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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
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<main>
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<h1><a class="header" href="#writing-automated-tests" id="writing-automated-tests">Writing Automated Tests</a></h1>
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<p>In his 1972 essay “The Humble Programmer,” Edsger W. Dijkstra said that
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“Program testing can be a very effective way to show the presence of bugs, but
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it is hopelessly inadequate for showing their absence.” That doesn’t mean we
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shouldn’t try to test as much as we can!</p>
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<p>Correctness in our programs is the extent to which our code does what we intend
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it to do. Rust is designed with a high degree of concern about the correctness
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of programs, but correctness is complex and not easy to prove. Rust’s type
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system shoulders a huge part of this burden, but the type system cannot catch
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every kind of incorrectness. As such, Rust includes support for writing
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automated software tests within the language.</p>
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<p>As an example, say we write a function called <code>add_two</code> that adds 2 to whatever
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number is passed to it. This function’s signature accepts an integer as a
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parameter and returns an integer as a result. When we implement and compile
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that function, Rust does all the type checking and borrow checking that you’ve
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learned so far to ensure that, for instance, we aren’t passing a <code>String</code> value
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or an invalid reference to this function. But Rust <em>can’t</em> check that this
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function will do precisely what we intend, which is return the parameter plus 2
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rather than, say, the parameter plus 10 or the parameter minus 50! That’s where
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tests come in.</p>
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<p>We can write tests that assert, for example, that when we pass <code>3</code> to the
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<code>add_two</code> function, the returned value is <code>5</code>. We can run these tests whenever
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we make changes to our code to make sure any existing correct behavior has not
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changed.</p>
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<p>Testing is a complex skill: although we can’t cover every detail about how to
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write good tests in one chapter, we’ll discuss the mechanics of Rust’s testing
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facilities. We’ll talk about the annotations and macros available to you when
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writing your tests, the default behavior and options provided for running your
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tests, and how to organize tests into unit tests and integration tests.</p>
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