<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Conditional Compilation - The Rust Programming Language</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<base href="">
<link rel="stylesheet" href="book.css">
<link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css">
<link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css">
<link rel="shortcut icon" href="favicon.png">
<!-- Font Awesome -->
<link rel="stylesheet" href="_FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<link rel="stylesheet" href="ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="src/theme/first-edition.css">
</head>
<body class="light">
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = 'light'; }
document.body.className = theme;
document.querySelector('html').className = theme + ' js';
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong aria-hidden="true">2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong aria-hidden="true">3.</strong> Syntax and Semantics</a></li><li><ol class="section"><li><a href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html" class="active"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="draft-warning" class="warning">
<span class="message">You are reading an <strong>outdated</strong> edition of TRPL. For more, go <a href="../index.html">here</a>.</span>
<button type="button" id="hide-draft-warning" title="Hide draft warning" class="button">
<i class="fa fa-times"></i>
</button>
</div>
<!-- Hide / unhide warning before it is displayed -->
<script type="text/javascript">
var warning = localStorage.getItem('trpl-first-edition-draft-warning');
if (warning === 'hidden') {
Array
.from(document.querySelectorAll('#page-wrapper'))
.forEach(function(block) { block.classList.remove('has-warning'); });
var elem = document.getElementById("draft-warning");
elem.parentNode.removeChild(elem);
}
document.addEventListener("DOMContentLoaded", function(event) {
document.getElementById("hide-draft-warning").addEventListener("click", function(e) {
var elem = document.getElementById("draft-warning");
elem.parentNode.removeChild(elem);
localStorage.setItem('trpl-first-edition-draft-warning', 'hidden');
});
});
</script>
<div id="menu-bar" class="menu-bar">
<div id="menu-bar-sticky-container">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light <span class="default">(default)</span></button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">The Rust Programming Language</h1>
<div class="right-buttons">
<a href="print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
</div>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<a class="header" href="conditional-compilation.html#conditional-compilation" id="conditional-compilation"><h1>Conditional Compilation</h1></a>
<p>Rust has a special attribute, <code>#[cfg]</code>, which allows you to compile code
based on a flag passed to the compiler. It has two forms:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[cfg(foo)]
# fn foo() {}
#[cfg(bar = "baz")]
# fn bar() {}
#}</code></pre></pre>
<p>They also have some helpers:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[cfg(any(unix, windows))]
# fn foo() {}
#[cfg(all(unix, target_pointer_width = "32"))]
# fn bar() {}
#[cfg(not(foo))]
# fn not_foo() {}
#}</code></pre></pre>
<p>These can nest arbitrarily:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[cfg(any(not(unix), all(target_os="macos", target_arch = "powerpc")))]
# fn foo() {}
#}</code></pre></pre>
<p>As for how to enable or disable these switches, if you’re using Cargo,
they get set in the <a href="http://doc.crates.io/manifest.html#the-features-section"><code>[features]</code> section</a> of your <code>Cargo.toml</code>:</p>
<pre><code class="language-toml">[features]
# no features by default
default = []
# Add feature "foo" here, then you can use it.
# Our "foo" feature depends on nothing else.
foo = []
</code></pre>
<p>When you do this, Cargo passes along a flag to <code>rustc</code>:</p>
<pre><code class="language-text">--cfg feature="${feature_name}"
</code></pre>
<p>The sum of these <code>cfg</code> flags will determine which ones get activated, and
therefore, which code gets compiled. Let’s take this code:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[cfg(feature = "foo")]
mod foo {
}
#}</code></pre></pre>
<p>If we compile it with <code>cargo build --features "foo"</code>, it will send the <code>--cfg feature="foo"</code> flag to <code>rustc</code>, and the output will have the <code>mod foo</code> in it.
If we compile it with a regular <code>cargo build</code>, no extra flags get passed on,
and so, no <code>foo</code> module will exist.</p>
<a class="header" href="conditional-compilation.html#cfg_attr" id="cfg_attr"><h1>cfg_attr</h1></a>
<p>You can also set another attribute based on a <code>cfg</code> variable with <code>cfg_attr</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[cfg_attr(a, b)]
# fn foo() {}
#}</code></pre></pre>
<p>Will be the same as <code>#[b]</code> if <code>a</code> is set by <code>cfg</code> attribute, and nothing otherwise.</p>
<a class="header" href="conditional-compilation.html#cfg" id="cfg"><h1>cfg!</h1></a>
<p>The <code>cfg!</code> macro lets you use these kinds of flags elsewhere in your code, too:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
println!("Think Different!");
}
#}</code></pre></pre>
<p>These will be replaced by a <code>true</code> or <code>false</code> at compile-time, depending on the
configuration settings.</p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="testing.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="documentation.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a href="testing.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a href="documentation.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script src="searchindex.js" type="text/javascript" charset="utf-8"></script>
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="book.js" type="text/javascript" charset="utf-8"></script>
<!-- Custom JS scripts -->
</body>
</html>
distrib
>
Mageia
>
6
>
armv7hl
>
media
>
core-updates
>
by-pkgid
>
4e2dbb669434a7691662cb2f0ad38972
>
files
>
648
