#unit #matcher #testing #assertions

bin+lib googletest

A rich assertion and matcher library inspired by GoogleTest for C++

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GoogleTest Rust

crates.io docs.rs 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 1.67

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

Assertions and matchers

Most assertions are made through the macro verify_that! which evaluates to a Result<()>. It takes two arguments: an actual value to be tested and a Matcher.

Unlike the macros used in other test assertion libraries in Rust, verify_that! does not panic when the test assertion fails. Instead, it evaluates to Result, which the caller can choose to handle by:

  • Returning immediately from the function with the ? operator (a fatal assertion), or
  • Logging the failure, marking the test as failed, and allowing execution to continue (see Non-fatal assertions below).

Fatal assertions are analogous to the ASSERT_* family of macros in GoogleTest.

For example, for fatal assertions:

use googletest::{matchers::eq, verify_that, Result};

fn more_than_one_failure() -> Result<()> {
    let value = 2;
    verify_that!(value, eq(4))?;  // Fails and ends execution of the test.
    verify_that!(value, eq(2)) // One can also just return the assertion result.

In case one wants behaviour closer to other Rust test libraries, the macro assert_that! has the same parameters as verify_that! but panics on failure.

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::{matchers::{contains, ge}, verify_that, Result};

fn contains_at_least_one_item_at_least_3() -> Result<()> {
    let value = vec![1, 2, 3];
    verify_that!(value, contains(ge(3)))

They can also be logically combined:

use googletest::{matchers::{gt, lt, not, AndMatcherExt}, verify_that, Result};

fn strictly_between_9_and_11() -> Result<()> {
    let value = 10;
    verify_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::{
    matchers::{contains_substring, eq, gt},

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

fn struct_has_expected_values() -> Result<()> {
    let value = AStruct {
        a_field: 10,
        another_field: 100,
        a_third_field: "A correct value",
    verify_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<T> for MyEqMatcher<T> {
    fn matches(&self, actual: &A) -> MatcherResult {
         (self.expected == *actual).into()

    fn describe(&self, matcher_result: MatcherResult) -> String {
        match matcher_result {
            MatcherResult::Matches => {
                format!("is equal to {:?} the way I define it", self.expected)
            MatcherResult::DoesNotMatch => {
                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<T> {
    MyEqMatcher { expected }

The new matcher can then be used in verify_that!:

fn should_be_equal_by_my_definition() -> Result<()> {
    verify_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 google_test instead of the Rust-standard #[test]. It must return Result<()>.

use googletest::{
    expect_that, google_test, matchers::eq, Result

fn more_than_one_failure() -> Result<()> {
    let value = 2;
    expect_that!(value, eq(3));  // Just marks the test as having failed.
    verify_that!(value, eq(2))?;  // Passes, but the test already failed.


You can use the #[google_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 #[google_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")

Contributing Changes

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


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