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<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html" class="active"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><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><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><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><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects that Allow for Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning our Single Threaded Server into a Multithreaded Server</a></li><li><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-newest-features.html"><strong aria-hidden="true">21.6.</strong> F - Newest Features</a></li><li><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="ch01-02-hello-world.html#hello-world" id="hello-world"><h2>Hello, World!</h2></a>
<p>Now that you’ve installed Rust, let’s write your first Rust program. It’s
traditional when learning a new language to write a little program that prints
the text “Hello, world!” to the screen, so we’ll do the same here!</p>
<blockquote>
<p>Note: This book assumes basic familiarity with the command line. Rust makes
no specific demands about your editing, tooling, or where your code lives, so
if you prefer to use an integrated development environment (IDE) instead of
the command line, feel free to use your favorite IDE. Many IDEs now have some
degree of Rust support; check the IDE’s documentation for details. Recently,
the Rust team has been focusing on enabling great IDE support, and progress
has been made rapidly on that front!</p>
</blockquote>
<a class="header" href="ch01-02-hello-world.html#creating-a-project-directory" id="creating-a-project-directory"><h3>Creating a Project Directory</h3></a>
<p>You’ll start by making a directory to store your Rust code. It doesn’t matter
to Rust where your code lives, but for the exercises and projects in this book,
we suggest making a <em>projects</em> directory in your home directory and keeping all
your projects there.</p>
<p>Open a terminal and enter the following commands to make a <em>projects</em> directory
and a directory for the “Hello, world!” project within the <em>projects</em> directory.</p>
<p>For Linux and macOS, enter this:</p>
<pre><code class="language-text">$ mkdir ~/projects
$ cd ~/projects
$ mkdir hello_world
$ cd hello_world
</code></pre>
<p>For Windows CMD, enter this:</p>
<pre><code class="language-cmd">> mkdir "%USERPROFILE%\projects"
> cd /d "%USERPROFILE%\projects"
> mkdir hello_world
> cd hello_world
</code></pre>
<p>For Windows PowerShell, enter this:</p>
<pre><code class="language-powershell">> mkdir $env:USERPROFILE\projects
> cd $env:USERPROFILE\projects
> mkdir hello_world
> cd hello_world
</code></pre>
<a class="header" href="ch01-02-hello-world.html#writing-and-running-a-rust-program" id="writing-and-running-a-rust-program"><h3>Writing and Running a Rust Program</h3></a>
<p>Next, make a new source file and call it <em>main.rs</em>. Rust files always end with
the <em>.rs</em> extension. If you’re using more than one word in your filename, use
an underscore to separate them. For example, use <em>hello_world.rs</em> rather than
<em>helloworld.rs</em>.</p>
<p>Now open the <em>main.rs</em> file you just created, and enter the code in Listing 1-1.</p>
<p><span class="filename">Filename: main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
println!("Hello, world!");
}
</code></pre></pre>
<p><span class="caption">Listing 1-1: A program that prints “Hello, world!”</span></p>
<p>Save the file, and go back to your terminal window. On Linux or macOS, enter
the following commands to compile and run the file:</p>
<pre><code class="language-text">$ rustc main.rs
$ ./main
Hello, world!
</code></pre>
<p>On Windows, enter the command <code>.\main.exe</code> instead of <code>./main</code>.</p>
<pre><code class="language-powershell">> rustc main.rs
> .\main.exe
Hello, world!
</code></pre>
<p>Regardless of your operating system, the string <code>Hello, world!</code> should print to
the terminal. If you don’t see this output, refer back to the “Troubleshooting”
section for ways to get help.</p>
<p>If <code>Hello, world!</code> did print, congratulations! You’ve officially written a Rust
program. That makes you a Rust programmer! Welcome!</p>
<a class="header" href="ch01-02-hello-world.html#anatomy-of-a-rust-program" id="anatomy-of-a-rust-program"><h3>Anatomy of a Rust Program</h3></a>
<p>Let’s review in detail what just happened in your “Hello, world!” program.
Here’s the first piece of the puzzle:</p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
}
</code></pre></pre>
<p>These lines define a <em>function</em> in Rust. The <code>main</code> function is special: it is
always the first code that runs in every executable Rust program. The first
line declares a function named <code>main</code> that has no parameters and returns
nothing. If there were parameters, they would go inside the parentheses, <code>(</code>
and <code>)</code>.</p>
<p>Also, note that the function body is wrapped in curly brackets, <code>{</code> and <code>}</code>.
Rust requires these around all function bodies. It’s good style to place the
opening curly bracket on the same line as the function declaration, adding one
space in between.</p>
<p>At the time of this writing, an automatic formatter tool called <code>rustfmt</code> is
under development. If you want to stick to a standard style across Rust
projects, <code>rustfmt</code> will format your code in a particular style. The Rust team
plans to eventually include it with the standard Rust distribution, like
<code>rustc</code>. So depending on when you read this book, it might already be installed
on your computer! Check the online documentation for more details.</p>
<p>Inside the <code>main</code> function is the following code:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
println!("Hello, world!");
#}</code></pre></pre>
<p>This line does all the work in this little program: it prints text to the
screen. There are four important details to notice here. First, Rust style is
to indent with four spaces, not a tab.</p>
<p>Second, <code>println!</code> calls a Rust <em>macro</em>. If it called a function instead, it
would be entered as <code>println</code> (without the <code>!</code>). We’ll discuss Rust macros in
more detail in Appendix D. For now, you just need to know that using a <code>!</code>
means that you’re calling a macro instead of a normal function.</p>
<p>Third, you see the <code>"Hello, world!"</code> <em>string</em>. We pass this string as an
argument to <code>println!</code>, and the string is printed to the screen.</p>
<p>Fourth, we end the line with a semicolon <code>;</code>, which indicates that this
expression is over and the next one is ready to begin. Most lines of Rust code
end with a semicolon.</p>
<a class="header" href="ch01-02-hello-world.html#compiling-and-running-are-separate-steps" id="compiling-and-running-are-separate-steps"><h3>Compiling and Running Are Separate Steps</h3></a>
<p>You’ve just run a newly created program, so let’s examine each step in the
process.</p>
<p>Before running a Rust program, you must compile it using the Rust compiler by
entering the <code>rustc</code> command and passing it the name of your source file, like
this:</p>
<pre><code class="language-text">$ rustc main.rs
</code></pre>
<p>If you have a C or C++ background, you’ll notice that this is similar to <code>gcc</code>
or <code>clang</code>. After compiling successfully, Rust outputs a binary executable.</p>
<p>On Linux, macOS, and PowerShell on Windows, you can see the executable by
entering the <code>ls</code> command in your shell as follows:</p>
<pre><code class="language-text">$ ls
main main.rs
</code></pre>
<p>With CMD on Windows, you would enter the following:</p>
<pre><code class="language-cmd">> dir /B %= the /B option says to only show the file names =%
main.exe
main.pdb
main.rs
</code></pre>
<p>This shows the source code file with the <em>.rs</em> extension, the executable file
(<em>main.exe</em> on Windows, but <em>main</em> on all other platforms), and, when using
CMD, a file containing debugging information with the <em>.pdb</em> extension. From
here, you run the <em>main</em> or <em>main.exe</em> file, like this:</p>
<pre><code class="language-text">$ ./main # or .\main.exe on Windows
</code></pre>
<p>If <em>main.rs</em> was your “Hello, world!” program, this line would print <code>Hello, world!</code> to your terminal.</p>
<p>If you’re more familiar with a dynamic language, such as Ruby, Python, or
JavaScript, you might not be used to compiling and running a program as
separate steps. Rust is an <em>ahead-of-time compiled</em> language, meaning you can
compile a program, give the executable to someone else, and they can run it
even without having Rust installed. If you give someone a <em>.rb</em>, <em>.py</em>, or
<em>.js</em> file, they need to have a Ruby, Python, or JavaScript implementation
installed (respectively). But in those languages, you only need one command to
compile and run your program. Everything is a trade-off in language design.</p>
<p>Just compiling with <code>rustc</code> is fine for simple programs, but as your project
grows, you’ll want to manage all the options and make it easy to share your
code. Next, we’ll introduce you to the Cargo tool, which will help you write
real-world Rust programs.</p>
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