<!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="API documentation for the Rust `Future` trait in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, Future"><title>core::future::Future - Rust</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 trait"><!--[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><p class='location'>Trait Future</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#associated-types">Associated Types</a><div class="sidebar-links"><a href="#associatedtype.Output">Output</a></div><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.poll">poll</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>future</a></p><script>window.sidebarCurrent = {name: 'Future', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></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"><h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>core</a>::<wbr><a href='index.html'>future</a>::<wbr><a class="trait" href=''>Future</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version '></span><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../../src/core/future.rs.html#37-94' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust trait'>pub trait Future {
type <a href='#associatedtype.Output' class="type">Output</a>;
fn <a href='#tymethod.poll' class='fnname'>poll</a>(self: <a class="struct" href="../../core/mem/struct.PinMut.html" title="struct core::mem::PinMut">PinMut</a><Self>, cx: &mut <a class="struct" href="../../core/task/struct.Context.html" title="struct core::task::Context">Context</a>) -> <a class="enum" href="../../core/task/enum.Poll.html" title="enum core::task::Poll">Poll</a><Self::<a class="type" href="../../core/future/trait.Future.html#associatedtype.Output" title="type core::future::Future::Output">Output</a>>;
}</pre></div><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>futures_api </code><a href="https://github.com/rust-lang/rust/issues/50547">#50547</a>)</summary><p>futures in libcore are unstable</p>
</details></div></div><div class='docblock'><p>A future represents an asychronous computation.</p>
<p>A future is a value that may not have finished computing yet. This kind of
"asynchronous value" makes it possible for a thread to continue doing useful
work while it waits for the value to become available.</p>
<h1 id="the-poll-method" class="section-header"><a href="#the-poll-method">The <code>poll</code> method</a></h1>
<p>The core method of future, <code>poll</code>, <em>attempts</em> to resolve the future into a
final value. This method does not block if the value is not ready. Instead,
the current task is scheduled to be woken up when it's possible to make
further progress by <code>poll</code>ing again. The wake up is performed using
<code>cx.waker()</code>, a handle for waking up the current task.</p>
<p>When using a future, you generally won't call <code>poll</code> directly, but instead
<code>await!</code> the value.</p>
</div>
<h2 id='associated-types' class='small-section-header'>
Associated Types<a href='#associated-types' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='associatedtype.Output' class='method'><span id='Output.t' class='invisible'><code>type <a href='#associatedtype.Output' class="type">Output</a></code></span></h3><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>futures_api </code><a href="https://github.com/rust-lang/rust/issues/50547">#50547</a>)</summary><p>futures in libcore are unstable</p>
</details></div></div><div class='docblock'><p>The result of the <code>Future</code>.</p>
</div></div>
<h2 id='required-methods' class='small-section-header'>
Required Methods<a href='#required-methods' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='tymethod.poll' class='method'><span id='poll.v' class='invisible'><code>fn <a href='#tymethod.poll' class='fnname'>poll</a>(self: <a class="struct" href="../../core/mem/struct.PinMut.html" title="struct core::mem::PinMut">PinMut</a><Self>, cx: &mut <a class="struct" href="../../core/task/struct.Context.html" title="struct core::task::Context">Context</a>) -> <a class="enum" href="../../core/task/enum.Poll.html" title="enum core::task::Poll">Poll</a><Self::<a class="type" href="../../core/future/trait.Future.html#associatedtype.Output" title="type core::future::Future::Output">Output</a>></code></span></h3><div class='stability'><div class='stab unstable'><details><summary><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>futures_api </code><a href="https://github.com/rust-lang/rust/issues/50547">#50547</a>)</summary><p>futures in libcore are unstable</p>
</details></div></div><div class='docblock'><p>Attempt to resolve the future to a final value, registering
the current task for wakeup if the value is not yet available.</p>
<h1 id="return-value" class="section-header"><a href="#return-value">Return value</a></h1>
<p>This function returns:</p>
<ul>
<li><code>Poll::Pending</code> if the future is not ready yet</li>
<li><code>Poll::Ready(val)</code> with the result <code>val</code> of this future if it finished
successfully.</li>
</ul>
<p>Once a future has finished, clients should not <code>poll</code> it again.</p>
<p>When a future is not ready yet, <code>poll</code> returns
<a href="../../core/task/enum.Poll.html"><code>Poll::Pending</code></a>. The future will <em>also</em> register the
interest of the current task in the value being produced. For example,
if the future represents the availability of data on a socket, then the
task is recorded so that when data arrives, it is woken up (via
<a href="../../core/task/struct.Context.html#method.waker"><code>cx.waker()</code></a>). Once a task has been woken up,
it should attempt to <code>poll</code> the future again, which may or may not
produce a final value.</p>
<p>Note that if <code>Pending</code> is returned it only means that the <em>current</em> task
(represented by the argument <code>cx</code>) will receive a notification. Tasks
from previous calls to <code>poll</code> will <em>not</em> receive notifications.</p>
<h1 id="runtime-characteristics" class="section-header"><a href="#runtime-characteristics">Runtime characteristics</a></h1>
<p>Futures alone are <em>inert</em>; they must be <em>actively</em> <code>poll</code>ed to make
progress, meaning that each time the current task is woken up, it should
actively re-<code>poll</code> pending futures that it still has an interest in.</p>
<p>The <code>poll</code> function is not called repeatedly in a tight loop for
futures, but only whenever the future itself is ready, as signaled via
the <code>Waker</code> inside <code>task::Context</code>. If you're familiar with the
<code>poll(2)</code> or <code>select(2)</code> syscalls on Unix it's worth noting that futures
typically do <em>not</em> suffer the same problems of "all wakeups must poll
all events"; they are more like <code>epoll(4)</code>.</p>
<p>An implementation of <code>poll</code> should strive to return quickly, and must
<em>never</em> block. Returning quickly prevents unnecessarily clogging up
threads or event loops. If it is known ahead of time that a call to
<code>poll</code> may end up taking awhile, the work should be offloaded to a
thread pool (or something similar) to ensure that <code>poll</code> can return
quickly.</p>
<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
<p>Once a future has completed (returned <code>Ready</code> from <code>poll</code>),
then any future calls to <code>poll</code> may panic, block forever, or otherwise
cause bad behavior. The <code>Future</code> trait itself provides no guarantees
about the behavior of <code>poll</code> after a future has completed.</p>
</div></div>
<h2 id='implementors' class='small-section-header'>
Implementors<a href='#implementors' class='anchor'></a>
</h2>
<ul class='item-list' id='implementors-list'>
<li><table class='table-display'><tbody><tr><td><code>impl<'a, F: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="../../core/future/trait.Future.html" title="trait core::future::Future">Future</a> + <a class="trait" href="../../core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>> Future for &'a mut F<span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = F::<a class="type" href="../../core/future/trait.Future.html#associatedtype.Output" title="type core::future::Future::Output">Output</a>;</span></code><td><div class='out-of-band'><a class='srclink' href='../../src/core/future.rs.html#96-102' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
<li><table class='table-display'><tbody><tr><td><code>impl<'a, F: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="../../core/future/trait.Future.html" title="trait core::future::Future">Future</a>> Future for <a class="struct" href="../../core/mem/struct.PinMut.html" title="struct core::mem::PinMut">PinMut</a><'a, F><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = F::<a class="type" href="../../core/future/trait.Future.html#associatedtype.Output" title="type core::future::Future::Output">Output</a>;</span></code><td><div class='out-of-band'><a class='srclink' href='../../src/core/future.rs.html#104-110' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
<li><table class='table-display'><tbody><tr><td><code>impl Future for <a class="struct" href="../../core/task/struct.TaskObj.html" title="struct core::task::TaskObj">TaskObj</a><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = ();</span></code><td><div class='out-of-band'><a class='srclink' href='../../src/core/task.rs.html#517-526' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
</ul><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
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