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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"><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" class="active"><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="types/closure.html#closure-types" id="closure-types"><h1>Closure types</h1></a>
<p>A <a href="expressions/closure-expr.html">closure expression</a> produces a closure value with a unique, anonymous type
that cannot be written out. A closure type is approximately equivalent to a
struct which contains the captured variables. For instance, the following
closure:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
fn f<F : FnOnce() -> String> (g: F) {
println!("{}", g());
}
let mut s = String::from("foo");
let t = String::from("bar");
f(|| {
s += &*t;
s
});
// Prints "foobar".
#}</code></pre></pre>
<p>generates a closure type roughly like the following:</p>
<pre><code class="language-rust ignore">struct Closure<'a> {
s : String,
t : &'a String,
}
impl<'a> (FnOnce() -> String) for Closure<'a> {
fn call_once(self) -> String {
self.s += &*self.t;
self.s
}
}
</code></pre>
<p>so that the call to <code>f</code> works as if it were:</p>
<pre><code class="language-rust ignore">f(Closure{s: s, t: &t});
</code></pre>
<a class="header" href="types/closure.html#capture-modes" id="capture-modes"><h2>Capture modes</h2></a>
<p>The compiler prefers to capture a closed-over variable by immutable borrow,
followed by unique immutable borrow (see below), by mutable borrow, and finally
by move. It will pick the first choice of these that allows the closure to
compile. The choice is made only with regards to the contents of the closure
expression; the compiler does not take into account surrounding code, such as
the lifetimes of involved variables.</p>
<p>If the <code>move</code> keyword is used, then all captures are by move or, for <code>Copy</code>
types, by copy, regardless of whether a borrow would work. The <code>move</code> keyword is
usually used to allow the closure to outlive the captured values, such as if the
closure is being returned or used to spawn a new thread.</p>
<p>Composite types such as structs, tuples, and enums are always captured entirely,
not by individual fields. It may be necessary to borrow into a local variable in
order to capture a single field:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# use std::collections::HashSet;
#
struct SetVec {
set: HashSet<u32>,
vec: Vec<u32>
}
impl SetVec {
fn populate(&mut self) {
let vec = &mut self.vec;
self.set.iter().for_each(|&n| {
vec.push(n);
})
}
}
#}</code></pre></pre>
<p>If, instead, the closure were to use <code>self.vec</code> directly, then it would attempt
to capture <code>self</code> by mutable reference. But since <code>self.set</code> is already
borrowed to iterate over, the code would not compile.</p>
<a class="header" href="types/closure.html#unique-immutable-borrows-in-captures" id="unique-immutable-borrows-in-captures"><h2>Unique immutable borrows in captures</h2></a>
<p>Captures can occur by a special kind of borrow called a <em>unique immutable
borrow</em>, which cannot be used anywhere else in the language and cannot be
written out explicitly. It occurs when modifying the referent of a mutable
reference, as in the following example:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut b = false;
let x = &mut b;
{
let mut c = || { *x = true; };
// The following line is an error:
// let y = &x;
c();
}
let z = &x;
#}</code></pre></pre>
<p>In this case, borrowing <code>x</code> mutably is not possible, because <code>x</code> is not <code>mut</code>.
But at the same time, borrowing <code>x</code> immutably would make the assignment illegal,
because a <code>& &mut</code> reference may not be unique, so it cannot safely be used to
modify a value. So a unique immutable borrow is used: it borrows <code>x</code> immutably,
but like a mutable borrow, it must be unique. In the above example, uncommenting
the declaration of <code>y</code> will produce an error because it would violate the
uniqueness of the closure's borrow of <code>x</code>; the declaration of z is valid because
the closure's lifetime has expired at the end of the block, releasing the borrow.</p>
<a class="header" href="types/closure.html#call-traits-and-coercions" id="call-traits-and-coercions"><h2>Call traits and coercions</h2></a>
<p>Closure types all implement <a href="../std/ops/trait.FnOnce.html"><code>FnOnce</code></a>, indicating that they can be called once
by consuming ownership of the closure. Additionally, some closures implement
more specific call traits:</p>
<ul>
<li>
<p>A closure which does not move out of any captured variables implements
<a href="../std/ops/trait.FnMut.html"><code>FnMut</code></a>, indicating that it can be called by mutable reference.</p>
</li>
<li>
<p>A closure which does not mutate or move out of any captured variables
implements <a href="../std/ops/trait.Fn.html"><code>Fn</code></a>, indicating that it can be called by shared reference.</p>
</li>
</ul>
<blockquote>
<p>Note: <code>move</code> closures may still implement <a href="../std/ops/trait.Fn.html"><code>Fn</code></a> or <a href="../std/ops/trait.FnMut.html"><code>FnMut</code></a>, even though
they capture variables by move. This is because the traits implemented by a
closure type are determined by what the closure does with captured values,
not how it captures them.</p>
</blockquote>
<p><em>Non-capturing closures</em> are closures that don't capture anything from their
environment. They can be coerced to function pointers (<code>fn</code>) with the matching
signature.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let add = |x, y| x + y;
let mut x = add(5,7);
type Binop = fn(i32, i32) -> i32;
let bo: Binop = add;
x = bo(5,7);
#}</code></pre></pre>
<a class="header" href="types/closure.html#other-traits" id="other-traits"><h2>Other traits</h2></a>
<p>All closure types implement <a href="special-types-and-traits.html#sized"><code>Sized</code></a>. Additionally, closure types implement the
following traits if allowed to do so by the types of the captures it stores:</p>
<ul>
<li><a href="special-types-and-traits.html#clone"><code>Clone</code></a></li>
<li><a href="special-types-and-traits.html#copy"><code>Copy</code></a></li>
<li><a href="special-types-and-traits.html#sync"><code>Sync</code></a></li>
<li><a href="special-types-and-traits.html#send"><code>Send</code></a></li>
</ul>
<p>The rules for <a href="special-types-and-traits.html#send"><code>Send</code></a> and <a href="special-types-and-traits.html#sync"><code>Sync</code></a> match those for normal struct types, while
<a href="special-types-and-traits.html#clone"><code>Clone</code></a> and <a href="special-types-and-traits.html#copy"><code>Copy</code></a> behave as if <a href="attributes/derive.html">derived</a>. For <a href="special-types-and-traits.html#clone"><code>Clone</code></a>, the order of
cloning of the captured variables is left unspecified.</p>
<p>Because captures are often by reference, the following general rules arise:</p>
<ul>
<li>A closure is <a href="special-types-and-traits.html#sync"><code>Sync</code></a> if all captured variables are <a href="special-types-and-traits.html#sync"><code>Sync</code></a>.</li>
<li>A closure is <a href="special-types-and-traits.html#send"><code>Send</code></a> if all variables captured by non-unique immutable
reference are <a href="special-types-and-traits.html#sync"><code>Sync</code></a>, and all values captured by unique immutable or mutable
reference, copy, or move are <a href="special-types-and-traits.html#send"><code>Send</code></a>.</li>
<li>A closure is <a href="special-types-and-traits.html#clone"><code>Clone</code></a> or <a href="special-types-and-traits.html#copy"><code>Copy</code></a> if it does not capture any values by
unique immutable or mutable reference, and if all values it captures by copy
or move are <a href="special-types-and-traits.html#clone"><code>Clone</code></a> or <a href="special-types-and-traits.html#copy"><code>Copy</code></a>, respectively.</li>
</ul>
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