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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="notation.html"><strong aria-hidden="true">1.</strong> Notation</a></li><li><a href="lexical-structure.html"><strong aria-hidden="true">2.</strong> Lexical structure</a></li><li><ol class="section"><li><a href="input-format.html"><strong aria-hidden="true">2.1.</strong> Input format</a></li><li><a href="keywords.html"><strong aria-hidden="true">2.2.</strong> Keywords</a></li><li><a href="identifiers.html"><strong aria-hidden="true">2.3.</strong> Identifiers</a></li><li><a href="comments.html"><strong aria-hidden="true">2.4.</strong> Comments</a></li><li><a href="whitespace.html"><strong aria-hidden="true">2.5.</strong> Whitespace</a></li><li><a href="tokens.html"><strong aria-hidden="true">2.6.</strong> Tokens</a></li><li><a href="paths.html"><strong aria-hidden="true">2.7.</strong> Paths</a></li></ol></li><li><a href="macros.html"><strong aria-hidden="true">3.</strong> Macros</a></li><li><ol class="section"><li><a href="macros-by-example.html"><strong aria-hidden="true">3.1.</strong> Macros By Example</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">3.2.</strong> Procedural Macros</a></li></ol></li><li><a href="crates-and-source-files.html"><strong aria-hidden="true">4.</strong> Crates and source files</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">5.</strong> Conditional compilation</a></li><li><a href="items.html"><strong aria-hidden="true">6.</strong> Items</a></li><li><ol class="section"><li><a href="items/modules.html"><strong aria-hidden="true">6.1.</strong> Modules</a></li><li><a href="items/extern-crates.html"><strong aria-hidden="true">6.2.</strong> Extern crates</a></li><li><a href="items/use-declarations.html"><strong aria-hidden="true">6.3.</strong> Use declarations</a></li><li><a href="items/functions.html"><strong aria-hidden="true">6.4.</strong> Functions</a></li><li><a href="items/type-aliases.html"><strong aria-hidden="true">6.5.</strong> Type aliases</a></li><li><a href="items/structs.html"><strong aria-hidden="true">6.6.</strong> Structs</a></li><li><a href="items/enumerations.html"><strong aria-hidden="true">6.7.</strong> Enumerations</a></li><li><a href="items/unions.html"><strong aria-hidden="true">6.8.</strong> Unions</a></li><li><a href="items/constant-items.html"><strong aria-hidden="true">6.9.</strong> Constant items</a></li><li><a href="items/static-items.html"><strong aria-hidden="true">6.10.</strong> Static items</a></li><li><a href="items/traits.html"><strong aria-hidden="true">6.11.</strong> Traits</a></li><li><a href="items/implementations.html"><strong aria-hidden="true">6.12.</strong> Implementations</a></li><li><a href="items/external-blocks.html"><strong aria-hidden="true">6.13.</strong> External blocks</a></li><li><a href="items/generics.html"><strong aria-hidden="true">6.14.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">6.15.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">6.16.</strong> Visibility and Privacy</a></li></ol></li><li><a href="attributes.html"><strong aria-hidden="true">7.</strong> Attributes</a></li><li><ol class="section"><li><a href="attributes/testing.html"><strong aria-hidden="true">7.1.</strong> Testing</a></li><li><a href="attributes/derive.html"><strong aria-hidden="true">7.2.</strong> Derive</a></li><li><a href="attributes/diagnostics.html"><strong aria-hidden="true">7.3.</strong> Diagnostics</a></li><li><a href="attributes/codegen.html" class="active"><strong aria-hidden="true">7.4.</strong> Code generation</a></li><li><a href="attributes/limits.html"><strong aria-hidden="true">7.5.</strong> Limits</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">8.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">8.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">8.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">8.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">8.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">8.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">8.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">8.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">8.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">8.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">8.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">8.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">8.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong aria-hidden="true">8.2.11.</strong> Method call expressions</a></li><li><a href="expressions/field-expr.html"><strong aria-hidden="true">8.2.12.</strong> Field access expressions</a></li><li><a href="expressions/closure-expr.html"><strong aria-hidden="true">8.2.13.</strong> Closure expressions</a></li><li><a href="expressions/loop-expr.html"><strong aria-hidden="true">8.2.14.</strong> Loop expressions</a></li><li><a href="expressions/range-expr.html"><strong aria-hidden="true">8.2.15.</strong> Range expressions</a></li><li><a href="expressions/if-expr.html"><strong aria-hidden="true">8.2.16.</strong> If and if let expressions</a></li><li><a href="expressions/match-expr.html"><strong aria-hidden="true">8.2.17.</strong> Match expressions</a></li><li><a href="expressions/return-expr.html"><strong aria-hidden="true">8.2.18.</strong> Return expressions</a></li></ol></li></ol></li><li><a href="patterns.html"><strong aria-hidden="true">9.</strong> Patterns</a></li><li><a href="type-system.html"><strong aria-hidden="true">10.</strong> Type system</a></li><li><ol class="section"><li><a href="types.html"><strong aria-hidden="true">10.1.</strong> Types</a></li><li><ol class="section"><li><a href="types/boolean.html"><strong aria-hidden="true">10.1.1.</strong> Boolean type</a></li><li><a href="types/numeric.html"><strong aria-hidden="true">10.1.2.</strong> Numeric types</a></li><li><a href="types/textual.html"><strong aria-hidden="true">10.1.3.</strong> Textual types</a></li><li><a href="types/never.html"><strong aria-hidden="true">10.1.4.</strong> Never type</a></li><li><a href="types/tuple.html"><strong aria-hidden="true">10.1.5.</strong> Tuple types</a></li><li><a href="types/array.html"><strong aria-hidden="true">10.1.6.</strong> Array types</a></li><li><a href="types/slice.html"><strong aria-hidden="true">10.1.7.</strong> Slice types</a></li><li><a href="types/struct.html"><strong aria-hidden="true">10.1.8.</strong> Struct types</a></li><li><a href="types/enum.html"><strong aria-hidden="true">10.1.9.</strong> Enumerated types</a></li><li><a href="types/union.html"><strong aria-hidden="true">10.1.10.</strong> Union types</a></li><li><a href="types/function-item.html"><strong aria-hidden="true">10.1.11.</strong> Function item types</a></li><li><a href="types/closure.html"><strong aria-hidden="true">10.1.12.</strong> Closure types</a></li><li><a href="types/pointer.html"><strong aria-hidden="true">10.1.13.</strong> Pointer types</a></li><li><a href="types/function-pointer.html"><strong aria-hidden="true">10.1.14.</strong> Function pointer types</a></li><li><a href="types/trait-object.html"><strong aria-hidden="true">10.1.15.</strong> Trait object types</a></li><li><a href="types/impl-trait.html"><strong aria-hidden="true">10.1.16.</strong> Impl trait type</a></li><li><a href="types/parameters.html"><strong aria-hidden="true">10.1.17.</strong> Type parameters</a></li><li><a href="types/inferred.html"><strong aria-hidden="true">10.1.18.</strong> Inferred type</a></li></ol></li><li><a href="dynamically-sized-types.html"><strong aria-hidden="true">10.2.</strong> Dynamically Sized Types</a></li><li><a href="type-layout.html"><strong aria-hidden="true">10.3.</strong> Type layout</a></li><li><a href="interior-mutability.html"><strong aria-hidden="true">10.4.</strong> Interior mutability</a></li><li><a href="subtyping.html"><strong aria-hidden="true">10.5.</strong> Subtyping and Variance</a></li><li><a href="trait-bounds.html"><strong aria-hidden="true">10.6.</strong> Trait and lifetime bounds</a></li><li><a href="type-coercions.html"><strong aria-hidden="true">10.7.</strong> Type coercions</a></li><li><a href="destructors.html"><strong aria-hidden="true">10.8.</strong> Destructors</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">10.9.</strong> Lifetime elision</a></li></ol></li><li><a href="special-types-and-traits.html"><strong aria-hidden="true">11.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">12.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">12.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">12.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">12.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">13.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">14.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">14.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">14.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">14.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">14.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="const_eval.html"><strong aria-hidden="true">15.</strong> Constant Evaluation</a></li><li><a href="abi.html"><strong aria-hidden="true">16.</strong> Application Binary Interface</a></li><li><a href="runtime.html"><strong aria-hidden="true">17.</strong> The Rust runtime</a></li><li><a href="appendices.html"><strong aria-hidden="true">18.</strong> Appendices</a></li><li><ol class="section"><li><a href="macro-ambiguity.html"><strong aria-hidden="true">18.1.</strong> Macro Follow-Set Ambiguity Formal Specification</a></li><li><a href="influences.html"><strong aria-hidden="true">18.2.</strong> Influences</a></li><li><a href="glossary.html"><strong aria-hidden="true">18.3.</strong> Glossary</a></li></ol></li></ol>
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For now, this reference is a best-effort document. We strive for validity
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teams will work together to figure out how best to do this. Until then, this
is a best-effort attempt. If you find something wrong or missing, file an
<a href="https://github.com/rust-lang-nursery/reference/issues">issue</a> or
send in a pull request.
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<a class="header" href="attributes/codegen.html#code-generation-attributes" id="code-generation-attributes"><h1>Code generation attributes</h1></a>
<p>The following <a href="attributes.html">attributes</a> are used for controlling code generation.</p>
<a class="header" href="attributes/codegen.html#optimization-hints" id="optimization-hints"><h2>Optimization hints</h2></a>
<p>The <code>cold</code> and <code>inline</code> <a href="attributes.html">attributes</a> give suggestions to generate code in a
way that may be faster than what it would do without the hint. The attributes
are only hints, and may be ignored.</p>
<p>Both attributes can be used on <a href="items/functions.html">functions</a>. When applied to a function in a
<a href="items/traits.html">trait</a>, they apply only to that function when used as a default function for
a trait implementation and not to all trait implementations. The attributes
have no effect on a trait function without a body.</p>
<a class="header" href="attributes/codegen.html#the-inline-attribute" id="the-inline-attribute"><h3>The <code>inline</code> attribute</h3></a>
<p>The <em><code>inline</code> <a href="attributes.html">attribute</a></em> suggests that a copy of the attributed function
should be placed in the caller, rather than generating code to call the
function where it is defined.</p>
<blockquote>
<p><strong><em>Note</em></strong>: The <code>rustc</code> compiler automatically inlines functions based on
internal heuristics. Incorrectly inlining functions can make the program
slower, so this attribute should be used with care.</p>
</blockquote>
<p>There are three ways to use the inline attribute:</p>
<ul>
<li><code>#[inline]</code> suggests performing an inline expansion.</li>
<li><code>#[inline(always)]</code> suggests that an inline expansion should always be
performed.</li>
<li><code>#[inline(never)]</code> suggests that an inline expansion should never be
performed.</li>
</ul>
<a class="header" href="attributes/codegen.html#the-cold-attribute" id="the-cold-attribute"><h3>The <code>cold</code> attribute</h3></a>
<p>The <em><code>cold</code> <a href="attributes.html">attribute</a></em> suggests that the attributed function is unlikely to
be called.</p>
<a class="header" href="attributes/codegen.html#the-no_builtins-attribute" id="the-no_builtins-attribute"><h2>The <code>no_builtins</code> attribute</h2></a>
<p>The <em><code>no_builtins</code> <a href="attributes.html">attribute</a></em> may be applied at the crate level to disable
optimizing certain code patterns to invocations of library functions that are
assumed to exist.</p>
<a class="header" href="attributes/codegen.html#the-target_feature-attribute" id="the-target_feature-attribute"><h2>The <code>target_feature</code> attribute</h2></a>
<p>The <em><code>target_feature</code> <a href="attributes.html">attribute</a></em> may be applied to an <a href="unsafe-functions.html">unsafe function</a> to
enable code generation of that function for specific platform architecture
features. It uses the <a href="attributes.html#meta-item-attribute-syntax"><em>MetaListNameValueStr</em></a> syntax with a single key of
<code>enable</code> whose value is a string of comma-separated feature names to enable.</p>
<pre><code class="language-rust ignore">#[target_feature(enable = "avx2")]
unsafe fn foo_avx2() {}
</code></pre>
<p>Each <a href="conditional-compilation.html#target_arch">target architecture</a> has a set of features that may be enabled. It is an
error to specify a feature for a target architecture that the crate is not
being compiled for.</p>
<p>It is <a href="behavior-considered-undefined.html">undefined behavior</a> to call a function that is compiled with a feature
that is not supported on the current platform the code is running on.</p>
<p>Functions marked with <code>target_feature</code> are not inlined into a context that
does not support the given features. The <code>#[inline(always)]</code> attribute may not
be used with a <code>target_feature</code> attribute.</p>
<a class="header" href="attributes/codegen.html#available-features" id="available-features"><h3>Available features</h3></a>
<p>The following is a list of the available feature names.</p>
<a class="header" href="attributes/codegen.html#x86-or-x86_64" id="x86-or-x86_64"><h4><code>x86</code> or <code>x86_64</code></h4></a>
<table><thead><tr><th>Feature </th><th> Implicitly Enables </th><th> Description</th></tr></thead><tbody>
<tr><td><code>aes</code> </td><td> <code>sse2</code> </td><td> <a href="https://en.wikipedia.org/wiki/AES_instruction_set">AES</a> — Advanced Encryption Standard</td></tr>
<tr><td><code>avx</code> </td><td> <code>sse4.2</code> </td><td> <a href="https://en.wikipedia.org/wiki/Advanced_Vector_Extensions">AVX</a> — Advanced Vector Extensions</td></tr>
<tr><td><code>avx2</code> </td><td> <code>avx</code> </td><td> <a href="https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#AVX2">AVX2</a> — Advanced Vector Extensions 2</td></tr>
<tr><td><code>bmi1</code> </td><td> </td><td> <a href="https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets">BMI1</a> — Bit Manipulation Instruction Sets</td></tr>
<tr><td><code>bmi2</code> </td><td> </td><td> <a href="https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets#BMI2">BMI2</a> — Bit Manipulation Instruction Sets 2</td></tr>
<tr><td><code>fma</code> </td><td> <code>avx</code> </td><td> <a href="https://en.wikipedia.org/wiki/FMA_instruction_set">FMA3</a> — Three-operand fused multiply-add</td></tr>
<tr><td><code>fxsr</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/fxsave"><code>fxsave</code></a> and <a href="https://www.felixcloutier.com/x86/fxrstor"><code>fxrstor</code></a> — Save and restore x87 FPU, MMX Technology, and SSE State</td></tr>
<tr><td><code>lzcnt</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/lzcnt"><code>lzcnt</code></a> — Leading zeros count</td></tr>
<tr><td><code>pclmulqdq</code> </td><td> <code>sse2</code> </td><td> <a href="https://www.felixcloutier.com/x86/pclmulqdq"><code>pclmulqdq</code></a> — Packed carry-less multiplication quadword</td></tr>
<tr><td><code>popcnt</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/popcnt"><code>popcnt</code></a> — Count of bits set to 1</td></tr>
<tr><td><code>rdrand</code> </td><td> </td><td> <a href="https://en.wikipedia.org/wiki/RdRand"><code>rdrand</code></a> — Read random number</td></tr>
<tr><td><code>rdseed</code> </td><td> </td><td> <a href="https://en.wikipedia.org/wiki/RdRand"><code>rdseed</code></a> — Read random seed</td></tr>
<tr><td><code>sha</code> </td><td> <code>sse2</code> </td><td> <a href="https://en.wikipedia.org/wiki/Intel_SHA_extensions">SHA</a> — Secure Hash Algorithm</td></tr>
<tr><td><code>sse</code> </td><td> </td><td> <a href="https://en.wikipedia.org/wiki/Streaming_SIMD_Extensions">SSE</a> — Streaming <abbr title="Single Instruction Multiple Data">SIMD</abbr> Extensions</td></tr>
<tr><td><code>sse2</code> </td><td> <code>sse</code> </td><td> <a href="https://en.wikipedia.org/wiki/SSE2">SSE2</a> — Streaming SIMD Extensions 2</td></tr>
<tr><td><code>sse3</code> </td><td> <code>sse2</code> </td><td> <a href="https://en.wikipedia.org/wiki/SSE3">SSE3</a> — Streaming SIMD Extensions 3</td></tr>
<tr><td><code>sse4.1</code> </td><td> <code>sse3</code> </td><td> <a href="https://en.wikipedia.org/wiki/SSE4#SSE4.1">SSE4.1</a> — Streaming SIMD Extensions 4.1</td></tr>
<tr><td><code>sse4.2</code> </td><td> <code>sse4.1</code> </td><td> <a href="https://en.wikipedia.org/wiki/SSE4#SSE4.2">SSE4.2</a> — Streaming SIMD Extensions 4.2</td></tr>
<tr><td><code>ssse3</code> </td><td> <code>sse3</code> </td><td> <a href="https://en.wikipedia.org/wiki/SSSE3">SSSE3</a> — Supplemental Streaming SIMD Extensions 3</td></tr>
<tr><td><code>xsave</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/xsave"><code>xsave</code></a> — Save processor extended states</td></tr>
<tr><td><code>xsavec</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/xsavec"><code>xsavec</code></a> — Save processor extended states with compaction</td></tr>
<tr><td><code>xsaveopt</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/xsaveopt"><code>xsaveopt</code></a> — Save processor extended states optimized</td></tr>
<tr><td><code>xsaves</code> </td><td> </td><td> <a href="https://www.felixcloutier.com/x86/xsaves"><code>xsaves</code></a> — Save processor extended states supervisor</td></tr>
</tbody></table>
<!-- Keep links near each table to make it easier to move and update. -->
<a class="header" href="attributes/codegen.html#additional-information" id="additional-information"><h3>Additional information</h3></a>
<p>See the <a href="conditional-compilation.html#target_feature"><code>target_feature</code> conditional compilation option</a> for selectively
enabling or disabling compilation of code based on compile-time settings. Note
that this option is not affected by the <code>target_feature</code> attribute, and is
only driven by the features enabled for the entire crate.</p>
<p>See the <a href="../std/macro.is_x86_feature_detected.html"><code>is_x86_feature_detected</code></a> macro in the standard library for runtime
feature detection on the x86 platforms.</p>
<blockquote>
<p>Note: <code>rustc</code> has a default set of features enabled for each target and CPU.
The CPU may be chosen with the <a href="../rustc/codegen-options/index.html#target-cpu"><code>-C target-cpu</code></a> flag. Individual features
may be enabled or disabled for an entire crate with the
<a href="../rustc/codegen-options/index.html#target-feature"><code>-C target-feature</code></a> flag.</p>
</blockquote>
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