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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" class="active"><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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<a class="header" href="items/functions.html#functions" id="functions"><h1>Functions</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>Function</em> :<br />
<em>FunctionQualifiers</em> <code>fn</code> <a href="identifiers.html">IDENTIFIER</a> <a href="items/generics.html"><em>Generics</em></a><sup>?</sup><br />
<code>(</code> <em>FunctionParameters</em><sup>?</sup> <code>)</code><br />
<em>FunctionReturnType</em><sup>?</sup> <a href="items/generics.html#where-clauses"><em>WhereClause</em></a><sup>?</sup><br />
<a href="expressions/block-expr.html"><em>BlockExpression</em></a></p>
<p><em>FunctionQualifiers</em> :<br />
<code>const</code><sup>?</sup> <code>unsafe</code><sup>?</sup> (<code>extern</code> <em>Abi</em><sup>?</sup>)<sup>?</sup></p>
<p><em>Abi</em> :<br />
<a href="tokens.html#string-literals">STRING_LITERAL</a> | <a href="tokens.html#raw-string-literals">RAW_STRING_LITERAL</a></p>
<p><em>FunctionParameters</em> :<br />
<em>FunctionParam</em> (<code>,</code> <em>FunctionParam</em>)<sup>*</sup> <code>,</code><sup>?</sup></p>
<p><em>FunctionParam</em> :<br />
<a href="patterns.html"><em>Pattern</em></a> <code>:</code> <a href="types.html#type-expressions"><em>Type</em></a></p>
<p><em>FunctionReturnType</em> :<br />
<code>-></code> <a href="types.html#type-expressions"><em>Type</em></a></p>
</blockquote>
<p>A <em>function</em> consists of a <a href="expressions/block-expr.html">block</a>, along with a name and a set of parameters.
Other than a name, all these are optional. Functions are declared with the
keyword <code>fn</code>. Functions may declare a set of <em>input</em> <a href="variables.html"><em>variables</em></a>
as parameters, through which the caller passes arguments into the function, and
the <em>output</em> <a href="types.html#type-expressions"><em>type</em></a> of the value the function will return to its caller
on completion.</p>
<p>When referred to, a <em>function</em> yields a first-class <em>value</em> of the
corresponding zero-sized <a href="types/function-item.html"><em>function item type</em></a>, which
when called evaluates to a direct call to the function.</p>
<p>For example, this is a simple function:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn answer_to_life_the_universe_and_everything() -> i32 {
return 42;
}
#}</code></pre></pre>
<p>As with <code>let</code> bindings, function arguments are irrefutable <a href="patterns.html">patterns</a>, so any
pattern that is valid in a let binding is also valid as an argument:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn first((value, _): (i32, i32)) -> i32 { value }
#}</code></pre></pre>
<p>The block of a function is conceptually wrapped in a block that binds the
argument patterns and then <code>return</code>s the value of the function's block. This
means that the tail expression of the block, if evaluated, ends up being
returned to the caller. As usual, an explicit return expression within
the body of the function will short-cut that implicit return, if reached.</p>
<p>For example, the function above behaves as if it was written as:</p>
<pre><code class="language-rust ignore">// argument_0 is the actual first argument passed from the caller
let (value, _) = argument_0;
return {
value
};
</code></pre>
<a class="header" href="items/functions.html#generic-functions" id="generic-functions"><h2>Generic functions</h2></a>
<p>A <em>generic function</em> allows one or more <em>parameterized types</em> to appear in its
signature. Each type parameter must be explicitly declared in an
angle-bracket-enclosed and comma-separated list, following the function name.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
// foo is generic over A and B
fn foo<A, B>(x: A, y: B) {
# }
#}</code></pre></pre>
<p>Inside the function signature and body, the name of the type parameter can be
used as a type name. <a href="items/traits.html">Trait</a> bounds can be specified for type
parameters to allow methods with that trait to be called on values of that
type. This is specified using the <code>where</code> syntax:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# use std::fmt::Debug;
fn foo<T>(x: T) where T: Debug {
# }
#}</code></pre></pre>
<p>When a generic function is referenced, its type is instantiated based on the
context of the reference. For example, calling the <code>foo</code> function here:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::fmt::Debug;
fn foo<T>(x: &[T]) where T: Debug {
// details elided
}
foo(&[1, 2]);
#}</code></pre></pre>
<p>will instantiate type parameter <code>T</code> with <code>i32</code>.</p>
<p>The type parameters can also be explicitly supplied in a trailing <a href="paths.html">path</a>
component after the function name. This might be necessary if there is not
sufficient context to determine the type parameters. For example,
<code>mem::size_of::<u32>() == 4</code>.</p>
<a class="header" href="items/functions.html#extern-functions" id="extern-functions"><h2>Extern functions</h2></a>
<p>Extern functions are part of Rust's foreign function interface, providing the
opposite functionality to <a href="items/external-blocks.html">external blocks</a>. Whereas external
blocks allow Rust code to call foreign code, extern functions with bodies
defined in Rust code <em>can be called by foreign code</em>. They are defined in the
same way as any other Rust function, except that they have the <code>extern</code>
qualifier.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
// Declares an extern fn, the ABI defaults to "C"
extern fn new_i32() -> i32 { 0 }
// Declares an extern fn with "stdcall" ABI
# #[cfg(target_arch = "x86_64")]
extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
#}</code></pre></pre>
<p>Unlike normal functions, extern fns have type <code>extern "ABI" fn()</code>. This is the
same type as the functions declared in an extern block.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# extern fn new_i32() -> i32 { 0 }
let fptr: extern "C" fn() -> i32 = new_i32;
#}</code></pre></pre>
<p>As non-Rust calling conventions do not support unwinding, unwinding past the end
of an extern function will cause the process to abort. In LLVM, this is
implemented by executing an illegal instruction.</p>
<a class="header" href="items/functions.html#const-functions" id="const-functions"><h2>Const functions</h2></a>
<p>Functions qualified with the <code>const</code> keyword are const functions. <em>Const
functions</em> can be called from within <a href="const_eval.html#const-context">const context</a>s. When called from a const
context, the function is interpreted by the compiler at compile time. The
interpretation happens in the environment of the compilation target and not the
host. So <code>usize</code> is <code>32</code> bits if you are compiling against a <code>32</code> bit system,
irrelevant of whether you are building on a <code>64</code> bit or a <code>32</code> bit system.</p>
<p>If a const function is called outside a <a href="const_eval.html#const-context">const context</a>, it is indistinguishable
from any other function. You can freely do anything with a const function that
you can do with a regular function.</p>
<p>Const functions have various restrictions to make sure that they can be
evaluated at compile-time. It is, for example, not possible to write a random
number generator as a const function. Calling a const function at compile-time
will always yield the same result as calling it at runtime, even when called
multiple times. There's one exception to this rule: if you are doing complex
floating point operations in extreme situations, then you might get (very
slightly) different results. It is advisable to not make array lengths and enum
discriminants depend on floating point computations.</p>
<p>Exhaustive list of permitted structures in const functions:</p>
<blockquote>
<p><strong>Note</strong>: this list is more restrictive than what you can write in
regular constants</p>
</blockquote>
<ul>
<li>
<p>Type parameters where the parameters only have any <a href="trait-bounds.html">trait bounds</a>
of the following kind:</p>
<ul>
<li>lifetimes</li>
<li><code>Sized</code> or <a href="trait-bounds.html#sized"><code>?Sized</code></a></li>
</ul>
<p>This means that <code><T: 'a + ?Sized></code>, <code><T: 'b + Sized></code> and <code><T></code>
are all permitted.</p>
<p>This rule also applies to type parameters of impl blocks that
contain const methods</p>
</li>
<li>
<p>Arithmetic and comparison operators on integers</p>
</li>
<li>
<p>All boolean operators except for <code>&&</code> and <code>||</code> which are banned since
they are short-circuiting.</p>
</li>
<li>
<p>Any kind of aggregate constructor (array, <code>struct</code>, <code>enum</code>, tuple, ...)</p>
</li>
<li>
<p>Calls to other <em>safe</em> const functions (whether by function call or method call)</p>
</li>
<li>
<p>Index expressions on arrays and slices</p>
</li>
<li>
<p>Field accesses on structs and tuples</p>
</li>
<li>
<p>Reading from constants (but not statics, not even taking a reference to a static)</p>
</li>
<li>
<p><code>&</code> and <code>*</code> (only dereferencing of references, not raw pointers)</p>
</li>
<li>
<p>Casts except for raw pointer to integer casts</p>
</li>
<li>
<p><code>unsafe</code> blocks and <code>const unsafe fn</code> are allowed, but the body/block may only do
the following unsafe operations:</p>
<ul>
<li>calls to const unsafe functions</li>
</ul>
</li>
</ul>
<a class="header" href="items/functions.html#attributes-on-functions" id="attributes-on-functions"><h2>Attributes on functions</h2></a>
<p><a href="attributes.html">Outer attributes</a> are allowed on functions. <a href="attributes.html">Inner
attributes</a> are allowed directly after the <code>{</code> inside its <a href="expressions/block-expr.html">block</a>.</p>
<p>This example shows an inner attribute on a function. The function will only be
available while running tests.</p>
<pre><code>fn test_only() {
#![test]
}
</code></pre>
<blockquote>
<p>Note: Except for lints, it is idiomatic to only use outer attributes on
function items.</p>
</blockquote>
<p>The attributes that have meaning on a function are <a href="conditional-compilation.html"><code>cfg</code></a>, <a href="attributes/diagnostics.html#the-deprecated-attribute"><code>deprecated</code></a>,
<a href="../rustdoc/the-doc-attribute.html"><code>doc</code></a>, <a href="abi.html#the-export_name-attribute"><code>export_name</code></a>, <a href="abi.html#the-link_section-attribute"><code>link_section</code></a>, <a href="abi.html#the-no_mangle-attribute"><code>no_mangle</code></a>, <a href="attributes/diagnostics.html#lint-check-attributes">the lint check
attributes</a>, <a href="attributes/diagnostics.html#the-must_use-attribute"><code>must_use</code></a>, <a href="procedural-macros.html">the procedural macro attributes</a>, <a href="attributes/testing.html">the testing
attributes</a>, and <a href="attributes/codegen.html#optimization-hints">the optimization hint attributes</a>. Functions also accept
attributes macros.</p>
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