# Tests

`rustc` has a built-in facility for building and running tests for a crate.
More information about writing and running tests may be found in the [Testing
Chapter] of the Rust Programming Language book.

Tests are written as free functions with the [`#[test]`
attribute][attribute-test]. For example:

```rust
#[test]
fn it_works() {
    assert_eq!(2 + 2, 4);
}
```

Tests "pass" if they return without an error. They "fail" if they [panic], or
return a [`Result`] with an error.

By passing the [`--test` option] to `rustc`, the compiler will build the crate
in a special mode to construct an executable that will run the tests in the
crate. The `--test` flag will make the following changes:

* The crate will be built as a `bin` [crate type], forcing it to be an
  executable.
* Links the executable with [`libtest`], the test harness that is part of the
  standard library, which handles running the tests.
* Synthesizes a [`main` function] which will process command-line arguments
  and run the tests. This new `main` function will replace any existing `main`
  function as the entry point of the executable, though the existing `main`
  will still be compiled.
* Enables the [`test` cfg option], which allows your code to use conditional
  compilation to detect if it is being built as a test.
* Enables building of functions annotated with the [`test`][attribute-test]
  and [`bench`](#benchmarks) attributes, which will be run by the test
  harness.

After the executable is created, you can run it to execute the tests and
receive a report on what passes and fails. If you are using [Cargo] to manage
your project, it has a built-in [`cargo test`] command which handles all of
this automatically. An example of the output looks like this:

```text
running 4 tests
test it_works ... ok
test check_valid_args ... ok
test invalid_characters ... ok
test walks_the_dog ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```

> **Note**: Tests must be built with the [`unwind` panic
> strategy][panic-strategy]. This is because all tests run in the same
> process, and they are intended to catch panics, which is not possible with
> the `abort` strategy. See the unstable [`-Z panic-abort-tests`] option for
> experimental support of the `abort` strategy by spawning tests in separate
> processes.

## Test attributes

Tests are indicated using attributes on free functions. The following
attributes are used for testing, see the linked documentation for more
details:

* [`#[test]`][attribute-test] — Indicates a function is a test to be run.
* `#[bench]` — Indicates a function is a benchmark to be
  run. Benchmarks are currently unstable and only available in the nightly
  channel, see the [unstable docs][bench-docs] for more details.
* [`#[should_panic]`][attribute-should_panic] — Indicates that the test
  function will only pass if the function [panics][panic].
* [`#[ignore]`][attribute-ignore] — Indicates that the test function will be
  compiled, but not run by default. See the [`--ignored`](#--ignored) and
  [`--include-ignored`](#--include-ignored) options to run these tests.

## CLI arguments

The libtest harness has several command-line arguments to control its
behavior.

> Note: When running with [`cargo test`], the libtest CLI arguments must be
> passed after the `--` argument to differentiate between flags for Cargo and
> those for the harness. For example: `cargo test -- --nocapture`

### Filters

Positional arguments (those without a `-` prefix) are treated as filters which
will only run tests whose name matches one of those strings. The filter will
match any substring found in the full path of the test function. For example,
if the test function `it_works` is located in the module
`utils::paths::tests`, then any of the filters `works`, `path`, `utils::`, or
`utils::paths::tests::it_works` will match that test.

See [Selection options](#selection-options) for more options to control which
tests are run.

### Action options

The following options perform different actions other than running tests.

#### `--list`

Prints a list of all tests and benchmarks. Does not run any of the tests.
[Filters](#filters) can be used to list only matching tests.

#### `-h`, `--help`

Displays usage information and command-line options.

### Selection options

The following options change how tests are selected.

#### `--test`

This is the default mode where all tests will be run as well as running all
benchmarks with only a single iteration (to ensure the benchmark works,
without taking the time to actually perform benchmarking). This can be
combined with the `--bench` flag to run both tests and perform full
benchmarking.

#### `--bench`

This runs in a mode where tests are ignored, and only runs benchmarks. This
can be combined with `--test` to run both benchmarks and tests.

#### `--exact`

This forces [filters](#filters) to match the full path of the test exactly.
For example, if the test `it_works` is in the module `utils::paths::tests`,
then only the string `utils::paths::tests::it_works` will match that test.

#### `--skip` _FILTER_

Skips any tests whose name contains the given _FILTER_ string. This flag may
be passed multiple times.

#### `--ignored`

Runs only tests that are marked with the [`ignore`
attribute][attribute-ignore].

#### `--include-ignored`

Runs both [ignored](#--ignored) and non-ignored tests.

#### `--exclude-should-panic`

Excludes tests marked with the [`should_panic`
attribute][attribute-should_panic].

⚠️ 🚧 This option is [unstable](#unstable-options), and requires the `-Z
unstable-options` flag. See [tracking issue
#82348](https://github.com/rust-lang/rust/issues/82348) for more information.

### Execution options

The following options affect how tests are executed.

#### `--test-threads` _NUM_THREADS_

Sets the number of threads to use for running tests in parallel. By default,
uses the amount of concurrency available on the hardware as indicated by
[`available_concurrency`].

This can also be specified with the `RUST_TEST_THREADS` environment variable.

#### `--force-run-in-process`

Forces the tests to run in a single process when using the [`abort` panic
strategy][panic-strategy].

⚠️ 🚧 This only works with the unstable [`-Z panic-abort-tests`] option, and
requires the `-Z unstable-options` flag. See [tracking issue
#67650](https://github.com/rust-lang/rust/issues/67650) for more information.

#### `--ensure-time`

⚠️ 🚧 This option is [unstable](#unstable-options), and requires the `-Z
unstable-options` flag. See [tracking issue
#64888](https://github.com/rust-lang/rust/issues/64888) and the [unstable
docs](../../unstable-book/compiler-flags/report-time.html) for more information.

### Output options

The following options affect the output behavior.

#### `-q`, `--quiet`

Displays one character per test instead of one line per test. This is an alias
for [`--format=terse`](#--format-format).

#### `--nocapture`

Does not capture the stdout and stderr of the test, and allows tests to print
to the console. Usually the output is captured, and only displayed if the test
fails.

This may also be specified by setting the `RUST_TEST_NOCAPTURE` environment
variable set to anything but `0`.

#### `--show-output`

Displays the stdout and stderr of successful tests after all tests have run.

Contrast this with [`--nocapture`](#--nocapture) which allows tests to print
*while they are running*, which can cause interleaved output if there are
multiple tests running in parallel, `--show-output` ensures the output is
contiguous, but requires waiting for all tests to finish.

#### `--color` _COLOR_

Control when colored terminal output is used. Valid options:

* `auto`: Colorize if stdout is a tty and [`--nocapture`](#--nocapture) is not
  used. This is the default.
* `always`: Always colorize the output.
* `never`: Never colorize the output.

#### `--format` _FORMAT_

Controls the format of the output. Valid options:

* `pretty`: This is the default format, with one line per test.
* `terse`: Displays only a single character per test. [`--quiet`](#-q---quiet)
  is an alias for this option.
* `json`: Emits JSON objects, one per line. ⚠️ 🚧 This option is
  [unstable](#unstable-options), and requires the `-Z unstable-options` flag.
  See [tracking issue #49359](https://github.com/rust-lang/rust/issues/49359)
  for more information.

#### `--logfile` _PATH_

Writes the results of the tests to the given file.

#### `--report-time` _FORMAT_

⚠️ 🚧 This option is [unstable](#unstable-options), and requires the `-Z
unstable-options` flag. See [tracking issue
#64888](https://github.com/rust-lang/rust/issues/64888) and the [unstable
docs](../../unstable-book/compiler-flags/report-time.html) for more information.

### Unstable options

Some CLI options are added in an "unstable" state, where they are intended for
experimentation and testing to determine if the option works correctly, has
the right design, and is useful. The option may not work correctly, break, or
change at any time. To signal that you acknowledge that you are using an
unstable option, they require passing the `-Z unstable-options` command-line
flag.

## Benchmarks

The libtest harness supports running benchmarks for functions annotated with
the `#[bench]` attribute. Benchmarks are currently unstable, and only
available on the [nightly channel]. More information may be found in the
[unstable book][bench-docs].

## Custom test frameworks

Experimental support for using custom test harnesses is available on the
[nightly channel]. See [tracking issue
#50297](https://github.com/rust-lang/rust/issues/50297) and the
[custom_test_frameworks documentation] for more information.

[`--test` option]: ../command-line-arguments.md#option-test
[`-Z panic-abort-tests`]: https://github.com/rust-lang/rust/issues/67650
[`available_concurrency`]: ../../std/thread/fn.available_concurrency.html
[`cargo test`]: ../../../cargo-doc/doc/commands/cargo-test.html
[`libtest`]: ../../test/index.html
[`main` function]: ../../reference/crates-and-source-files.html#main-functions
[`Result`]: ../../std/result/index.html
[`test` cfg option]: ../../reference/conditional-compilation.html#test
[attribute-ignore]: ../../reference/attributes/testing.html#the-ignore-attribute
[attribute-should_panic]: ../../reference/attributes/testing.html#the-should_panic-attribute
[attribute-test]: ../../reference/attributes/testing.html#the-test-attribute
[bench-docs]: ../../unstable-book/library-features/test.html
[Cargo]: ../../../cargo-doc/doc/index.html
[crate type]: ../../reference/linkage.html
[custom_test_frameworks documentation]: ../../unstable-book/language-features/custom-test-frameworks.html
[nightly channel]: ../../book/appendix-07-nightly-rust.html
[panic-strategy]: ../../book/ch09-01-unrecoverable-errors-with-panic.html
[panic]: ../../book/ch09-01-unrecoverable-errors-with-panic.html
[Testing Chapter]: ../../book/ch11-00-testing.html
