# 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)

Correctness in our programs means that our code does what we intend for it to do. Rust is a programming language that cares a lot about correctness, but correctness is a complex topic and isn't easy to prove. Rust's type system shoulders a huge part of this burden, but the type system cannot catch every kind of incorrectness. As such, Rust includes support for writing software tests within the language itself.

As an example, say we write a function called `add_two` that adds two to a number passed to it. This function's signature accepts an integer as a parameter and returns an integer as a result. When we implement and compile that function, Rust will do all the type checking and borrow checking that we've seen so far. Those checks will make sure that, for instance, we aren't passing a `String` value or an invalid reference to this function. What Rust can't check is that this function will do precisely what we intend: return the parameter plus two, rather than, say, the parameter plus 10 or the parameter minus 50! That's where tests come in.

We can write tests that assert, for example, that when we pass `3` to the `add_two` function, we get `5` back. We can run these tests whenever we make changes to our code to make sure any existing correct behavior has not changed.

Testing is a complex skill, and we cannot hope to cover everything about how to write good tests in one chapter of a book, so here we'll just discuss 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.