# Unit testing

Tests 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][mod] with the `#[cfg(test)]` [attribute][attribute].
Test functions are marked with the `#[test]` attribute.

Tests fail when something in the test function [panics][panic]. There are some
helper [macros][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.

```rust,ignore
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`.

```shell
$ 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.

```rust,editable
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(())
    }
}
```

See ["The Edition Guide"][editionguide] for more details.

## Testing panics

To 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.

```rust,ignore
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:

```shell
$ 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 tests

To run specific tests one may specify the test name to `cargo test` command.

```shell
$ 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.

```shell
$ 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 tests

Tests can be marked with the `#[ignore]` attribute to exclude some tests. Or to run
them with command `cargo test -- --ignored`

```rust,ignore
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);
    }
}
```

```shell
$ 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
```

[attribute]: ../attribute.md
[panic]: ../std/panic.md
[macros]: ../macros.md
[mod]: ../mod.md
[editionguide]: https://doc.rust-lang.org/edition-guide/rust-2018/error-handling-and-panics/question-mark-in-main-and-tests.html
