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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 `scope` fn in crate `rayon`."><meta name="keywords" content="rust, rustlang, rust-lang, scope"><title>rayon::scope - 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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[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='../rayon/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>rayon</a></p><script>window.sidebarCurrent = {name: 'scope', ty: 'fn', 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="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.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="../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/rayon_core/scope/mod.rs.html#288-297' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='index.html'>rayon</a>::<wbr><a class="fn" href=''>scope</a></span></h1><pre class='rust fn'>pub fn scope<'scope, OP, R>(op: OP) -> R <span class="where fmt-newline">where<br> OP: for<'s> <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>(&'s <a class="struct" href="../rayon/struct.Scope.html" title="struct rayon::Scope">Scope</a><'scope>) -> R + 'scope + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>,<br> R: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>, </span></pre><div class='docblock'><p>Create a "fork-join" scope <code>s</code> and invokes the closure with a
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reference to <code>s</code>. This closure can then spawn asynchronous tasks
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into <code>s</code>. Those tasks may run asynchronously with respect to the
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closure; they may themselves spawn additional tasks into <code>s</code>. When
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the closure returns, it will block until all tasks that have been
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spawned into <code>s</code> complete.</p>
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<p><code>scope()</code> is a more flexible building block compared to <code>join()</code>,
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since a loop can be used to spawn any number of tasks without
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recursing. However, that flexibility comes at a performance price:
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tasks spawned using <code>scope()</code> must be allocated onto the heap,
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whereas <code>join()</code> can make exclusive use of the stack. <strong>Prefer
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<code>join()</code> (or, even better, parallel iterators) where possible.</strong></p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>The Rayon <code>join()</code> function launches two closures and waits for them
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to stop. One could implement <code>join()</code> using a scope like so, although
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it would be less efficient than the real implementation:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">join</span><span class="op"><</span><span class="ident">A</span>,<span class="ident">B</span>,<span class="ident">RA</span>,<span class="ident">RB</span><span class="op">></span>(<span class="ident">oper_a</span>: <span class="ident">A</span>, <span class="ident">oper_b</span>: <span class="ident">B</span>) <span class="op">-</span><span class="op">></span> (<span class="ident">RA</span>, <span class="ident">RB</span>)
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<span class="kw">where</span> <span class="ident">A</span>: <span class="ident">FnOnce</span>() <span class="op">-</span><span class="op">></span> <span class="ident">RA</span> <span class="op">+</span> <span class="ident">Send</span>,
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<span class="ident">B</span>: <span class="ident">FnOnce</span>() <span class="op">-</span><span class="op">></span> <span class="ident">RB</span> <span class="op">+</span> <span class="ident">Send</span>,
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<span class="ident">RA</span>: <span class="ident">Send</span>,
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<span class="ident">RB</span>: <span class="ident">Send</span>,
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{
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">result_a</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">RA</span><span class="op">></span> <span class="op">=</span> <span class="prelude-val">None</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">result_b</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">RB</span><span class="op">></span> <span class="op">=</span> <span class="prelude-val">None</span>;
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<span class="ident">rayon</span>::<span class="ident">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> <span class="ident">result_a</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">oper_a</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="ident">result_b</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">oper_b</span>()));
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});
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(<span class="ident">result_a</span>.<span class="ident">unwrap</span>(), <span class="ident">result_b</span>.<span class="ident">unwrap</span>())
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}</pre></div>
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<h1 id="a-note-on-threading" class="section-header"><a href="#a-note-on-threading">A note on threading</a></h1>
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<p>The closure given to <code>scope()</code> executes in the Rayon thread-pool,
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as do those given to <code>spawn()</code>. This means that you can't access
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thread-local variables (well, you can, but they may have
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unexpected values).</p>
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<h1 id="task-execution" class="section-header"><a href="#task-execution">Task execution</a></h1>
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<p>Task execution potentially starts as soon as <code>spawn()</code> is called.
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The task will end sometime before <code>scope()</code> returns. Note that the
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<em>closure</em> given to scope may return much earlier. In general
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the lifetime of a scope created like `scope(body) goes something like this:</p>
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<ul>
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<li>Scope begins when <code>scope(body)</code> is called</li>
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<li>Scope body <code>body()</code> is invoked
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<ul>
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<li>Scope tasks may be spawned</li>
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</ul>
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</li>
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<li>Scope body returns</li>
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<li>Scope tasks execute, possibly spawning more tasks</li>
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<li>Once all tasks are done, scope ends and <code>scope()</code> returns</li>
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</ul>
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<p>To see how and when tasks are joined, consider this example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// point start</span>
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<span class="ident">rayon</span>::<span class="ident">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="ident">s</span><span class="op">|</span> { <span class="comment">// task s.1</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> { <span class="comment">// task s.1.1</span>
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<span class="ident">rayon</span>::<span class="ident">scope</span>(<span class="op">|</span><span class="ident">t</span><span class="op">|</span> {
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<span class="ident">t</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> ()); <span class="comment">// task t.1</span>
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<span class="ident">t</span>.<span class="ident">spawn</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> ()); <span class="comment">// task t.2</span>
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});
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});
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});
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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> { <span class="comment">// task s.2</span>
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});
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<span class="comment">// point mid</span>
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});
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<span class="comment">// point end</span></pre></div>
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<p>The various tasks that are run will execute roughly like so:</p>
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<pre><code class="language-notrust">| (start)
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| (scope `s` created)
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+-----------------------------------------------+ (task s.2)
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+-------+ (task s.1) |
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| | |
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| +---+ (task s.1.1) |
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| | | |
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| | | (scope `t` created) |
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| | +----------------+ (task t.2) |
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| | +---+ (task t.1) | |
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| (mid) | | | | |
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: | + <-+------------+ (scope `t` ends) |
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: | | |
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|<------+---+-----------------------------------+ (scope `s` ends)
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| (end)
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</code></pre>
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<p>The point here is that everything spawned into scope <code>s</code> will
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terminate (at latest) at the same point -- right before the
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original call to <code>rayon::scope</code> returns. This includes new
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subtasks created by other subtasks (e.g., task <code>s.1.1</code>). If a new
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scope is created (such as <code>t</code>), the things spawned into that scope
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will be joined before that scope returns, which in turn occurs
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before the creating task (task <code>s.1.1</code> in this case) finishes.</p>
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<p>There is no guaranteed order of execution for spawns in a scope,
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given that other threads may steal tasks at any time. However, they
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are generally prioritized in a LIFO order on the thread from which
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they were spawned. So in this example, absent any stealing, we can
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expect <code>s.2</code> to execute before <code>s.1</code>, and <code>t.2</code> before <code>t.1</code>. Other
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threads always steal from the other end of the deque, like FIFO
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order. The idea is that "recent" tasks are most likely to be fresh
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in the local CPU's cache, while other threads can steal older
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"stale" tasks. For an alternate approach, consider
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<a href="fn.scope_fifo.html"><code>scope_fifo()</code></a> instead.</p>
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<h1 id="accessing-stack-data" class="section-header"><a href="#accessing-stack-data">Accessing stack data</a></h1>
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<p>In general, spawned tasks may access stack data in place that
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outlives the scope itself. Other data must be fully owned by the
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spawned task.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ok</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</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>];
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<span class="ident">rayon</span>::<span class="ident">scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="ident">bad</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</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">// We can access `ok` because outlives the scope `s`.</span>
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>);
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<span class="comment">// If we just try to use `bad` here, the closure will borrow `bad`</span>
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<span class="comment">// (because we are just printing it out, and that only requires a</span>
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<span class="comment">// borrow), which will result in a compilation error. Read on</span>
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<span class="comment">// for options.</span>
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<span class="comment">// println!("bad: {:?}", bad);</span>
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});
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});</pre></div>
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<p>As the comments example above suggest, to reference <code>bad</code> we must
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take ownership of it. One way to do this is to detach the closure
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from the surrounding stack frame, using the <code>move</code> keyword. This
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will cause it to take ownership of <em>all</em> the variables it touches,
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in this case including both <code>ok</code> <em>and</em> <code>bad</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ok</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</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>];
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<span class="ident">rayon</span>::<span class="ident">scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="ident">bad</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</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="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>);
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"bad: {:?}"</span>, <span class="ident">bad</span>);
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});
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<span class="comment">// That closure is fine, but now we can't use `ok` anywhere else,</span>
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<span class="comment">// since it is owend by the previous task:</span>
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<span class="comment">// s.spawn(|_| println!("ok: {:?}", ok));</span>
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});</pre></div>
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<p>While this works, it could be a problem if we want to use <code>ok</code> elsewhere.
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There are two choices. We can keep the closure as a <code>move</code> closure, but
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instead of referencing the variable <code>ok</code>, we create a shadowed variable that
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is a borrow of <code>ok</code> and capture <em>that</em>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">ok</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</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>];
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<span class="ident">rayon</span>::<span class="ident">scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="ident">bad</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>];
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<span class="kw">let</span> <span class="ident">ok</span>: <span class="kw-2">&</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">ok</span>; <span class="comment">// shadow the original `ok`</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="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>); <span class="comment">// captures the shadowed version</span>
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"bad: {:?}"</span>, <span class="ident">bad</span>);
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});
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<span class="comment">// Now we too can use the shadowed `ok`, since `&Vec<i32>` references</span>
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<span class="comment">// can be shared freely. Note that we need a `move` closure here though,</span>
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<span class="comment">// because otherwise we'd be trying to borrow the shadowed `ok`,</span>
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<span class="comment">// and that doesn't outlive `scope`.</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> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>));
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});</pre></div>
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<p>Another option is not to use the <code>move</code> keyword but instead to take ownership
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of individual variables:</p>
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|
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|
<div class="example-wrap"><pre class="rust rust-example-rendered">
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|
<span class="kw">let</span> <span class="ident">ok</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</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>];
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<span class="ident">rayon</span>::<span class="ident">scope</span>(<span class="op">|</span><span class="ident">s</span><span class="op">|</span> {
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|
<span class="kw">let</span> <span class="ident">bad</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">4</span>, <span class="number">5</span>, <span class="number">6</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">// Transfer ownership of `bad` into a local variable (also named `bad`).</span>
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|
<span class="comment">// This will force the closure to take ownership of `bad` from the environment.</span>
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|
<span class="kw">let</span> <span class="ident">bad</span> <span class="op">=</span> <span class="ident">bad</span>;
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|
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>); <span class="comment">// `ok` is only borrowed.</span>
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|
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"bad: {:?}"</span>, <span class="ident">bad</span>); <span class="comment">// refers to our local variable, above.</span>
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});
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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="macro">!</span>(<span class="string">"ok: {:?}"</span>, <span class="ident">ok</span>)); <span class="comment">// we too can borrow `ok`</span>
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});</pre></div>
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<h1 id="panics" class="section-header"><a href="#panics">Panics</a></h1>
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<p>If a panic occurs, either in the closure given to <code>scope()</code> or in
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any of the spawned jobs, that panic will be propagated and the
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call to <code>scope()</code> will panic. If multiple panics occurs, it is
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non-deterministic which of their panic values will propagate.
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Regardless, once a task is spawned using <code>scope.spawn()</code>, it will
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execute, even if the spawning task should later panic. <code>scope()</code>
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returns once all spawned jobs have completed, and any panics are
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propagated at that point.</p>
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