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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="Threads that can borrow variables from the stack."><meta name="keywords" content="rust, rustlang, rust-lang, thread"><title>crossbeam_utils::thread - 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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<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 mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu" role="button">☰</div><a class="sidebar-logo" href="../../crossbeam_utils/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.png" alt="logo"></div>
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</a><h2 class="location">Module thread</h2><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></div><div id="sidebar-vars" data-name="thread" data-ty="mod" 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="../../crossbeam_utils/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">Module <a href="../index.html">crossbeam_utils</a>::<wbr><a class="mod" href="#">thread</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/crossbeam_utils/thread.rs.html#1-585" title="goto source code">[src]</a></span></h1><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Threads that can borrow variables from the stack.</p>
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<p>Create a scope when spawned threads need to access variables on the stack:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">crossbeam_utils::thread</span>;
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<span class="kw">let</span> <span class="ident">people</span> <span class="op">=</span> <span class="macro">vec!</span>[
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<span class="string">"Alice"</span>.<span class="ident">to_string</span>(),
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<span class="string">"Bob"</span>.<span class="ident">to_string</span>(),
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<span class="string">"Carol"</span>.<span class="ident">to_string</span>(),
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];
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<span class="ident">thread::scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="kw">for</span> <span class="ident">person</span> <span class="kw">in</span> <span class="kw-2">&</span><span class="ident">people</span> {
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<span class="ident">s</span>.<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="kw">_</span><span class="op">|</span> {
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<span class="macro">println!</span>(<span class="string">"Hello, {}!"</span>, <span class="ident">person</span>);
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});
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}
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}).<span class="ident">unwrap</span>();</code></pre></div>
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<h2 id="why-scoped-threads" class="section-header"><a href="#why-scoped-threads">Why scoped threads?</a></h2>
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<p>Suppose we wanted to re-write the previous example using plain threads:</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">use</span> <span class="ident">std::thread</span>;
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<span class="kw">let</span> <span class="ident">people</span> <span class="op">=</span> <span class="macro">vec!</span>[
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<span class="string">"Alice"</span>.<span class="ident">to_string</span>(),
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<span class="string">"Bob"</span>.<span class="ident">to_string</span>(),
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<span class="string">"Carol"</span>.<span class="ident">to_string</span>(),
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];
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">threads</span> <span class="op">=</span> <span class="ident">Vec::new</span>();
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<span class="kw">for</span> <span class="ident">person</span> <span class="kw">in</span> <span class="kw-2">&</span><span class="ident">people</span> {
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<span class="ident">threads</span>.<span class="ident">push</span>(<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="macro">println!</span>(<span class="string">"Hello, {}!"</span>, <span class="ident">person</span>);
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}));
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}
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<span class="kw">for</span> <span class="ident">thread</span> <span class="kw">in</span> <span class="ident">threads</span> {
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<span class="ident">thread</span>.<span class="ident">join</span>().<span class="ident">unwrap</span>();
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}</code></pre></div>
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<p>This doesn’t work because the borrow checker complains about <code>people</code> not living long enough:</p>
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<div class="example-wrap"><pre class="language-text"><code>error[E0597]: `people` does not live long enough
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--> src/main.rs:12:20
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12 | for person in &people {
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| ^^^^^^ borrowed value does not live long enough
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...
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21 | }
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| - borrowed value only lives until here
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|
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= note: borrowed value must be valid for the static lifetime...</code></pre></div>
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<p>The problem here is that spawned threads are not allowed to borrow variables on stack because
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the compiler cannot prove they will be joined before <code>people</code> is destroyed.</p>
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<p>Scoped threads are a mechanism to guarantee to the compiler that spawned threads will be joined
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before the scope ends.</p>
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<h2 id="how-scoped-threads-work" class="section-header"><a href="#how-scoped-threads-work">How scoped threads work</a></h2>
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<p>If a variable is borrowed by a thread, the thread must complete before the variable is
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destroyed. Threads spawned using <a href="https://doc.rust-lang.org/1.59.0/std/thread/fn.spawn.html" title="std::thread::spawn"><code>std::thread::spawn</code></a> can only borrow variables with the
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<code>'static</code> lifetime because the borrow checker cannot be sure when the thread will complete.</p>
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<p>A scope creates a clear boundary between variables outside the scope and threads inside the
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scope. Whenever a scope spawns a thread, it promises to join the thread before the scope ends.
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This way we guarantee to the borrow checker that scoped threads only live within the scope and
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can safely access variables outside it.</p>
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<h2 id="nesting-scoped-threads" class="section-header"><a href="#nesting-scoped-threads">Nesting scoped threads</a></h2>
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<p>Sometimes scoped threads need to spawn more threads within the same scope. This is a little
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tricky because argument <code>s</code> lives <em>inside</em> the invocation of <code>thread::scope()</code> and as such
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cannot be borrowed by scoped threads:</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">use</span> <span class="ident">crossbeam_utils::thread</span>;
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<span class="ident">thread::scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="ident">s</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> {
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<span class="comment">// Not going to compile because we're trying to borrow `s`,</span>
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<span class="comment">// which lives *inside* the scope! :(</span>
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<span class="ident">s</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> <span class="macro">println!</span>(<span class="string">"nested thread"</span>));
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});
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});</code></pre></div>
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<p>Fortunately, there is a solution. Every scoped thread is passed a reference to its scope as an
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argument, which can be used for spawning nested threads:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">crossbeam_utils::thread</span>;
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<span class="ident">thread::scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="comment">// Note the `|s|` here.</span>
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<span class="ident">s</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="comment">// Yay, this works because we're using a fresh argument `s`! :)</span>
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<span class="ident">s</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> <span class="macro">println!</span>(<span class="string">"nested thread"</span>));
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});
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}).<span class="ident">unwrap</span>();</code></pre></div>
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</div></details><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Scope.html" title="crossbeam_utils::thread::Scope struct">Scope</a></div><div class="item-right docblock-short"><p>A scope for spawning threads.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.ScopedJoinHandle.html" title="crossbeam_utils::thread::ScopedJoinHandle struct">ScopedJoinHandle</a></div><div class="item-right docblock-short"><p>A handle that can be used to join its scoped thread.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.ScopedThreadBuilder.html" title="crossbeam_utils::thread::ScopedThreadBuilder struct">ScopedThreadBuilder</a></div><div class="item-right docblock-short"><p>Configures the properties of a new thread.</p>
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</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.scope.html" title="crossbeam_utils::thread::scope fn">scope</a></div><div class="item-right docblock-short"><p>Creates a new scope for spawning threads.</p>
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