way I define it", self.expected)
            MatcherResult::NoMatch => {
                format!("isn't equal to {:?} the way I define it", self.expected)

It is recommended to expose a function which constructs the matcher:

pub fn eq_my_way<T: PartialEq + Debug>(expected: T) -> impl Matcher<ActualT = T> {
    MyEqMatcher { expected }

The new matcher can then be used in the assertion macros:

fn should_be_equal_by_my_definition() {
    expect_that!(10, eq_my_way(10));

Non-fatal assertions

Using non-fatal assertions, a single test is able to log multiple assertion failures. Any single assertion failure causes the test to be considered having failed, but execution continues until the test completes or otherwise aborts.

This is analogous to the EXPECT_* family of macros in GoogleTest.

To make a non-fatal assertion, use the macro expect_that!. The test must also be marked with googletest::test instead of the Rust-standard #[test].

use googletest::prelude::*;

fn three_non_fatal_assertions() {
    let value = 2;
    expect_that!(value, eq(2));  // Passes; test still considered passing.
    expect_that!(value, eq(3));  // Fails; logs failure and marks the test failed.
    expect_that!(value, eq(4));  // A second failure, also logged.

This can be used in the same tests as verify_that!, in which case the test function must also return Result<()>:

use googletest::prelude::*;

fn failing_non_fatal_assertion() -> Result<()> {
    let value = 2;
    expect_that!(value, eq(3));  // Just marks the test as having failed.
    verify_that!(value, eq(2))?;  // Passes, so does not abort the test.
    Ok(())        // Because of the failing expect_that! call above, the
                  // test fails despite returning Ok(())
use googletest::prelude::*;

fn failing_fatal_assertion_after_non_fatal_assertion() -> Result<()> {
    let value = 2;
    verify_that!(value, eq(3))?; // Fails and aborts the test.
    expect_that!(value, eq(3));  // Never executes, since the test already aborted.


You can use the #[googletest::test] macro together with many other libraries such as rstest. Just apply both attribute macros to the test:

fn rstest_works_with_google_test(#[case] value: u32) -> Result<()> {
   verify_that!(value, gt(0))

Make sure to put #[googletest::test] before #[rstest]. Otherwise the annotated test will run twice, since both macros will attempt to register a test with the Rust test harness.

The macro also works together with async tests with Tokio in the same way:

async fn should_work_with_tokio() -> Result<()> {
    verify_that!(3, gt(0))

There is one caveat when running async tests: test failure reporting through and_log_failure will not work properly if the assertion occurs on a different thread than runs the test.

Predicate assertions

The macro verify_pred! provides predicate assertions analogous to GoogleTest's EXPECT_PRED family of macros. Wrap an invocation of a predicate in a verify_pred! invocation to turn that into a test assertion which passes precisely when the predicate returns true:

fn stuff_is_correct(x: i32, y: i32) -> bool {
    x == y

let x = 3;
let y = 4;
verify_pred!(stuff_is_correct(x, y))?;

The assertion failure message shows the arguments and the values to which they evaluate:

stuff_is_correct(x, y) was false with
  x = 3,
  y = 4

The verify_pred! invocation evaluates to a Result<()> just like verify_that!. There is also a macro expect_pred! to make a non-fatal predicaticate assertion.

Unconditionally generating a test failure

The macro fail! unconditionally evaluates to a Result indicating a test failure. It can be used analogously to verify_that! and verify_pred! to cause a test to fail, with an optional formatted message:

fn always_fails() -> Result<()> {
    fail!("This test must fail with {}", "today")


This library is configurable through environment variables. Since the configuration does not impact whether a test fails or not but how a failure is displayed, we recommend setting those variables in the personal ~/.cargo/config.toml instead of in the project-scoped Cargo.toml.

Configuration variable list

Variable name Description
NO_COLOR Disables colored output. See
FORCE_COLOR Forces colors even when the output is piped to a file.

Contributing Changes

Please read for details on how to contribute to this project.

Commit count: 334

cargo fmt