| <!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="Pins a value on the stack."><meta name="keywords" content="rust, rustlang, rust-lang, pin"><title>pin in tokio - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc macro"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In tokio</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Macro <a href="index.html">tokio</a>::<wbr><a class="macro" href="#">pin</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/tokio/macros/pin.rs.html#125-144">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules! </span>pin { |
| ($(<span class="macro-nonterminal">$x</span>:ident),<span class="kw-2">*</span>) => { ... }; |
| ($( |
| <span class="kw">let </span><span class="macro-nonterminal">$x</span>:ident = <span class="macro-nonterminal">$init</span>:expr; |
| )<span class="kw-2">*</span>) => { ... }; |
| }</code></pre></div> |
| </div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Pins a value on the stack.</p> |
| <p>Calls to <code>async fn</code> return anonymous <a href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html"><code>Future</code></a> values that are <code>!Unpin</code>. |
| These values must be pinned before they can be polled. Calling <code>.await</code> will |
| handle this, but consumes the future. If it is required to call <code>.await</code> on |
| a <code>&mut _</code> reference, the caller is responsible for pinning the future.</p> |
| <p>Pinning may be done by allocating with <a href="https://doc.rust-lang.org/nightly/alloc/boxed/struct.Box.html#method.pin"><code>Box::pin</code></a> or by using the stack |
| with the <code>pin!</code> macro.</p> |
| <p>The following will <strong>fail to compile</strong>:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>my_async_fn() { |
| <span class="comment">// async logic here |
| </span>} |
| |
| <span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="kw">let </span><span class="kw-2">mut </span>future = my_async_fn(); |
| (<span class="kw-2">&mut </span>future).<span class="kw">await</span>; |
| }</code></pre></div> |
| <p>To make this work requires pinning:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::pin; |
| |
| <span class="kw">async fn </span>my_async_fn() { |
| <span class="comment">// async logic here |
| </span>} |
| |
| <span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="kw">let </span>future = my_async_fn(); |
| <span class="macro">pin!</span>(future); |
| |
| (<span class="kw-2">&mut </span>future).<span class="kw">await</span>; |
| }</code></pre></div> |
| <p>Pinning is useful when using <code>select!</code> and stream operators that require <code>T: Stream + Unpin</code>.</p> |
| <h2 id="usage"><a href="#usage">Usage</a></h2> |
| <p>The <code>pin!</code> macro takes <strong>identifiers</strong> as arguments. It does <strong>not</strong> work |
| with expressions.</p> |
| <p>The following does not compile as an expression is passed to <code>pin!</code>.</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">async fn </span>my_async_fn() { |
| <span class="comment">// async logic here |
| </span>} |
| |
| <span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="kw">let </span><span class="kw-2">mut </span>future = <span class="macro">pin!</span>(my_async_fn()); |
| (<span class="kw-2">&mut </span>future).<span class="kw">await</span>; |
| }</code></pre></div> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <p>Using with select:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::{pin, select}; |
| <span class="kw">use </span>tokio_stream::{<span class="self">self </span><span class="kw">as </span>stream, StreamExt}; |
| |
| <span class="kw">async fn </span>my_async_fn() { |
| <span class="comment">// async logic here |
| </span>} |
| |
| <span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="kw">let </span><span class="kw-2">mut </span>stream = stream::iter(<span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]); |
| |
| <span class="kw">let </span>future = my_async_fn(); |
| <span class="macro">pin!</span>(future); |
| |
| <span class="kw">loop </span>{ |
| <span class="macro">select! </span>{ |
| <span class="kw">_ </span>= <span class="kw-2">&mut </span>future => { |
| <span class="comment">// Stop looping `future` will be polled after completion |
| </span><span class="kw">break</span>; |
| } |
| <span class="prelude-val">Some</span>(val) = stream.next() => { |
| <span class="macro">println!</span>(<span class="string">"got value = {}"</span>, val); |
| } |
| } |
| } |
| }</code></pre></div> |
| <p>Because assigning to a variable followed by pinning is common, there is also |
| a variant of the macro that supports doing both in one go.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::{pin, select}; |
| |
| <span class="kw">async fn </span>my_async_fn() { |
| <span class="comment">// async logic here |
| </span>} |
| |
| <span class="attribute">#[tokio::main] |
| </span><span class="kw">async fn </span>main() { |
| <span class="macro">pin! </span>{ |
| <span class="kw">let </span>future1 = my_async_fn(); |
| <span class="kw">let </span>future2 = my_async_fn(); |
| } |
| |
| <span class="macro">select! </span>{ |
| <span class="kw">_ </span>= <span class="kw-2">&mut </span>future1 => {} |
| <span class="kw">_ </span>= <span class="kw-2">&mut </span>future2 => {} |
| } |
| }</code></pre></div> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
