# Testing

As we know testing is integral to any piece of software! Rust has first-class support for unit and integration testing (see this chapter in TRPL).

From the testing chapters linked above, we see how to write unit tests and integration tests. Organizationally, we can place unit tests in the modules they test and integration tests in their own `tests/` directory:

    foo
    ├── Cargo.toml
    ├── src
    │   └── main.rs
    └── tests
        ├── my_test.rs
        └── my_other_test.rs

Each file in `tests` is a separate integration test.

`cargo` naturally provides an easy way to run all of your tests!

    $ cargo test

You should see output like this:

    $ cargo test
       Compiling blah v0.1.0 (file:///nobackup/blah)
        Finished dev [unoptimized + debuginfo] target(s) in 0.89 secs
         Running target/debug/deps/blah-d3b32b97275ec472

    running 3 tests
    test test_bar ... ok
    test test_baz ... ok
    test test_foo_bar ... ok
    test test_foo ... ok

    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

You can also run tests whose name matches a pattern:

    $ cargo test test_foo

    $ cargo test test_foo
       Compiling blah v0.1.0 (file:///nobackup/blah)
        Finished dev [unoptimized + debuginfo] target(s) in 0.35 secs
         Running target/debug/deps/blah-d3b32b97275ec472

    running 2 tests
    test test_foo ... ok
    test test_foo_bar ... ok

    test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 2 filtered out

One word of caution: Cargo may run multiple tests concurrently, so make sure that they don't race with each other. For example, if they all output to a file, you should make them write to different files.