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<ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong>1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong>2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong>3.</strong> Syntax and Semantics</a></li><li><ul class="section"><li><a href="variable-bindings.html"><strong>3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong>3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong>3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="if.html"><strong>3.5.</strong> if</a></li><li><a href="loops.html"><strong>3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong>3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong>3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong>3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong>3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong>3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong>3.12.</strong> Structs</a></li><li><a href="enums.html"><strong>3.13.</strong> Enums</a></li><li><a href="match.html"><strong>3.14.</strong> Match</a></li><li><a href="patterns.html"><strong>3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong>3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong>3.17.</strong> Strings</a></li><li><a href="generics.html"><strong>3.18.</strong> Generics</a></li><li><a href="traits.html"><strong>3.19.</strong> Traits</a></li><li><a href="drop.html"><strong>3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong>3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong>3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong>3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong>3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong>3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong>3.26.</strong> <code>const</code> and <code>static</code></a></li><li><a href="attributes.html"><strong>3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong>3.28.</strong> <code>type</code> aliases</a></li><li><a href="casting-between-types.html"><strong>3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong>3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong>3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong>3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong>3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong>3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong>3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong>3.36.</strong> <code>unsafe</code></a></li></ul></li><li><a href="effective-rust.html"><strong>4.</strong> Effective Rust</a></li><li><ul class="section"><li><a href="the-stack-and-the-heap.html"><strong>4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong>4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html" class="active"><strong>4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong>4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong>4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong>4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong>4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong>4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong>4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong>4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong>4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong>4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong>4.13.</strong> Procedural Macros (and custom derive)</a></li></ul></li><li><a href="glossary.html"><strong>5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong>6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong>7.</strong> Bibliography</a></li></ul>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<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>
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