# Google Rust testing library
This library brings the rich assertion types of Google's C++ testing library
[GoogleTest](https://github.com/google/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](https://google.github.io/googletest/reference/matchers.html),
and
* A new set of assertion macros offering similar functionality to those of
[GoogleTest](https://google.github.io/googletest/primer.html#assertions).
> :warning: 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 other Rust assertion libraries, this library does not panic when a test
assertion fails. Instead, a test assertion 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](#non-fatal-assertions) below).
Fatal assertions are analogous to the `ASSERT_*` family of macros in GoogleTest.
For example, for fatal assertions:
```rust
use googletest::{matchers::eq, verify_that, Result};
#[test]
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.
}
```
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:
```rust
use googletest::{matchers::{contains, ge}, verify_that, Result};
#[test]
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:
```rust
use googletest::{matchers::{gt, lt, not, AndMatcherExt}, verify_that, Result};
#[test]
fn strictly_between_9_and_11() -> Result<()> {
let value = 10;
verify_that!(value, gt(9).and(not(ge(11))))
}
```
## Pattern-matching
One can use the macro `matches_pattern!` to create a composite matcher for a
struct or enum that matches fields with other matchers:
```rust
use googletest::{
matchers::{contains_substring, eq, gt},
matches_pattern,
verify_that,
Result,
};
struct AStruct {
a_field: i32,
another_field: i32,
a_third_field: &'static str,
}
#[test]
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 traits `Matcher` and
`Display`:
```rust
struct MyEqMatcher<T> {
expected: T,
}
impl<T: PartialEq + Debug> Matcher<T> for MyEqMatcher<T> {
fn matches(&self, actual: &A) -> MatcherResult {
if self.expected == *actual { MatcherResult::Matches } else { MatcherResult::DoesNotMatch }
}
}
impl<T: Debug> Display for MyEqMatcher<T> {
fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
write!(f, "equal to {:?} the way I define it", self.expected)
}
}
```
It is recommended to expose a function which constructs the matcher:
```rust
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!`:
```rust
#[test]
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, invoke the method `and_log_failure()` on the
output of the assertion macro. The test must also be marked with
`#[google_test]` instead of the Rust-standard `#[test]`. It must return
`googletest::Result`.
```rust
use googletest::{
google_test, matchers::eq, verify_that, GoogleTestSupport, Result
};
#[google_test]
fn more_than_one_failure() -> Result<()> {
let value = 2;
verify_that!(value, eq(3)).and_log_failure(); // Just marks the test as
// having failed.
verify_that!(value, eq(2))?; // Passes, but the test already failed.
Ok(())
}
```
### Interoperability
You can use the `#[google_test]` macro together with many other libraries such
as [rstest](https://crates.io/crates/rstest). Just apply both attribute macros
to the test:
```rust
#[rstest]
#[case(1)]
#[case(2)]
#[case(3)]
#[google_test]
fn rstest_works_with_google_test(#[case] value: u32) -> Result<()> {
verify_that!(value, gt(0))
}
```
The macro also works together with
[async tests with Tokio](https://docs.rs/tokio/latest/tokio/attr.test.html) in
the same way:
```rust
#[tokio::test]
#[google_test]
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`:
```rust
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!`.
## 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:
```rust
#[test]
fn always_fails() -> Result<()> {
fail!("This test must fail with {}", "today")
}
```
## Contributing Changes
Please read [CONTRIBUTING.md](CONTRIBUTING.md) for details on how to contribute
to this project.