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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="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" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script>
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</a><div class="sidebar-elems"><h2 class="location">Other items in<br><a href="index.html">tokio</a></h2><div id="sidebar-vars" data-name="pin" data-ty="macro" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../tokio/index.html"><img class="rust-logo" src="../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><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" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">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></span><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></h1><div class="docblock item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules!</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>) => { ... };
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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> ;) <span class="kw-2">*</span>) => { ... };
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}</code></pre></div>
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</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>
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<p>Calls to <code>async fn</code> return anonymous <a href="https://doc.rust-lang.org/1.59.0/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="https://doc.rust-lang.org/1.59.0/alloc/boxed/struct.Box.html#method.pin"><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'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">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::main</span>]</span>
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<span class="kw">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">&mut</span> <span class="ident">future</span>).<span class="kw">await</span>;
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}</code></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"><code><span class="kw">use</span> <span class="ident">tokio::pin</span>;
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<span class="kw">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::main</span>]</span>
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<span class="kw">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="ident">future</span>);
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(<span class="kw-2">&mut</span> <span class="ident">future</span>).<span class="kw">await</span>;
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}</code></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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<h2 id="usage" class="section-header"><a href="#usage">Usage</a></h2>
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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'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">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::main</span>]</span>
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<span class="kw">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="ident">my_async_fn</span>());
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(<span class="kw-2">&mut</span> <span class="ident">future</span>).<span class="kw">await</span>;
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}</code></pre></div>
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<h2 id="examples" class="section-header"><a href="#examples">Examples</a></h2>
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<p>Using with select:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><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="kw">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::main</span>]</span>
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<span class="kw">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::iter</span>(<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>]);
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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="ident">future</span>);
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<span class="kw">loop</span> {
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<span class="macro">select!</span> {
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&mut</span> <span class="ident">future</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>() => {
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<span class="macro">println!</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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}</code></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"><code><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">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::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="macro">pin!</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> {
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&mut</span> <span class="ident">future1</span> => {}
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<span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&mut</span> <span class="ident">future2</span> => {}
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}
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}</code></pre></div>
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