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<!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 `pin` macro in crate `tokio`."><meta name="keywords" content="rust, rustlang, rust-lang, pin"><title>tokio::pin - 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="../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../ayu.css" disabled ><script id="default-settings"></script><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc macro"><!--[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='../tokio/index.html'><div class='logo-container rust-logo'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class="location"><a href="index.html">tokio</a></p><script>window.sidebarCurrent = {name: "pin", ty: "macro", relpath: ""};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<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="out-of-band"><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/tokio/macros/pin.rs.html#125-144" title="goto source code">[src]</a></span><span class="in-band">Macro <a href="index.html">tokio</a>::<wbr><a class="macro" href="">pin</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">pin</span> {
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">x</span>:<span class="ident">ident</span>),<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> { ... };
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($(
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<span class="kw">let</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">x</span>:<span class="ident">ident</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">init</span>:<span class="ident">expr</span>;
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)<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> { ... };
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}</pre></div>
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</div><div class="docblock"><p>Pins a value on the stack.</p>
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<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>.
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These values must be pinned before they can be polled. Calling <code>.await</code> will
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handle this, but consumes the future. If it is required to call <code>.await</code> on
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a <code>&mut _</code> reference, the caller is responsible for pinning the future.</p>
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<p>Pinning may be done by allocating with <a href="#"><code>Box::pin</code></a> or by using the stack
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with the <code>pin!</code> macro.</p>
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<p>The following will <strong>fail to compile</strong>:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">my_async_fn</span>() {
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<span class="comment">// async logic here</span>
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}
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<span class="attribute">#[<span class="ident">tokio</span>::<span class="ident">main</span>]</span>
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">future</span> <span class="op">=</span> <span class="ident">my_async_fn</span>();
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(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future</span>).<span class="ident">await</span>;
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}</pre></div>
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<p>To make this work requires pinning:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">tokio</span>::<span class="ident">pin</span>;
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">my_async_fn</span>() {
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<span class="comment">// async logic here</span>
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}
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<span class="attribute">#[<span class="ident">tokio</span>::<span class="ident">main</span>]</span>
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">future</span> <span class="op">=</span> <span class="ident">my_async_fn</span>();
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<span class="macro">pin</span><span class="macro">!</span>(<span class="ident">future</span>);
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(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future</span>).<span class="ident">await</span>;
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}</pre></div>
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<p>Pinning is useful when using <code>select!</code> and stream operators that require <code>T: Stream + Unpin</code>.</p>
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<h1 id="usage" class="section-header"><a href="#usage">Usage</a></h1>
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<p>The <code>pin!</code> macro takes <strong>identifiers</strong> as arguments. It does <strong>not</strong> work
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with expressions.</p>
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<p>The following does not compile as an expression is passed to <code>pin!</code>.</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">my_async_fn</span>() {
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<span class="comment">// async logic here</span>
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}
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<span class="attribute">#[<span class="ident">tokio</span>::<span class="ident">main</span>]</span>
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">future</span> <span class="op">=</span> <span class="macro">pin</span><span class="macro">!</span>(<span class="ident">my_async_fn</span>());
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(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future</span>).<span class="ident">await</span>;
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}</pre></div>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Using with select:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">tokio</span>::{<span class="ident">pin</span>, <span class="ident">select</span>};
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<span class="kw">use</span> <span class="ident">tokio_stream</span>::{<span class="self">self</span> <span class="kw">as</span> <span class="ident">stream</span>, <span class="ident">StreamExt</span>};
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">my_async_fn</span>() {
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<span class="comment">// async logic here</span>
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}
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<span class="attribute">#[<span class="ident">tokio</span>::<span class="ident">main</span>]</span>
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">stream</span> <span class="op">=</span> <span class="ident">stream</span>::<span class="ident">iter</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]);
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<span class="kw">let</span> <span class="ident">future</span> <span class="op">=</span> <span class="ident">my_async_fn</span>();
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<span class="macro">pin</span><span class="macro">!</span>(<span class="ident">future</span>);
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<span class="kw">loop</span> {
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<span class="macro">select</span><span class="macro">!</span> {
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future</span> <span class="op">=</span><span class="op">></span> {
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<span class="comment">// Stop looping `future` will be polled after completion</span>
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<span class="kw">break</span>;
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}
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<span class="prelude-val">Some</span>(<span class="ident">val</span>) <span class="op">=</span> <span class="ident">stream</span>.<span class="ident">next</span>() <span class="op">=</span><span class="op">></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"got value = {}"</span>, <span class="ident">val</span>);
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}
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}
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}
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}</pre></div>
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<p>Because assigning to a variable followed by pinning is common, there is also
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a variant of the macro that supports doing both in one go.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">tokio</span>::{<span class="ident">pin</span>, <span class="ident">select</span>};
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">my_async_fn</span>() {
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<span class="comment">// async logic here</span>
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}
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<span class="attribute">#[<span class="ident">tokio</span>::<span class="ident">main</span>]</span>
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<span class="ident">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="macro">pin</span><span class="macro">!</span> {
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<span class="kw">let</span> <span class="ident">future1</span> <span class="op">=</span> <span class="ident">my_async_fn</span>();
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<span class="kw">let</span> <span class="ident">future2</span> <span class="op">=</span> <span class="ident">my_async_fn</span>();
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}
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<span class="macro">select</span><span class="macro">!</span> {
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future1</span> <span class="op">=</span><span class="op">></span> {}
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">future2</span> <span class="op">=</span><span class="op">></span> {}
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}
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}</pre></div>
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</div></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../";window.currentCrate = "tokio";</script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html>
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