Testing

Program testing can be a very effective way to show the presence of bugs, but it is hopelessly inadequate for showing their absence.

Edsger W. Dijkstra, "The Humble Programmer" (1972)

Rust is a programming language that cares a lot about correctness, but correctness is a complex topic and isn't easy to prove. Rust places a lot of weight on its type system to help ensure that our programs do what we intend, but it cannot help with everything. As such, Rust also includes support for writing software tests in the language itself.

For example, we can write a function called add_two with a signature that has an integer as a parameter and returns an integer as a result. We can implement and compile that function, and Rust can do all the type checking and borrow checking that we've seen it's capable of doing. What Rust can't check for us is that we've implemented this function to return the parameter plus two and not the parameter plus 10 or the parameter minus 50! That's where tests come in. We can write tests that, for example, pass 3 to the add_two function and check that we get 5 back. We can run the tests whenever we make changes to our code to make sure we didn't change any existing behavior from what the tests specify it should be.

Testing is a skill, and we cannot hope to cover everything about how to write good tests in one chapter of a book. What we can discuss, however, are the mechanics of Rust's testing facilities. We'll talk about the annotations and macros available to you when writing your tests, the default behavior and options provided for running your tests, and how to organize tests into unit tests and integration tests.