<!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 `std`."><meta name="keywords" content="rust, rustlang, rust-lang, Future"><title>std::future::Future - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../light1.35.0.css" id="themeStyle"><script src="../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow1.35.0.svg");}</style></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='../../std/index.html'><img src='../../rust-logo1.35.0.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'>std</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="../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../../settings.html"><img src="../../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><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/future.rs.html#29-96' title='goto source code'>[src]</a></span><span class='in-band'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>future</a>::<wbr><a class="trait" href=''>Future</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class='rust trait'><div class="docblock attributes">#[must_use = "futures do nothing unless polled"]
</div>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="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><<a class="primitive" href="../primitive.reference.html">&mut </a>Self>, cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><Self::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>>;
}</pre></div><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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 asynchronous 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
the <code>waker</code> argument of the <code>poll()</code> method, which is 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'><code id='Output.t'>type <a href='#associatedtype.Output' class="type">Output</a></code></h3><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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 type of value produced on completion.</p>
</div></div><span class='loading-content'>Loading content...</span>
<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'><code id='poll.v'>fn <a href='#tymethod.poll' class='fnname'>poll</a>(self: <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><<a class="primitive" href="../primitive.reference.html">&mut </a>Self>, cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><Self::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>></code></h3><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><a href="../task/enum.Poll.html#variant.Pending"><code>Poll::Pending</code></a> if the future is not ready yet</li>
<li><a href="../task/enum.Poll.html#variant.Ready"><code>Poll::Ready(val)</code></a> 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 <code>Poll::Pending</code> and
stores a clone of the <a href="../task/struct.Waker.html"><code>Waker</code></a> copied from the current <a href="../task/struct.Context.html"><code>Context</code></a>.
This <a href="../task/struct.Waker.html"><code>Waker</code></a> is then woken once the future can make progress.
For example, a future waiting for a socket to become
readable would call <code>.clone()</code> on the <a href="../task/struct.Waker.html"><code>Waker</code></a> and store it.
When a signal arrives elsewhere indicating that the socket is readable,
<code>[Waker::wake]</code> is called and the socket future's task is awoken.
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 on multiple calls to <code>poll</code>, only the most recent
<a href="../task/struct.Waker.html"><code>Waker</code></a> passed to <code>poll</code> should be scheduled to receive a
wakeup.</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 -- instead,
it should only be called when the future indicates that it is ready to
make progress (by calling <code>wake()</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 should
not 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>
<p>An implementation of <code>poll</code> may also never cause memory unsafety.</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 any kind of bad behavior except causing memory unsafety.
The <code>Future</code> trait itself provides no guarantees about the behavior
of <code>poll</code> after a future has completed.</p>
</div></div><span class='loading-content'>Loading content...</span>
<h2 id='implementors' class='small-section-header'>Implementors<a href='#implementors' class='anchor'></a></h2><div class='item-list' id='implementors-list'><h3 id='impl-Future' class='impl'><code class='in-band'>impl<'_, F> Future for <a class="primitive" href="../primitive.reference.html">&'_ mut </a>F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/marker/trait.Unpin.html" title="trait std::marker::Unpin">Unpin</a> + <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-Future' class='anchor'></a><a class='srclink' href='../../src/core/future/future.rs.html#98-104' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-1' class="type"><code id='Output.t-1'>type <a href='#associatedtype.Output-1' class="type">Output</a> = <F as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a></code></h4><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h4 id='method.poll' class="method hidden"><code id='poll.v-1'>fn <a href='#method.poll' class='fnname'>poll</a>(<br> self: <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><&mut <a class="primitive" href="../primitive.reference.html">&'_ mut </a>F>, <br> cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a><br>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><<<a class="primitive" href="../primitive.reference.html">&'_ mut </a>F as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>></code><a class='srclink' href='../../src/core/future/future.rs.html#101-103' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h3 id='impl-Future-1' class='impl'><code class='in-band'>impl<F> Future for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/marker/trait.Unpin.html" title="trait std::marker::Unpin">Unpin</a> + <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-Future-1' class='anchor'></a><a class='srclink' href='../../src/alloc/boxed.rs.html#912-918' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-2' class="type"><code id='Output.t-2'>type <a href='#associatedtype.Output-2' class="type">Output</a> = <F as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a></code></h4><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h4 id='method.poll-1' class="method hidden"><code id='poll.v-2'>fn <a href='#method.poll-1' class='fnname'>poll</a>(<br> self: <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><&mut <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><F>>, <br> cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a><br>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><<<a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><F> as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>></code><a class='srclink' href='../../src/alloc/boxed.rs.html#915-917' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h3 id='impl-Future-2' class='impl'><code class='in-band'>impl<F: <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>> Future for <a class="struct" href="../../std/panic/struct.AssertUnwindSafe.html" title="struct std::panic::AssertUnwindSafe">AssertUnwindSafe</a><F></code><a href='#impl-Future-2' class='anchor'></a><a class='srclink' href='../../src/std/panic.rs.html#323-330' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-3' class="type"><code id='Output.t-3'>type <a href='#associatedtype.Output-3' class="type">Output</a> = F::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a></code></h4><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h4 id='method.poll-2' class="method hidden"><code id='poll.v-3'>fn <a href='#method.poll-2' class='fnname'>poll</a>(self: <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><<a class="primitive" href="../primitive.reference.html">&mut </a>Self>, cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><Self::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>></code><a class='srclink' href='../../src/std/panic.rs.html#326-329' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h3 id='impl-Future-3' class='impl'><code class='in-band'>impl<P> Future for <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><P> <span class="where fmt-newline">where<br> P: <a class="trait" href="../../std/marker/trait.Unpin.html" title="trait std::marker::Unpin">Unpin</a> + <a class="trait" href="../../std/ops/trait.DerefMut.html" title="trait std::ops::DerefMut">DerefMut</a>,<br> <P as <a class="trait" href="../../std/ops/trait.Deref.html" title="trait std::ops::Deref">Deref</a>>::<a class="type" href="../../std/ops/trait.Deref.html#associatedtype.Target" title="type std::ops::Deref::Target">Target</a>: <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>, </span></code><a href='#impl-Future-3' class='anchor'></a><a class='srclink' href='../../src/core/future/future.rs.html#106-116' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-4' class="type"><code id='Output.t-4'>type <a href='#associatedtype.Output-4' class="type">Output</a> = <<P as <a class="trait" href="../../std/ops/trait.Deref.html" title="trait std::ops::Deref">Deref</a>>::<a class="type" href="../../std/ops/trait.Deref.html#associatedtype.Target" title="type std::ops::Deref::Target">Target</a> as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a></code></h4><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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><h4 id='method.poll-3' class="method hidden"><code id='poll.v-4'>fn <a href='#method.poll-3' class='fnname'>poll</a>(<br> self: <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><&mut <a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><P>>, <br> cx: &mut <a class="struct" href="../../std/task/struct.Context.html" title="struct std::task::Context">Context</a><br>) -> <a class="enum" href="../../std/task/enum.Poll.html" title="enum std::task::Poll">Poll</a><<<a class="struct" href="../../std/pin/struct.Pin.html" title="struct std::pin::Pin">Pin</a><P> as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>></code><a class='srclink' href='../../src/core/future/future.rs.html#113-115' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><details><summary><span class='emoji'>🔬</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>
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