<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `libcore/ops/function.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>function.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../../normalize.css"><link rel="stylesheet" type="text/css" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../../dark.css"><link rel="stylesheet" type="text/css" href="../../../light.css" id="themeStyle"><script src="../../../storage.js"></script><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../../../settings.html"><img src="../../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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</pre><pre class="rust ">
<span class="comment">// Copyright 2012 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">/// The version of the call operator that takes an immutable receiver.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Instances of `Fn` can be called repeatedly without mutating state.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// *This trait (`Fn`) is not to be confused with [function pointers][]</span>
<span class="doccomment">/// (`fn`).*</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `Fn` is implemented automatically by closures which only take immutable</span>
<span class="doccomment">/// references to captured variables or don't capture anything at all, as well</span>
<span class="doccomment">/// as (safe) [function pointers][] (with some caveats, see their documentation</span>
<span class="doccomment">/// for more details). Additionally, for any type `F` that implements `Fn`, `&F`</span>
<span class="doccomment">/// implements `Fn`, too.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Since both [`FnMut`] and [`FnOnce`] are supertraits of `Fn`, any</span>
<span class="doccomment">/// instance of `Fn` can be used as a parameter where a [`FnMut`] or [`FnOnce`]</span>
<span class="doccomment">/// is expected.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `Fn` as a bound when you want to accept a parameter of function-like</span>
<span class="doccomment">/// type and need to call it repeatedly and without mutating state (e.g. when</span>
<span class="doccomment">/// calling it concurrently). If you do not need such strict requirements, use</span>
<span class="doccomment">/// [`FnMut`] or [`FnOnce`] as bounds.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// See the [chapter on closures in *The Rust Programming Language*][book] for</span>
<span class="doccomment">/// some more information on this topic.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Also of note is the special syntax for `Fn` traits (e.g.</span>
<span class="doccomment">/// `Fn(usize, bool) -> usize`). Those interested in the technical details of</span>
<span class="doccomment">/// this can refer to [the relevant section in the *Rustonomicon*][nomicon].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [book]: ../../book/second-edition/ch13-01-closures.html</span>
<span class="doccomment">/// [`FnMut`]: trait.FnMut.html</span>
<span class="doccomment">/// [`FnOnce`]: trait.FnOnce.html</span>
<span class="doccomment">/// [function pointers]: ../../std/primitive.fn.html</span>
<span class="doccomment">/// [nomicon]: ../../nomicon/hrtb.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Calling a closure</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let square = |x| x * x;</span>
<span class="doccomment">/// assert_eq!(square(5), 25);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Using a `Fn` parameter</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn call_with_one<F>(func: F) -> usize</span>
<span class="doccomment">/// where F: Fn(usize) -> usize {</span>
<span class="doccomment">/// func(1)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let double = |x| x * 2;</span>
<span class="doccomment">/// assert_eq!(call_with_one(double), 2);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"fn"</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">rustc_paren_sugar</span>]</span>
<span class="attribute">#[<span class="ident">fundamental</span>]</span> <span class="comment">// so that regex can rely that `&str: !FnMut`</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Fn</span><span class="op"><</span><span class="ident">Args</span><span class="op">></span> : <span class="ident">FnMut</span><span class="op"><</span><span class="ident">Args</span><span class="op">></span> {
<span class="doccomment">/// Performs the call operation.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fn_traits"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"29625"</span>)]</span>
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">args</span>: <span class="ident">Args</span>) <span class="op">-></span> <span class="self">Self</span>::<span class="ident">Output</span>;
}
<span class="doccomment">/// The version of the call operator that takes a mutable receiver.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Instances of `FnMut` can be called repeatedly and may mutate state.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `FnMut` is implemented automatically by closures which take mutable</span>
<span class="doccomment">/// references to captured variables, as well as all types that implement</span>
<span class="doccomment">/// [`Fn`], e.g. (safe) [function pointers][] (since `FnMut` is a supertrait of</span>
<span class="doccomment">/// [`Fn`]). Additionally, for any type `F` that implements `FnMut`, `&mut F`</span>
<span class="doccomment">/// implements `FnMut`, too.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Since [`FnOnce`] is a supertrait of `FnMut`, any instance of `FnMut` can be</span>
<span class="doccomment">/// used where a [`FnOnce`] is expected, and since [`Fn`] is a subtrait of</span>
<span class="doccomment">/// `FnMut`, any instance of [`Fn`] can be used where `FnMut` is expected.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `FnMut` as a bound when you want to accept a parameter of function-like</span>
<span class="doccomment">/// type and need to call it repeatedly, while allowing it to mutate state.</span>
<span class="doccomment">/// If you don't want the parameter to mutate state, use [`Fn`] as a</span>
<span class="doccomment">/// bound; if you don't need to call it repeatedly, use [`FnOnce`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// See the [chapter on closures in *The Rust Programming Language*][book] for</span>
<span class="doccomment">/// some more information on this topic.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Also of note is the special syntax for `Fn` traits (e.g.</span>
<span class="doccomment">/// `Fn(usize, bool) -> usize`). Those interested in the technical details of</span>
<span class="doccomment">/// this can refer to [the relevant section in the *Rustonomicon*][nomicon].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [book]: ../../book/second-edition/ch13-01-closures.html</span>
<span class="doccomment">/// [`Fn`]: trait.Fn.html</span>
<span class="doccomment">/// [`FnOnce`]: trait.FnOnce.html</span>
<span class="doccomment">/// [function pointers]: ../../std/primitive.fn.html</span>
<span class="doccomment">/// [nomicon]: ../../nomicon/hrtb.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Calling a mutably capturing closure</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let mut x = 5;</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// let mut square_x = || x *= x;</span>
<span class="doccomment">/// square_x();</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// assert_eq!(x, 25);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Using a `FnMut` parameter</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn do_twice<F>(mut func: F)</span>
<span class="doccomment">/// where F: FnMut()</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// func();</span>
<span class="doccomment">/// func();</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut x: usize = 1;</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// let add_two_to_x = || x += 2;</span>
<span class="doccomment">/// do_twice(add_two_to_x);</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(x, 5);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"fn_mut"</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">rustc_paren_sugar</span>]</span>
<span class="attribute">#[<span class="ident">fundamental</span>]</span> <span class="comment">// so that regex can rely that `&str: !FnMut`</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">FnMut</span><span class="op"><</span><span class="ident">Args</span><span class="op">></span> : <span class="ident">FnOnce</span><span class="op"><</span><span class="ident">Args</span><span class="op">></span> {
<span class="doccomment">/// Performs the call operation.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fn_traits"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"29625"</span>)]</span>
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">args</span>: <span class="ident">Args</span>) <span class="op">-></span> <span class="self">Self</span>::<span class="ident">Output</span>;
}
<span class="doccomment">/// The version of the call operator that takes a by-value receiver.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Instances of `FnOnce` can be called, but might not be callable multiple</span>
<span class="doccomment">/// times. Because of this, if the only thing known about a type is that it</span>
<span class="doccomment">/// implements `FnOnce`, it can only be called once.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `FnOnce` is implemented automatically by closure that might consume captured</span>
<span class="doccomment">/// variables, as well as all types that implement [`FnMut`], e.g. (safe)</span>
<span class="doccomment">/// [function pointers][] (since `FnOnce` is a supertrait of [`FnMut`]).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Since both [`Fn`] and [`FnMut`] are subtraits of `FnOnce`, any instance of</span>
<span class="doccomment">/// [`Fn`] or [`FnMut`] can be used where a `FnOnce` is expected.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Use `FnOnce` as a bound when you want to accept a parameter of function-like</span>
<span class="doccomment">/// type and only need to call it once. If you need to call the parameter</span>
<span class="doccomment">/// repeatedly, use [`FnMut`] as a bound; if you also need it to not mutate</span>
<span class="doccomment">/// state, use [`Fn`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// See the [chapter on closures in *The Rust Programming Language*][book] for</span>
<span class="doccomment">/// some more information on this topic.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Also of note is the special syntax for `Fn` traits (e.g.</span>
<span class="doccomment">/// `Fn(usize, bool) -> usize`). Those interested in the technical details of</span>
<span class="doccomment">/// this can refer to [the relevant section in the *Rustonomicon*][nomicon].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [book]: ../../book/second-edition/ch13-01-closures.html</span>
<span class="doccomment">/// [`Fn`]: trait.Fn.html</span>
<span class="doccomment">/// [`FnMut`]: trait.FnMut.html</span>
<span class="doccomment">/// [function pointers]: ../../std/primitive.fn.html</span>
<span class="doccomment">/// [nomicon]: ../../nomicon/hrtb.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Calling a by-value closure</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let x = 5;</span>
<span class="doccomment">/// let square_x = move || x * x;</span>
<span class="doccomment">/// assert_eq!(square_x(), 25);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Using a `FnOnce` parameter</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn consume_with_relish<F>(func: F)</span>
<span class="doccomment">/// where F: FnOnce() -> String</span>
<span class="doccomment">/// {</span>
<span class="doccomment">/// // `func` consumes its captured variables, so it cannot be run more</span>
<span class="doccomment">/// // than once.</span>
<span class="doccomment">/// println!("Consumed: {}", func());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// println!("Delicious!");</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Attempting to invoke `func()` again will throw a `use of moved</span>
<span class="doccomment">/// // value` error for `func`.</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let x = String::from("x");</span>
<span class="doccomment">/// let consume_and_return_x = move || x;</span>
<span class="doccomment">/// consume_with_relish(consume_and_return_x);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // `consume_and_return_x` can no longer be invoked at this point</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"fn_once"</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">rustc_paren_sugar</span>]</span>
<span class="attribute">#[<span class="ident">fundamental</span>]</span> <span class="comment">// so that regex can rely that `&str: !FnMut`</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">FnOnce</span><span class="op"><</span><span class="ident">Args</span><span class="op">></span> {
<span class="doccomment">/// The returned type after the call operator is used.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fn_once_output"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.12.0"</span>)]</span>
<span class="kw">type</span> <span class="ident">Output</span>;
<span class="doccomment">/// Performs the call operation.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fn_traits"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"29625"</span>)]</span>
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_once</span>(<span class="self">self</span>, <span class="ident">args</span>: <span class="ident">Args</span>) <span class="op">-></span> <span class="self">Self</span>::<span class="ident">Output</span>;
}
<span class="kw">mod</span> <span class="ident">impls</span> {
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>,<span class="ident">A</span>,<span class="ident">F</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">Fn</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">F</span>
<span class="kw">where</span> <span class="ident">F</span> : <span class="ident">Fn</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>
{
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">args</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">F</span>::<span class="ident">Output</span> {
(<span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span>).<span class="ident">call</span>(<span class="ident">args</span>)
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>,<span class="ident">A</span>,<span class="ident">F</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">FnMut</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">F</span>
<span class="kw">where</span> <span class="ident">F</span> : <span class="ident">Fn</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>
{
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">args</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">F</span>::<span class="ident">Output</span> {
(<span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span>).<span class="ident">call</span>(<span class="ident">args</span>)
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>,<span class="ident">A</span>,<span class="ident">F</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">FnOnce</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">F</span>
<span class="kw">where</span> <span class="ident">F</span> : <span class="ident">Fn</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>
{
<span class="kw">type</span> <span class="ident">Output</span> <span class="op">=</span> <span class="ident">F</span>::<span class="ident">Output</span>;
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_once</span>(<span class="self">self</span>, <span class="ident">args</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">F</span>::<span class="ident">Output</span> {
(<span class="kw-2">*</span><span class="self">self</span>).<span class="ident">call</span>(<span class="ident">args</span>)
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>,<span class="ident">A</span>,<span class="ident">F</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">FnMut</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">F</span>
<span class="kw">where</span> <span class="ident">F</span> : <span class="ident">FnMut</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>
{
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">args</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">F</span>::<span class="ident">Output</span> {
(<span class="kw-2">*</span><span class="self">self</span>).<span class="ident">call_mut</span>(<span class="ident">args</span>)
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>,<span class="ident">A</span>,<span class="ident">F</span>:<span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">FnOnce</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">F</span>
<span class="kw">where</span> <span class="ident">F</span> : <span class="ident">FnMut</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>
{
<span class="kw">type</span> <span class="ident">Output</span> <span class="op">=</span> <span class="ident">F</span>::<span class="ident">Output</span>;
<span class="kw">extern</span> <span class="string">"rust-call"</span> <span class="kw">fn</span> <span class="ident">call_once</span>(<span class="self">self</span>, <span class="ident">args</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">F</span>::<span class="ident">Output</span> {
(<span class="kw-2">*</span><span class="self">self</span>).<span class="ident">call_mut</span>(<span class="ident">args</span>)
}
}
}
</pre>
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