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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" class="active"><strong aria-hidden="true">4.</strong> Crates and source files</a></li><li><a href="items-and-attributes.html"><strong aria-hidden="true">5.</strong> Items and attributes</a></li><li><ol class="section"><li><a href="items.html"><strong aria-hidden="true">5.1.</strong> Items</a></li><li><ol class="section"><li><a href="items/modules.html"><strong aria-hidden="true">5.1.1.</strong> Modules</a></li><li><a href="items/extern-crates.html"><strong aria-hidden="true">5.1.2.</strong> Extern crates</a></li><li><a href="items/use-declarations.html"><strong aria-hidden="true">5.1.3.</strong> Use declarations</a></li><li><a href="items/functions.html"><strong aria-hidden="true">5.1.4.</strong> Functions</a></li><li><a href="items/type-aliases.html"><strong aria-hidden="true">5.1.5.</strong> Type aliases</a></li><li><a href="items/structs.html"><strong aria-hidden="true">5.1.6.</strong> Structs</a></li><li><a href="items/enumerations.html"><strong aria-hidden="true">5.1.7.</strong> Enumerations</a></li><li><a href="items/unions.html"><strong aria-hidden="true">5.1.8.</strong> Unions</a></li><li><a href="items/constant-items.html"><strong aria-hidden="true">5.1.9.</strong> Constant items</a></li><li><a href="items/static-items.html"><strong aria-hidden="true">5.1.10.</strong> Static items</a></li><li><a href="items/traits.html"><strong aria-hidden="true">5.1.11.</strong> Traits</a></li><li><a href="items/implementations.html"><strong aria-hidden="true">5.1.12.</strong> Implementations</a></li><li><a href="items/external-blocks.html"><strong aria-hidden="true">5.1.13.</strong> External blocks</a></li></ol></li><li><a href="items/generics.html"><strong aria-hidden="true">5.2.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">5.3.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">5.4.</strong> Visibility and Privacy</a></li><li><a href="attributes.html"><strong aria-hidden="true">5.5.</strong> Attributes</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">6.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">6.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">6.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">6.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">6.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">6.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">6.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">6.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">6.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">6.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">6.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">6.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">6.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong aria-hidden="true">6.2.11.</strong> Method call expressions</a></li><li><a href="expressions/field-expr.html"><strong aria-hidden="true">6.2.12.</strong> Field access expressions</a></li><li><a href="expressions/closure-expr.html"><strong aria-hidden="true">6.2.13.</strong> Closure expressions</a></li><li><a href="expressions/loop-expr.html"><strong aria-hidden="true">6.2.14.</strong> Loop expressions</a></li><li><a href="expressions/range-expr.html"><strong aria-hidden="true">6.2.15.</strong> Range expressions</a></li><li><a href="expressions/if-expr.html"><strong aria-hidden="true">6.2.16.</strong> If and if let expressions</a></li><li><a href="expressions/match-expr.html"><strong aria-hidden="true">6.2.17.</strong> Match expressions</a></li><li><a href="expressions/return-expr.html"><strong aria-hidden="true">6.2.18.</strong> Return expressions</a></li></ol></li></ol></li><li><a href="type-system.html"><strong aria-hidden="true">7.</strong> Type system</a></li><li><ol class="section"><li><a href="types.html"><strong aria-hidden="true">7.1.</strong> Types</a></li><li><a href="dynamically-sized-types.html"><strong aria-hidden="true">7.2.</strong> Dynamically Sized Types</a></li><li><a href="type-layout.html"><strong aria-hidden="true">7.3.</strong> Type layout</a></li><li><a href="interior-mutability.html"><strong aria-hidden="true">7.4.</strong> Interior mutability</a></li><li><a href="subtyping.html"><strong aria-hidden="true">7.5.</strong> Subtyping and Variance</a></li><li><a href="trait-bounds.html"><strong aria-hidden="true">7.6.</strong> Trait and lifetime bounds</a></li><li><a href="type-coercions.html"><strong aria-hidden="true">7.7.</strong> Type coercions</a></li><li><a href="destructors.html"><strong aria-hidden="true">7.8.</strong> Destructors</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">7.9.</strong> Lifetime elision</a></li></ol></li><li><a href="special-types-and-traits.html"><strong aria-hidden="true">8.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">9.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">9.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">9.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">9.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">10.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">11.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">11.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">11.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">11.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">11.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="influences.html">Appendix: Influences</a></li><li class="affix"><a href="undocumented.html">Appendix: As-yet-undocumented Features</a></li><li class="affix"><a href="glossary.html">Appendix: Glossary</a></li></ol>
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is a best-effort attempt. If you find something wrong or missing, file an
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<a class="header" href="crates-and-source-files.html#crates-and-source-files" id="crates-and-source-files"><h1>Crates and source files</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>Crate</em> :<br />
UTF8BOM<sup>?</sup><br />
SHEBANG<sup>?</sup><br />
<a href="attributes.html"><em>InnerAttribute</em></a><sup>*</sup><br />
<a href="items.html"><em>Item</em></a><sup>*</sup></p>
</blockquote>
<blockquote>
<p><strong><sup>Lexer</sup></strong><br />
UTF8BOM : <code>\uFEFF</code><br />
SHEBANG : <code>#!</code> ~[<code>[</code> <code>\n</code>] ~<code>\n</code><sup>*</sup></p>
</blockquote>
<blockquote>
<p>Note: Although Rust, like any other language, can be implemented by an
interpreter as well as a compiler, the only existing implementation is a
compiler,and the language has always been designed to be compiled. For these
reasons, this section assumes a compiler.</p>
</blockquote>
<p>Rust's semantics obey a <em>phase distinction</em> between compile-time and
run-time.<sup class="footnote-reference"><a href="crates-and-source-files.html#phase-distinction">1</a></sup> Semantic rules that have a <em>static
interpretation</em> govern the success or failure of compilation, while
semantic rules
that have a <em>dynamic interpretation</em> govern the behavior of the program at
run-time.</p>
<p>The compilation model centers on artifacts called <em>crates</em>. Each compilation
processes a single crate in source form, and if successful, produces a single
crate in binary form: either an executable or some sort of
library.<sup class="footnote-reference"><a href="crates-and-source-files.html#cratesourcefile">2</a></sup></p>
<p>A <em>crate</em> is a unit of compilation and linking, as well as versioning,
distribution and runtime loading. A crate contains a <em>tree</em> of nested
<a href="items/modules.html">module</a> scopes. The top level of this tree is a module that is
anonymous (from the point of view of paths within the module) and any item
within a crate has a canonical <a href="paths.html">module path</a> denoting its location
within the crate's module tree.</p>
<p>The Rust compiler is always invoked with a single source file as input, and
always produces a single output crate. The processing of that source file may
result in other source files being loaded as modules. Source files have the
extension <code>.rs</code>.</p>
<p>A Rust source file describes a module, the name and location of which —
in the module tree of the current crate — are defined from outside the
source file: either by an explicit <code>mod_item</code> in a referencing source file, or
by the name of the crate itself. Every source file is a module, but not every
module needs its own source file: <a href="items/modules.html">module definitions</a> can be nested
within one file.</p>
<p>Each source file contains a sequence of zero or more <code>item</code> definitions, and
may optionally begin with any number of <a href="attributes.html">attributes</a>
that apply to the containing module, most of which influence the behavior of
the compiler. The anonymous crate module can have additional attributes that
apply to the crate as a whole.</p>
<pre><pre class="playpen"><code class="language-rust no_run">
# #![allow(unused_variables)]
#fn main() {
// Specify the crate name.
#![crate_name = "projx"]
// Specify the type of output artifact.
#![crate_type = "lib"]
// Turn on a warning.
// This can be done in any module, not just the anonymous crate module.
#![warn(non_camel_case_types)]
#}</code></pre></pre>
<p>A crate that contains a <code>main</code> <a href="items/functions.html">function</a> can be compiled to an executable. If a
<code>main</code> function is present, it must take no arguments, must not declare any
<a href="trait-bounds.html">trait or lifetime bounds</a>, must not have any [where clauses], and its return
type must be one of the following:</p>
<ul>
<li><code>()</code></li>
</ul>
<!-- * `!` -->
* `Result<T, E> where T: on this list, E: Error`
<blockquote>
<p>Note: The implementation of which return types are allowed is determined by
the unstable <a href="../std/process/trait.Termination.html"><code>Termination</code></a> trait.</p>
</blockquote>
<p>The optional <a href="https://en.wikipedia.org/wiki/Byte_order_mark#UTF-8"><em>UTF8 byte order mark</em></a> (UTF8BOM production) indicates that the
file is encoded in UTF8. It can only occur at the beginning of the file and
is ignored by the compiler.</p>
<p>A source file can have a <a href="https://en.wikipedia.org/wiki/Shebang_(Unix)"><em>shebang</em></a> (SHEBANG production), which indicates
to the operating system what program to use to execute this file. It serves
essentially to treat the source file as an executable script. The shebang
can only occur at the beginning of the file (but after the optional
<em>UTF8BOM</em>). It is ignored by the compiler. For example:</p>
<pre><code class="language-text ignore">#!/usr/bin/env rustx
fn main() {
println!("Hello!");
}
</code></pre>
<div class="footnote-definition" id="phase-distinction"><sup class="footnote-definition-label">1</sup>
<p>This distinction would also exist in an interpreter.
Static checks like syntactic analysis, type checking, and lints should
happen before the program is executed regardless of when it is executed.</p>
</div>
<div class="footnote-definition" id="cratesourcefile"><sup class="footnote-definition-label">2</sup>
<p>A crate is somewhat analogous to an <em>assembly</em> in the
ECMA-335 CLI model, a <em>library</em> in the SML/NJ Compilation Manager, a <em>unit</em>
in the Owens and Flatt module system, or a <em>configuration</em> in Mesa.</p>
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