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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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<h2><a class="header" href="#appendix-a-keywords" id="appendix-a-keywords">Appendix A: Keywords</a></h2>
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<p>The following list contains keywords that are reserved for current or future
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use by the Rust language. As such, they cannot be used as identifiers (except
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as raw identifiers as we’ll discuss in the “<a href="#raw-identifiers">Raw
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Identifiers</a><!-- ignore -->” section), including names of
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functions, variables, parameters, struct fields, modules, crates, constants,
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macros, static values, attributes, types, traits, or lifetimes.</p>
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<h3><a class="header" href="#keywords-currently-in-use" id="keywords-currently-in-use">Keywords Currently in Use</a></h3>
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<p>The following keywords currently have the functionality described.</p>
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<ul>
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<li><code>as</code> - perform primitive casting, disambiguate the specific trait containing
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an item, or rename items in <code>use</code> and <code>extern crate</code> statements</li>
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<li><code>break</code> - exit a loop immediately</li>
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<li><code>const</code> - define constant items or constant raw pointers</li>
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<li><code>continue</code> - continue to the next loop iteration</li>
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<li><code>crate</code> - link an external crate or a macro variable representing the crate in
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which the macro is defined</li>
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<li><code>dyn</code> - dynamic dispatch to a trait object</li>
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<li><code>else</code> - fallback for <code>if</code> and <code>if let</code> control flow constructs</li>
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<li><code>enum</code> - define an enumeration</li>
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<li><code>extern</code> - link an external crate, function, or variable</li>
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<li><code>false</code> - Boolean false literal</li>
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<li><code>fn</code> - define a function or the function pointer type</li>
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<li><code>for</code> - loop over items from an iterator, implement a trait, or specify a
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higher-ranked lifetime</li>
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<li><code>if</code> - branch based on the result of a conditional expression</li>
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<li><code>impl</code> - implement inherent or trait functionality</li>
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<li><code>in</code> - part of <code>for</code> loop syntax</li>
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<li><code>let</code> - bind a variable</li>
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<li><code>loop</code> - loop unconditionally</li>
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<li><code>match</code> - match a value to patterns</li>
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<li><code>mod</code> - define a module</li>
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<li><code>move</code> - make a closure take ownership of all its captures</li>
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<li><code>mut</code> - denote mutability in references, raw pointers, or pattern bindings</li>
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<li><code>pub</code> - denote public visibility in struct fields, <code>impl</code> blocks, or modules</li>
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<li><code>ref</code> - bind by reference</li>
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<li><code>return</code> - return from function</li>
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<li><code>Self</code> - a type alias for the type implementing a trait</li>
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<li><code>self</code> - method subject or current module</li>
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<li><code>static</code> - global variable or lifetime lasting the entire program execution</li>
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<li><code>struct</code> - define a structure</li>
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<li><code>super</code> - parent module of the current module</li>
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<li><code>trait</code> - define a trait</li>
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<li><code>true</code> - Boolean true literal</li>
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<li><code>type</code> - define a type alias or associated type</li>
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<li><code>unsafe</code> - denote unsafe code, functions, traits, or implementations</li>
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<li><code>use</code> - bring symbols into scope</li>
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<li><code>where</code> - denote clauses that constrain a type</li>
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<li><code>while</code> - loop conditionally based on the result of an expression</li>
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</ul>
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<h3><a class="header" href="#keywords-reserved-for-future-use" id="keywords-reserved-for-future-use">Keywords Reserved for Future Use</a></h3>
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<p>The following keywords do not have any functionality but are reserved by Rust
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for potential future use.</p>
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<ul>
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<li><code>abstract</code></li>
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<li><code>async</code></li>
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<li><code>await</code></li>
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<li><code>become</code></li>
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<li><code>box</code></li>
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<li><code>do</code></li>
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<li><code>final</code></li>
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<li><code>macro</code></li>
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<li><code>override</code></li>
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<li><code>priv</code></li>
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<li><code>try</code></li>
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<li><code>typeof</code></li>
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<li><code>unsized</code></li>
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<li><code>virtual</code></li>
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<li><code>yield</code></li>
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</ul>
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<h3><a class="header" href="#raw-identifiers" id="raw-identifiers">Raw Identifiers</a></h3>
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<p><em>Raw identifiers</em> are the syntax that lets you use keywords where they wouldn’t
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normally be allowed. You use a raw identifier by prefixing a keyword with <code>r#</code>.</p>
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<p>For example, <code>match</code> is a keyword. If you try to compile the following function
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that uses <code>match</code> as its name:</p>
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<p><span class="filename">Filename: src/main.rs</span></p>
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<pre><code class="language-rust ignore does_not_compile">fn match(needle: &str, haystack: &str) -> bool {
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haystack.contains(needle)
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}
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</code></pre>
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<p>you’ll get this error:</p>
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<pre><code class="language-text">error: expected identifier, found keyword `match`
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--> src/main.rs:4:4
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4 | fn match(needle: &str, haystack: &str) -> bool {
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| ^^^^^ expected identifier, found keyword
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</code></pre>
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<p>The error shows that you can’t use the keyword <code>match</code> as the function
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identifier. To use <code>match</code> as a function name, you need to use the raw
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identifier syntax, like this:</p>
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<p><span class="filename">Filename: src/main.rs</span></p>
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<pre><pre class="playpen"><code class="language-rust">fn r#match(needle: &str, haystack: &str) -> bool {
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haystack.contains(needle)
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}
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fn main() {
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assert!(r#match("foo", "foobar"));
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}
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</code></pre></pre>
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<p>This code will compile without any errors. Note the <code>r#</code> prefix on the function
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name in its definition as well as where the function is called in <code>main</code>.</p>
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<p>Raw identifiers allow you to use any word you choose as an identifier, even if
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that word happens to be a reserved keyword. In addition, raw identifiers allow
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you to use libraries written in a different Rust edition than your crate uses.
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For example, <code>try</code> isn’t a keyword in the 2015 edition but is in the 2018
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edition. If you depend on a library that’s written using the 2015 edition and
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has a <code>try</code> function, you’ll need to use the raw identifier syntax, <code>r#try</code> in
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this case, to call that function from your 2018 edition code. See <a href="appendix-05-editions.html">Appendix
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E</a><!-- ignore --> for more information on editions.</p>
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