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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_project` macro in crate `pin_project_lite`."><meta name="keywords" content="rust, rustlang, rust-lang, pin_project"><title>pin_project_lite::pin_project - 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='../pin_project_lite/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">pin_project_lite</a></p><div id="sidebar-vars" data-name="pin_project" data-ty="macro" data-relpath=""></div><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><button type="button" class="help-button">?</button>
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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="in-band">Macro <a href="index.html">pin_project_lite</a>::<wbr><a class="macro" href="">pin_project</a></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/pin_project_lite/lib.rs.html#296-303" title="goto source code">[src]</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_project</span> {
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tt</span>:<span class="ident">tt</span>)<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>A macro that creates a projection type covering all the fields of struct.</p>
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<p>This macro creates a projection type according to the following rules:</p>
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<ul>
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<li>For the field that uses <code>#[pin]</code> attribute, makes the pinned reference to the field.</li>
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<li>For the other fields, makes the unpinned reference to the field.</li>
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</ul>
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<p>And the following methods are implemented on the original type:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">fn</span> <span class="ident">project</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>) <span class="op">-</span><span class="op">></span> <span class="ident">Projection</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>;
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<span class="kw">fn</span> <span class="ident">project_ref</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="self">Self</span><span class="op">></span>) <span class="op">-</span><span class="op">></span> <span class="ident">ProjectionRef</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>;</pre></div>
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<p>By passing an attribute with the same name as the method to the macro,
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you can name the projection type returned from the method. This allows you
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to use pattern matching on the projected types.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">pin_project</span><span class="macro">!</span> {
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<span class="attribute">#[<span class="ident">project</span> <span class="op">=</span> <span class="ident">EnumProj</span>]</span>
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<span class="kw">enum</span> <span class="ident">Enum</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">Variant</span> { <span class="attribute">#[<span class="ident">pin</span>]</span> <span class="ident">field</span>: <span class="ident">T</span> },
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}
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Enum</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">method</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>) {
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<span class="kw">let</span> <span class="ident">this</span>: <span class="ident">EnumProj</span><span class="op"><</span><span class="lifetime">'_</span>, <span class="ident">T</span><span class="op">></span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">project</span>();
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<span class="kw">match</span> <span class="ident">this</span> {
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<span class="ident">EnumProj</span>::<span class="ident">Variant</span> { <span class="ident">field</span> } <span class="op">=</span><span class="op">></span> {
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<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span><span class="op">></span> <span class="op">=</span> <span class="ident">field</span>;
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}
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}
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}
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}</pre></div>
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<p>By passing the <code>#[project_replace = MyProjReplace]</code> attribute you may create an additional
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method which allows the contents of <code>Pin<&mut Self></code> to be replaced while simultaneously moving
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out all unpinned fields in <code>Self</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">fn</span> <span class="ident">project_replace</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>, <span class="ident">replacement</span>: <span class="self">Self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">MyProjReplace</span>;</pre></div>
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<p>Also, note that the projection types returned by <code>project</code> and <code>project_ref</code> have
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an additional lifetime at the beginning of generics.</p>
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<pre><code class="language-text">let this: EnumProj<'_, T> = self.project();
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^^
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</code></pre>
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<p>The visibility of the projected types and projection methods is based on the
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original type. However, if the visibility of the original type is <code>pub</code>, the
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visibility of the projected types and the projection methods is downgraded
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to <code>pub(crate)</code>.</p>
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<h1 id="safety" class="section-header"><a href="#safety">Safety</a></h1>
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<p><code>pin_project!</code> macro guarantees safety in much the same way as <a href="https://github.com/taiki-e/pin-project">pin-project</a> crate.
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Both are completely safe unless you write other unsafe code.</p>
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<p>See <a href="https://github.com/taiki-e/pin-project">pin-project</a> crate for more details.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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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">std</span>::<span class="ident">pin</span>::<span class="ident">Pin</span>;
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<span class="kw">use</span> <span class="ident">pin_project_lite</span>::<span class="ident">pin_project</span>;
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<span class="macro">pin_project</span><span class="macro">!</span> {
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<span class="kw">struct</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> {
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<span class="attribute">#[<span class="ident">pin</span>]</span>
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<span class="ident">pinned</span>: <span class="ident">T</span>,
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<span class="ident">unpinned</span>: <span class="ident">U</span>,
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}
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">method</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>) {
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<span class="kw">let</span> <span class="ident">this</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">project</span>();
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<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span><span class="op">></span> <span class="op">=</span> <span class="ident">this</span>.<span class="ident">pinned</span>; <span class="comment">// Pinned reference to the field</span>
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<span class="kw">let</span> <span class="kw">_</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">U</span> <span class="op">=</span> <span class="ident">this</span>.<span class="ident">unpinned</span>; <span class="comment">// Normal reference to the field</span>
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}
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}</pre></div>
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<p>To use <code>pin_project!</code> on enums, you need to name the projection type
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returned from the method.</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">std</span>::<span class="ident">pin</span>::<span class="ident">Pin</span>;
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<span class="kw">use</span> <span class="ident">pin_project_lite</span>::<span class="ident">pin_project</span>;
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<span class="macro">pin_project</span><span class="macro">!</span> {
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<span class="attribute">#[<span class="ident">project</span> <span class="op">=</span> <span class="ident">EnumProj</span>]</span>
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<span class="kw">enum</span> <span class="ident">Enum</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">Struct</span> {
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<span class="attribute">#[<span class="ident">pin</span>]</span>
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<span class="ident">field</span>: <span class="ident">T</span>,
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},
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<span class="ident">Unit</span>,
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}
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Enum</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">method</span>(<span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>) {
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<span class="kw">match</span> <span class="self">self</span>.<span class="ident">project</span>() {
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<span class="ident">EnumProj</span>::<span class="ident">Struct</span> { <span class="ident">field</span> } <span class="op">=</span><span class="op">></span> {
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<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span><span class="op">></span> <span class="op">=</span> <span class="ident">field</span>;
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}
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<span class="ident">EnumProj</span>::<span class="ident">Unit</span> <span class="op">=</span><span class="op">></span> {}
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}
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}
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}</pre></div>
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<p>If you want to call the <code>project()</code> method multiple times or later use the
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original <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a> type, it needs to use <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html#method.as_mut"><code>.as_mut()</code></a> to avoid
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consuming the <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a>.</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">std</span>::<span class="ident">pin</span>::<span class="ident">Pin</span>;
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<span class="kw">use</span> <span class="ident">pin_project_lite</span>::<span class="ident">pin_project</span>;
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<span class="macro">pin_project</span><span class="macro">!</span> {
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<span class="kw">struct</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="attribute">#[<span class="ident">pin</span>]</span>
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<span class="ident">field</span>: <span class="ident">T</span>,
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}
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">call_project_twice</span>(<span class="kw-2">mut</span> <span class="self">self</span>: <span class="ident">Pin</span><span class="op"><</span><span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span><span class="op">></span>) {
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<span class="comment">// `project` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.</span>
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<span class="self">self</span>.<span class="ident">as_mut</span>().<span class="ident">project</span>();
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<span class="self">self</span>.<span class="ident">as_mut</span>().<span class="ident">project</span>();
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}
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}</pre></div>
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<h1 id="unpin" class="section-header"><a href="#unpin"><code>!Unpin</code></a></h1>
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<p>If you want to ensure that <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> is not implemented, use <code>#[pin]</code>
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attribute for a <a href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomPinned.html"><code>PhantomPinned</code></a> field.</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">std</span>::<span class="ident">marker</span>::<span class="ident">PhantomPinned</span>;
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<span class="kw">use</span> <span class="ident">pin_project_lite</span>::<span class="ident">pin_project</span>;
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<span class="macro">pin_project</span><span class="macro">!</span> {
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<span class="kw">struct</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">field</span>: <span class="ident">T</span>,
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<span class="attribute">#[<span class="ident">pin</span>]</span> <span class="comment">// <------ This `#[pin]` is required to make `Struct` to `!Unpin`.</span>
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<span class="ident">_pin</span>: <span class="ident">PhantomPinned</span>,
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}
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}</pre></div>
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<p>Note that using <a href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomPinned.html"><code>PhantomPinned</code></a> without <code>#[pin]</code> attribute has no effect.</p>
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