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# 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
    │   └── lib.rs
    └── tests
        ├── my_test.rs
        └── my_other_test.rs

Each file in `tests` is a separate integration test, i.e. a test that is meant to test your library as if it were being called from a dependent crate.

The Testing chapter elaborates on the three different testing styles: Unit, Doc, and Integration.

`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 4 tests
    test test_bar ... ok
    test test_baz ... ok
    test test_foo_bar ... ok
    test test_foo ... ok

    test result: ok. 4 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.

One example of this concurrency causing issues is if two tests output to a file, such as below:

    #![allow(unused)]
    fn main() {
    #[cfg(test)]
    mod tests {
        // Import the necessary modules
        use std::fs::OpenOptions;
        use std::io::Write;

        // This test writes to a file
        #[test]
        fn test_file() {
            // Opens the file ferris.txt or creates one if it doesn't exist.
            let mut file = OpenOptions::new()
                .append(true)
                .create(true)
                .open("ferris.txt")
                .expect("Failed to open ferris.txt");

            // Print "Ferris" 5 times.
            for _ in 0..5 {
                file.write_all("Ferris\n".as_bytes())
                    .expect("Could not write to ferris.txt");
            }
        }

        // This test tries to write to the same file
        #[test]
        fn test_file_also() {
            // Opens the file ferris.txt or creates one if it doesn't exist.
            let mut file = OpenOptions::new()
                .append(true)
                .create(true)
                .open("ferris.txt")
                .expect("Failed to open ferris.txt");

            // Print "Corro" 5 times.
            for _ in 0..5 {
                file.write_all("Corro\n".as_bytes())
                    .expect("Could not write to ferris.txt");
            }
        }
    }
    }

Although the intent is to get the following:

    $ cat ferris.txt
    Ferris
    Ferris
    Ferris
    Ferris
    Ferris
    Corro
    Corro
    Corro
    Corro
    Corro

What actually gets put into `ferris.txt` is this:

    $ cargo test test_file && cat ferris.txt
    Corro
    Ferris
    Corro
    Ferris
    Corro
    Ferris
    Corro
    Ferris
    Corro
    Ferris