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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="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" class="active"><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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<a class="header" href="expressions/call-expr.html#call-expressions" id="call-expressions"><h1>Call expressions</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>CallExpression</em> :<br />
<a href="expressions.html"><em>Expression</em></a> <code>(</code> <em>CallParams</em><sup>?</sup> <code>)</code></p>
<p><em>CallParams</em> :<br />
<a href="expressions.html"><em>Expression</em></a> ( <code>,</code> <a href="expressions.html"><em>Expression</em></a> )<sup>*</sup> <code>,</code><sup>?</sup></p>
</blockquote>
<p>A <em>call expression</em> consists of an expression followed by a parenthesized
expression-list. It invokes a function, providing zero or more input variables.
If the function eventually returns, then the expression completes. For
<a href="types.html#function-item-types">non-function types</a>, the expression f(...) uses
the method on one of the <a href="../std/ops/trait.Fn.html"><code>std::ops::Fn</code></a>, <a href="../std/ops/trait.FnMut.html"><code>std::ops::FnMut</code></a> or
<a href="../std/ops/trait.FnOnce.html"><code>std::ops::FnOnce</code></a> traits, which differ in whether they take the type by
reference, mutable reference, or take ownership respectively. An automatic
borrow will be taken if needed. Rust will also automatically dereference <code>f</code> as
required. Some examples of call expressions:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# fn add(x: i32, y: i32) -> i32 { 0 }
let three: i32 = add(1i32, 2i32);
let name: &'static str = (|| "Rust")();
#}</code></pre></pre>
<a class="header" href="expressions/call-expr.html#disambiguating-function-calls" id="disambiguating-function-calls"><h2>Disambiguating Function Calls</h2></a>
<p>Rust treats all function calls as sugar for a more explicit, fully-qualified
syntax. Upon compilation, Rust will desugar all function calls into the explicit
form. Rust may sometimes require you to qualify function calls with trait,
depending on the ambiguity of a call in light of in-scope items.</p>
<blockquote>
<p><strong>Note</strong>: In the past, the Rust community used the terms "Unambiguous
Function Call Syntax", "Universal Function Call Syntax", or "UFCS", in
documentation, issues, RFCs, and other community writings. However, the term
lacks descriptive power and potentially confuses the issue at hand. We mention
it here for searchability's sake.</p>
</blockquote>
<p>Several situations often occur which result in ambiguities about the receiver or
referent of method or associated function calls. These situations may include:</p>
<ul>
<li>Multiple in-scope traits define methods with the same name for the same types</li>
<li>Auto-<code>deref</code> is undesirable; for example, distinguishing between methods on a
smart pointer itself and the pointer's referent</li>
<li>Methods which take no arguments, like <code>default()</code>, and return properties of a
type, like <code>size_of()</code></li>
</ul>
<p>To resolve the ambiguity, the programmer may refer to their desired method or
function using more specific paths, types, or traits.</p>
<p>For example,</p>
<pre><pre class="playpen"><code class="language-rust">trait Pretty {
fn print(&self);
}
trait Ugly {
fn print(&self);
}
struct Foo;
impl Pretty for Foo {
fn print(&self) {}
}
struct Bar;
impl Pretty for Bar {
fn print(&self) {}
}
impl Ugly for Bar{
fn print(&self) {}
}
fn main() {
let f = Foo;
let b = Bar;
// we can do this because we only have one item called `print` for `Foo`s
f.print();
// more explicit, and, in the case of `Foo`, not necessary
Foo::print(&f);
// if you're not into the whole brevity thing
<Foo as Pretty>::print(&f);
// b.print(); // Error: multiple 'print' found
// Bar::print(&b); // Still an error: multiple `print` found
// necessary because of in-scope items defining `print`
<Bar as Pretty>::print(&b);
}
</code></pre></pre>
<p>Refer to <a href="https://github.com/rust-lang/rfcs/blob/master/text/0132-ufcs.md">RFC 132</a> for further details and motivations.</p>
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