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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` attr in crate `pin_project`."><meta name="keywords" content="rust, rustlang, rust-lang, pin_project"><title>pin_project::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 attr"><!--[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/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</a></p><script>window.sidebarCurrent = {name: "pin_project", ty: "attr", 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><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="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_internal/lib.rs.html#486" title="goto source code">[src]</a></span><span class="in-band">Attribute Macro <a href="index.html">pin_project</a>::<wbr><a class="attr" href="">pin_project</a></span></h1><pre class="rust attr">#[pin_project]</pre><div class="docblock"><p>An attribute that creates projection types covering all the fields of
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struct or enum.</p>
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<p>This attribute creates projection types according to the following rules:</p>
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<ul>
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<li>For the fields that use <code>#[pin]</code> attribute, create the pinned reference to
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the field.</li>
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<li>For the other fields, create a normal 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 argument with the same name as the method to the attribute,
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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="attribute">#[<span class="ident">pin_project</span>(<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">T</span>),
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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">x</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">x</span>;
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}
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}
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}
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}</pre></div>
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<p>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>This attribute is completely safe. In the absence of other <code>unsafe</code> code
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<em>that you write</em>, it is impossible to cause <a href="https://doc.rust-lang.org/reference/behavior-considered-undefined.html">undefined
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behavior</a> with this attribute.</p>
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<p>This is accomplished by enforcing the four requirements for pin projection
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stated in <a href="https://doc.rust-lang.org/nightly/core/pin/index.html#projections-and-structural-pinning">the Rust documentation</a>:</p>
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<ol>
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<li>
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<p>The struct must only be <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> if all the structural fields are
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<a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a>.</p>
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<p>To enforce this, this attribute will automatically generate an <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a>
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implementation for you, which will require that all structurally pinned
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fields be <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a>.</p>
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<p>If you attempt to provide an <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> impl, the blanket impl will then
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apply to your type, causing a compile-time error due to the conflict with
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the second impl.</p>
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<p>If you wish to provide a manual <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> impl, you can do so via the
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<a href="../pin_project/attr.pin_project.html#unsafeunpin"><code>UnsafeUnpin</code></a> argument.</p>
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</li>
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<li>
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<p>The destructor of the struct must not move structural fields out of its
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argument.</p>
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<p>To enforce this, this attribute will generate code like this:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">struct</span> <span class="ident">MyStruct</span> {}
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<span class="kw">trait</span> <span class="ident">MyStructMustNotImplDrop</span> {}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Drop</span><span class="op">></span> <span class="ident">MyStructMustNotImplDrop</span> <span class="kw">for</span> <span class="ident">T</span> {}
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<span class="kw">impl</span> <span class="ident">MyStructMustNotImplDrop</span> <span class="kw">for</span> <span class="ident">MyStruct</span> {}</pre></div>
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<p>If you attempt to provide an <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl, the blanket impl will then
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apply to your type, causing a compile-time error due to the conflict with
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the second impl.</p>
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<p>If you wish to provide a custom <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl, you can annotate an impl
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with <a href="../pin_project/attr.pin_project.html#pinned_drop"><code>#[pinned_drop]</code></a>. This impl takes a pinned version of
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your struct - that is, <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a><code><&mut MyStruct></code> where <code>MyStruct</code> is the
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type of your struct.</p>
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<p>You can call <code>.project()</code> on this type as usual, along with any other
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methods you have defined. Because your code is never provided with
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a <code>&mut MyStruct</code>, it is impossible to move out of pin-projectable
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fields in safe code in your destructor.</p>
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</li>
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<li>
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<p>You must make sure that you uphold the <a href="https://doc.rust-lang.org/nightly/core/pin/index.html#drop-guarantee"><code>Drop</code>
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guarantee</a>: once your struct is pinned, the memory that
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contains the content is not overwritten or deallocated without calling
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the content's destructors.</p>
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<p>Safe code doesn't need to worry about this - the only way to violate
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this requirement is to manually deallocate memory (which is <code>unsafe</code>),
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or to overwrite a field with something else.
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Because your custom destructor takes <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a><code><&mut MyStruct></code>, it's
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impossible to obtain a mutable reference to a pin-projected field in safe
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code.</p>
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</li>
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<li>
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<p>You must not offer any other operations that could lead to data being
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moved out of the structural fields when your type is pinned.</p>
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<p>As with requirement 3, it is impossible for safe code to violate this.
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This crate ensures that safe code can never obtain a mutable reference to
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<code>#[pin]</code> fields, which prevents you from ever moving out of them in safe
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code.</p>
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</li>
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</ol>
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<p>Pin projections are also incompatible with <a href="https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked"><code>#[repr(packed)]</code></a>
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types. Attempting to use this attribute on a <code>#[repr(packed)]</code> type results
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in a compile-time error.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p><code>#[pin_project]</code> can be used on structs and enums.</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</span>::<span class="ident">pin_project</span>;
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<span class="attribute">#[<span class="ident">pin_project</span>]</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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<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>;
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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>;
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}
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}</pre></div>
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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</span>::<span class="ident">pin_project</span>;
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<span class="attribute">#[<span class="ident">pin_project</span>]</span>
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<span class="kw">struct</span> <span class="ident">TupleStruct</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span>(<span class="attribute">#[<span class="ident">pin</span>]</span> <span class="ident">T</span>, <span class="ident">U</span>);
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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">TupleStruct</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="number">0</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="number">1</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</span>::<span class="ident">pin_project</span>;
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<span class="attribute">#[<span class="ident">pin_project</span>(<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="ident">U</span><span class="op">></span> {
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<span class="ident">Tuple</span>(<span class="attribute">#[<span class="ident">pin</span>]</span> <span class="ident">T</span>),
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<span class="ident">Struct</span> { <span class="ident">field</span>: <span class="ident">U</span> },
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<span class="ident">Unit</span>,
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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">Enum</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">match</span> <span class="self">self</span>.<span class="ident">project</span>() {
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<span class="ident">EnumProj</span>::<span class="ident">Tuple</span>(<span class="ident">x</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">x</span>;
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}
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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="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">U</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>When <code>#[pin_project]</code> is used on enums, only named projection types and
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methods are generated because there is no way to access variants of
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projected types without naming it.
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For example, in the above example, only the <code>project</code> method is generated,
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and the <code>project_ref</code> method is not generated.
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(When <code>#[pin_project]</code> is used on structs, both methods are always generated.)</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">
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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">Enum</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">call_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>) {
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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_ref</span>();
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<span class="comment">//~^ ERROR no method named `project_ref` found for struct `Pin<&Enum<T, U>>` in the current scope</span>
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}
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}</pre></div>
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<p>If you want to call <code>.project()</code> 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</span>::<span class="ident">pin_project</span>;
|
||
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<span class="attribute">#[<span class="ident">pin_project</span>]</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>
|
||
<span class="ident">field</span>: <span class="ident">T</span>,
|
||
}
|
||
|
||
<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> {
|
||
<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>) {
|
||
<span class="comment">// `project` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.</span>
|
||
<span class="self">self</span>.<span class="ident">as_mut</span>().<span class="ident">project</span>();
|
||
<span class="self">self</span>.<span class="ident">as_mut</span>().<span class="ident">project</span>();
|
||
}
|
||
}</pre></div>
|
||
<h1 id="unpin" class="section-header"><a href="#unpin"><code>!Unpin</code></a></h1>
|
||
<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 the <code>!Unpin</code>
|
||
argument to <code>#[pin_project]</code>.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::<span class="ident">pin_project</span>;
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>(<span class="op">!</span><span class="ident">Unpin</span>)]</span>
|
||
<span class="kw">struct</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
|
||
<span class="ident">field</span>: <span class="ident">T</span>,
|
||
}</pre></div>
|
||
<p>This is equivalent to using <code>#[pin]</code> attribute for the <a href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomPinned.html"><code>PhantomPinned</code></a>
|
||
field.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">marker</span>::<span class="ident">PhantomPinned</span>;
|
||
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::<span class="ident">pin_project</span>;
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>]</span>
|
||
<span class="kw">struct</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
|
||
<span class="ident">field</span>: <span class="ident">T</span>,
|
||
<span class="attribute">#[<span class="ident">pin</span>]</span> <span class="comment">// <------ This `#[pin]` is required to make `Struct` to `!Unpin`.</span>
|
||
<span class="ident">_pin</span>: <span class="ident">PhantomPinned</span>,
|
||
}</pre></div>
|
||
<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>
|
||
<h1 id="unsafeunpin" class="section-header"><a href="#unsafeunpin"><code>UnsafeUnpin</code></a></h1>
|
||
<p>If you want to implement <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> manually, you must use the <code>UnsafeUnpin</code>
|
||
argument to <code>#[pin_project]</code>.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::{<span class="ident">pin_project</span>, <span class="ident">UnsafeUnpin</span>};
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>(<span class="ident">UnsafeUnpin</span>)]</span>
|
||
<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> {
|
||
<span class="attribute">#[<span class="ident">pin</span>]</span>
|
||
<span class="ident">pinned</span>: <span class="ident">T</span>,
|
||
<span class="ident">unpinned</span>: <span class="ident">U</span>,
|
||
}
|
||
|
||
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Unpin</span>, <span class="ident">U</span><span class="op">></span> <span class="ident">UnsafeUnpin</span> <span class="kw">for</span> <span class="ident">Struct</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> {}</pre></div>
|
||
<p>Note the usage of the unsafe <a href="https://docs.rs/pin-project/1/pin_project/trait.UnsafeUnpin.html"><code>UnsafeUnpin</code></a> trait, instead of the usual
|
||
<a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a> trait. <a href="https://docs.rs/pin-project/1/pin_project/trait.UnsafeUnpin.html"><code>UnsafeUnpin</code></a> behaves exactly like <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a>, except that
|
||
is unsafe to implement. This unsafety comes from the fact that pin
|
||
projections are being used. If you implement <a href="https://docs.rs/pin-project/1/pin_project/trait.UnsafeUnpin.html"><code>UnsafeUnpin</code></a>, you must
|
||
ensure that it is only implemented when all pin-projected fields implement
|
||
<a href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="Unpin"><code>Unpin</code></a>.</p>
|
||
<p>See <a href="https://docs.rs/pin-project/1/pin_project/trait.UnsafeUnpin.html"><code>UnsafeUnpin</code></a> trait for more details.</p>
|
||
<h1 id="pinned_drop" class="section-header"><a href="#pinned_drop"><code>#[pinned_drop]</code></a></h1>
|
||
<p>In order to correctly implement pin projections, a type's <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl must
|
||
not move out of any structurally pinned fields. Unfortunately,
|
||
<a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html#tymethod.drop" title="Drop::drop"><code>Drop::drop</code></a> takes <code>&mut Self</code>, not <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a><code><&mut Self></code>.</p>
|
||
<p>To ensure that this requirement is upheld, the <code>#[pin_project]</code> attribute
|
||
will provide a <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl for you. This <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl will delegate to
|
||
an impl block annotated with <code>#[pinned_drop]</code> if you use the <code>PinnedDrop</code>
|
||
argument to <code>#[pin_project]</code>.</p>
|
||
<p>This impl block acts just like a normal <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> impl,
|
||
except for the following two:</p>
|
||
<ul>
|
||
<li><code>drop</code> method takes <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html"><code>Pin</code></a><code><&mut Self></code></li>
|
||
<li>Name of the trait is <code>PinnedDrop</code>.</li>
|
||
</ul>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">PinnedDrop</span> {
|
||
<span class="kw">fn</span> <span class="ident">drop</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>);
|
||
}</pre></div>
|
||
<p><code>#[pin_project]</code> implements the actual <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> trait via <code>PinnedDrop</code> you
|
||
implemented. To drop a type that implements <code>PinnedDrop</code>, use the <a href="https://doc.rust-lang.org/nightly/core/mem/fn.drop.html" title="drop"><code>drop</code></a>
|
||
function just like dropping a type that directly implements <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a>.</p>
|
||
<p>In particular, it will never be called more than once, just like
|
||
<a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html#tymethod.drop" title="Drop::drop"><code>Drop::drop</code></a>.</p>
|
||
<p>For example:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">fmt</span>::<span class="ident">Debug</span>, <span class="ident">pin</span>::<span class="ident">Pin</span>};
|
||
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::{<span class="ident">pin_project</span>, <span class="ident">pinned_drop</span>};
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>(<span class="ident">PinnedDrop</span>)]</span>
|
||
<span class="kw">struct</span> <span class="ident">PrintOnDrop</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Debug</span>, <span class="ident">U</span>: <span class="ident">Debug</span><span class="op">></span> {
|
||
<span class="attribute">#[<span class="ident">pin</span>]</span>
|
||
<span class="ident">pinned_field</span>: <span class="ident">T</span>,
|
||
<span class="ident">unpin_field</span>: <span class="ident">U</span>,
|
||
}
|
||
|
||
<span class="attribute">#[<span class="ident">pinned_drop</span>]</span>
|
||
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Debug</span>, <span class="ident">U</span>: <span class="ident">Debug</span><span class="op">></span> <span class="ident">PinnedDrop</span> <span class="kw">for</span> <span class="ident">PrintOnDrop</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> {
|
||
<span class="kw">fn</span> <span class="ident">drop</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="macro">println</span><span class="macro">!</span>(<span class="string">"Dropping pinned field: {:?}"</span>, <span class="self">self</span>.<span class="ident">pinned_field</span>);
|
||
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Dropping unpin field: {:?}"</span>, <span class="self">self</span>.<span class="ident">unpin_field</span>);
|
||
}
|
||
}
|
||
|
||
<span class="kw">fn</span> <span class="ident">main</span>() {
|
||
<span class="kw">let</span> <span class="ident">_x</span> <span class="op">=</span> <span class="ident">PrintOnDrop</span> { <span class="ident">pinned_field</span>: <span class="bool-val">true</span>, <span class="ident">unpin_field</span>: <span class="number">40</span> };
|
||
}</pre></div>
|
||
<p>See also <a href="../pin_project/attr.pinned_drop.html" title="pinned_drop"><code>#[pinned_drop]</code></a> attribute.</p>
|
||
<h1 id="project_replace-method" class="section-header"><a href="#project_replace-method"><code>project_replace</code> method</a></h1>
|
||
<p>In addition to the <code>project</code> and <code>project_ref</code> methods which are always
|
||
provided when you use the <code>#[pin_project]</code> attribute, there is a third
|
||
method, <code>project_replace</code> which can be useful in some situations. It is
|
||
equivalent to <a href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html#method.set"><code>Pin::set</code></a>, except that the unpinned fields are moved and
|
||
returned, instead of being dropped in-place.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<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">other</span>: <span class="self">Self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">ProjectionOwned</span>;</pre></div>
|
||
<p>The <code>ProjectionOwned</code> type is identical to the <code>Self</code> type, except that
|
||
all pinned fields have been replaced by equivalent <a href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomData.html"><code>PhantomData</code></a> types.</p>
|
||
<p>This method is opt-in, because it is only supported for <a href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="Sized"><code>Sized</code></a> types, and
|
||
because it is incompatible with the <a href="../pin_project/attr.pin_project.html#pinned_drop"><code>#[pinned_drop]</code></a>
|
||
attribute described above. It can be enabled by using
|
||
<code>#[pin_project(project_replace)]</code>.</p>
|
||
<p>For example:</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">marker</span>::<span class="ident">PhantomData</span>, <span class="ident">pin</span>::<span class="ident">Pin</span>};
|
||
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::<span class="ident">pin_project</span>;
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>(<span class="ident">project_replace</span>)]</span>
|
||
<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> {
|
||
<span class="attribute">#[<span class="ident">pin</span>]</span>
|
||
<span class="ident">pinned_field</span>: <span class="ident">T</span>,
|
||
<span class="ident">unpinned_field</span>: <span class="ident">U</span>,
|
||
}
|
||
|
||
<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> {
|
||
<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>, <span class="ident">other</span>: <span class="self">Self</span>) {
|
||
<span class="kw">let</span> <span class="ident">this</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">project_replace</span>(<span class="ident">other</span>);
|
||
<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">U</span> <span class="op">=</span> <span class="ident">this</span>.<span class="ident">unpinned_field</span>;
|
||
<span class="kw">let</span> <span class="kw">_</span>: <span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="op">=</span> <span class="ident">this</span>.<span class="ident">pinned_field</span>;
|
||
}
|
||
}</pre></div>
|
||
<p>By passing the value to the <code>project_replace</code> argument, you can name the
|
||
returned type of the <code>project_replace</code> method. This is necessary whenever
|
||
destructuring the return type of the <code>project_replace</code> method, and work in exactly
|
||
the same way as the <code>project</code> and <code>project_ref</code> arguments.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">pin_project</span>::<span class="ident">pin_project</span>;
|
||
|
||
<span class="attribute">#[<span class="ident">pin_project</span>(<span class="ident">project_replace</span> <span class="op">=</span> <span class="ident">EnumProjOwn</span>)]</span>
|
||
<span class="kw">enum</span> <span class="ident">Enum</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> {
|
||
<span class="ident">A</span> {
|
||
<span class="attribute">#[<span class="ident">pin</span>]</span>
|
||
<span class="ident">pinned_field</span>: <span class="ident">T</span>,
|
||
<span class="ident">unpinned_field</span>: <span class="ident">U</span>,
|
||
},
|
||
<span class="ident">B</span>,
|
||
}
|
||
|
||
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">pin</span>(<span class="ident">Enum</span>::<span class="ident">A</span> { <span class="ident">pinned_field</span>: <span class="number">42</span>, <span class="ident">unpinned_field</span>: <span class="string">"hello"</span> });
|
||
|
||
<span class="kw">match</span> <span class="ident">x</span>.<span class="ident">as_mut</span>().<span class="ident">project_replace</span>(<span class="ident">Enum</span>::<span class="ident">B</span>) {
|
||
<span class="ident">EnumProjOwn</span>::<span class="ident">A</span> { <span class="ident">unpinned_field</span>, .. } <span class="op">=</span><span class="op">></span> <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">unpinned_field</span>, <span class="string">"hello"</span>),
|
||
<span class="ident">EnumProjOwn</span>::<span class="ident">B</span> <span class="op">=</span><span class="op">></span> <span class="macro">unreachable</span><span class="macro">!</span>(),
|
||
}</pre></div>
|
||
</div></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../";window.currentCrate = "pin_project";</script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html> |