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<ol class="chapter"><li class="affix"><a href="introduction.html" class="active">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"><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
and completeness, but are not yet there. In the future, the docs and lang
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
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<a class="header" href="introduction.html#introduction" id="introduction"><h1>Introduction</h1></a>
<p>This book is the primary reference for the Rust programming language. It
provides three kinds of material:</p>
<ul>
<li>Chapters that informally describe each language construct and their use.</li>
<li>Chapters that informally describe the memory model, concurrency model,
runtime services, linkage model and debugging facilities.</li>
<li>Appendix chapters providing rationale and references to languages that
influenced the design.</li>
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<div class="warning">
<p>Warning: This book is incomplete. Documenting everything takes a while. See
the <a href="https://github.com/rust-lang-nursery/reference/issues">GitHub issues</a> for what is not documented in this book.</p>
</div>
<a class="header" href="introduction.html#what-the-reference-is-not" id="what-the-reference-is-not"><h2>What <em>The Reference</em> is Not</h2></a>
<p>This book does not serve as an introduction to the language. Background
familiarity with the language is assumed. A separate <a href="../book/index.html">book</a> is available to
help acquire such background familiarity.</p>
<p>This book also does not serve as a reference to the <a href="../std/index.html">standard library</a>
included in the language distribution. Those libraries are documented
separately by extracting documentation attributes from their source code. Many
of the features that one might expect to be language features are library
features in Rust, so what you're looking for may be there, not here.</p>
<p>Similarly, this book does not usually document the specifics of <code>rustc</code> as a
tool or of Cargo. <code>rustc</code> has its own <a href="../rustc/index.html">book</a>. Cargo has a
<a href="../cargo/index.html">book</a> that contains a <a href="../cargo/reference/index.html">reference</a>. There are a few
pages such as <a href="linkage.html">linkage</a> that still describe how <code>rustc</code> works.</p>
<p>This book also only serves as a reference to what is available in stable
Rust. For unstable features being worked on, see the <a href="https://doc.rust-lang.org/nightly/unstable-book/">Unstable Book</a>.</p>
<p>Finally, this book is not normative. It may include details that are
specific to <code>rustc</code> itself, and should not be taken as a specification for
the Rust language. We intend to produce such a book someday, and until then,
the reference is the closest thing we have to one.</p>
<a class="header" href="introduction.html#how-to-use-this-book" id="how-to-use-this-book"><h2>How to Use This Book</h2></a>
<p>This book does not assume you are reading this book sequentially. Each
chapter generally can be read standalone, but will cross-link to other chapters
for facets of the language they refer to, but do not discuss.</p>
<p>There are two main ways to read this document.</p>
<p>The first is to answer a specific question. If you know which chapter answers
that question, you can jump to that chapter in the table of contents. Otherwise,
you can press <code>s</code> or the click the magnifying glass on the top bar to search for
keywords related to your question. For example, say you wanted to know when a
temporary value created in a let statement is dropped. If you didn't already
know that the <a href="expressions.html#temporary-lifetimes">lifetime of temporaries</a> is defined in the <a href="expressions.html">expressions chapter</a>,
you could search "temporary let" and the first search result will take you to
that section.</p>
<p>The second is to generally improve your knowledge of a facet of the language.
In that case, just browse the table of contents until you see something you
want to know more about, and just start reading. If a link looks interesting,
click it, and read about that section.</p>
<p>That said, there is no wrong way to read this book. Read it however you feel
helps you best.</p>
<a class="header" href="introduction.html#conventions" id="conventions"><h3>Conventions</h3></a>
<p>Like all technical books, this book has certain conventions in how it displays
information. These conventions are documented here.</p>
<ul>
<li>
<p>Statements that define a term contain that term in <em>italics</em>. Whenever that
term is used outside of that chapter, it is usually a link to the section that
has this definition.</p>
<p>An <em>example term</em> is an example of a term being defined.</p>
</li>
<li>
<p>Differences in the language by which edition the crate is compiled under are
in a blockquote that start with the words "Edition Differences:" in <strong>bold</strong>.</p>
<blockquote>
<p><strong>Edition Differences</strong>: In the 2015 edition, this syntax is valid that is
disallowed as of the 2018 edition.</p>
</blockquote>
</li>
<li>
<p>Notes that contain useful information about the state of the book or point out
useful, but mostly out of scope, information are in blockquotes that start
with the word "Note:" in <strong>bold</strong>.</p>
<blockquote>
<p><strong>Note</strong>: This is an example note.</p>
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</li>
<li>
<p>Warnings that show unsound behavior in the language or possibly confusing
interactions of language features are in a special warning box.</p>
<div class="warning">
<p>Warning: This is an example warning.</p>
</div>
</li>
<li>
<p>Code snippets inline in the text are inside <code><code></code> tags.</p>
<p>Longer code examples are in a syntax highlighted box that has controls for
copying, executing, and showing hidden lines in the top right corner.</p>
<pre><pre class="playpen"><code class="language-rust"># // This is a hidden line.
fn main() {
println!("This is a code example");
}
</code></pre></pre>
</li>
<li>
<p>The grammar and lexical structure is in blockquotes with either "Lexer" or
"Syntax" in <sup><strong>bold superscript</strong></sup> as the first line.</p>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>ExampleGrammar</em>:<br />
<code>~</code> <a href="expressions.html"><em>Expression</em></a><br />
| <code>box</code> <a href="expressions.html"><em>Expression</em></a></p>
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<p>See <a href="notation.html">Notation</a> for more detail.</p>
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<a class="header" href="introduction.html#contributing" id="contributing"><h2>Contributing</h2></a>
<p>We welcome contributions of all kinds.</p>
<p>You can contribute to this book by opening an issue or sending a pull
request to <a href="https://github.com/rust-lang-nursery/reference/">the Rust Reference repository</a>. If this book does not answer
your question, and you think its answer is in scope of it, please do not
hesitate to file an issue or ask about it in the <code>#docs</code> channels on
<a href="https://discord.gg/rust-lang">Discord</a>. Knowing what people use this book for the most helps direct our
attention to making those sections the best that they can be.</p>
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