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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="A macro that creates a projection type covering all the fields of struct."><title>pin_project in pin_project_lite - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-492a78a4a87dcc01.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="pin_project_lite" data-themes="" data-resource-suffix="" data-rustdoc-version="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-3b12f09e550e0385.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc macro"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../pin_project_lite/index.html">pin_<wbr>project_<wbr>lite</a><span class="version">0.2.14</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Macro <a href="index.html">pin_project_lite</a>::<wbr><a class="macro" href="#">pin_project</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/pin_project_lite/lib.rs.html#383-390">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules!</span> pin_project {
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($(<span class="macro-nonterminal">$tt</span>:tt)<span class="kw-2">*</span>) => { ... };
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}</pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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"><code><span class="kw">fn </span>project(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) -> Projection<<span class="lifetime">'_</span>>;
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<span class="kw">fn </span>project_ref(<span class="self">self</span>: Pin<<span class="kw-2">&</span><span class="self">Self</span>>) -> ProjectionRef<<span class="lifetime">'_</span>>;</code></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"><code><span class="macro">pin_project!</span> {
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<span class="attr">#[project = EnumProj]
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</span><span class="kw">enum </span>Enum<T> {
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Variant { <span class="attr">#[pin] </span>field: T },
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}
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}
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<span class="kw">impl</span><T> Enum<T> {
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<span class="kw">fn </span>method(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) {
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<span class="kw">let </span>this: EnumProj<<span class="lifetime">'_</span>, T> = <span class="self">self</span>.project();
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<span class="kw">match </span>this {
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EnumProj::Variant { field } => {
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<span class="kw">let _</span>: Pin<<span class="kw-2">&mut </span>T> = field;
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}
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}
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}
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}</code></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"><code><span class="kw">fn </span>project_replace(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>, replacement: <span class="self">Self</span>) -> MyProjReplace;</code></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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<div class="example-wrap"><pre class="language-text"><code>let this: EnumProj<'_, T> = self.project();
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^^
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</code></pre></div>
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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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<h2 id="safety"><a class="doc-anchor" href="#safety">§</a>Safety</h2>
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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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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::pin::Pin;
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<span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="kw">struct </span>Struct<T, U> {
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<span class="attr">#[pin]
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</span>pinned: T,
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unpinned: U,
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}
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}
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<span class="kw">impl</span><T, U> Struct<T, U> {
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<span class="kw">fn </span>method(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) {
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<span class="kw">let </span>this = <span class="self">self</span>.project();
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<span class="kw">let _</span>: Pin<<span class="kw-2">&mut </span>T> = this.pinned; <span class="comment">// Pinned reference to the field
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</span><span class="kw">let _</span>: <span class="kw-2">&mut </span>U = this.unpinned; <span class="comment">// Normal reference to the field
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</span>}
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}</code></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"><code><span class="kw">use </span>std::pin::Pin;
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<span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="attr">#[project = EnumProj]
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</span><span class="kw">enum </span>Enum<T> {
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Struct {
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<span class="attr">#[pin]
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</span>field: T,
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},
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Unit,
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}
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}
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<span class="kw">impl</span><T> Enum<T> {
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<span class="kw">fn </span>method(<span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) {
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<span class="kw">match </span><span class="self">self</span>.project() {
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EnumProj::Struct { field } => {
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<span class="kw">let _</span>: Pin<<span class="kw-2">&mut </span>T> = field;
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}
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EnumProj::Unit => {}
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}
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}
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}</code></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/1.82.0/core/pin/struct.Pin.html" title="struct core::pin::Pin"><code>Pin</code></a> type, it needs to use <a href="https://doc.rust-lang.org/1.82.0/core/pin/struct.Pin.html#method.as_mut" title="method core::pin::Pin::as_mut"><code>.as_mut()</code></a> to avoid
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consuming the <a href="https://doc.rust-lang.org/1.82.0/core/pin/struct.Pin.html" title="struct core::pin::Pin"><code>Pin</code></a>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::pin::Pin;
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<span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="kw">struct </span>Struct<T> {
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<span class="attr">#[pin]
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</span>field: T,
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}
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}
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<span class="kw">impl</span><T> Struct<T> {
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<span class="kw">fn </span>call_project_twice(<span class="kw-2">mut </span><span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) {
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<span class="comment">// `project` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
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</span><span class="self">self</span>.as_mut().project();
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<span class="self">self</span>.as_mut().project();
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}
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}</code></pre></div>
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<h2 id="unpin"><a class="doc-anchor" href="#unpin">§</a><code>!Unpin</code></h2>
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<p>If you want to make sure <code>Unpin</code> is not implemented, use the <code>#[project(!Unpin)]</code>
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attribute.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="attr">#[project(!Unpin)]
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</span><span class="kw">struct </span>Struct<T> {
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<span class="attr">#[pin]
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</span>field: T,
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}
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}</code></pre></div>
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<p>This is equivalent to using <code>#[pin]</code> attribute for a <a href="https://doc.rust-lang.org/1.82.0/core/marker/struct.PhantomPinned.html" title="struct core::marker::PhantomPinned"><code>PhantomPinned</code></a> field.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::marker::PhantomPinned;
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<span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="kw">struct </span>Struct<T> {
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field: T,
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<span class="attr">#[pin]
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</span>_pin: PhantomPinned,
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}
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}</code></pre></div>
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<p>Note that using <a href="https://doc.rust-lang.org/1.82.0/core/marker/struct.PhantomPinned.html" title="struct core::marker::PhantomPinned"><code>PhantomPinned</code></a> without <code>#[pin]</code> or <code>#[project(!Unpin)]</code>
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attribute has no effect.</p>
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<h2 id="pinned-drop"><a class="doc-anchor" href="#pinned-drop">§</a>Pinned Drop</h2>
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<p>In order to correctly implement pin projections, a type’s <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html" title="trait core::ops::drop::Drop"><code>Drop</code></a> impl must not move out of any
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structurally pinned fields. Unfortunately, <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html#tymethod.drop" title="method core::ops::drop::Drop::drop"><code>Drop::drop</code></a> takes <code>&mut Self</code>, not <code>Pin<&mut Self></code>.</p>
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<p>To implement <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html" title="trait core::ops::drop::Drop"><code>Drop</code></a> for type that has pin, add an <code>impl PinnedDrop</code> block at the end of the
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<a href="macro.pin_project.html" title="macro pin_project_lite::pin_project"><code>pin_project</code></a> macro block. PinnedDrop has the following interface:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">trait </span>PinnedDrop {
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<span class="kw">fn </span>drop(this: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>);
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}</code></pre></div>
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<p>Note that the argument to <code>PinnedDrop::drop</code> cannot be named <code>self</code>.</p>
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<p><code>pin_project!</code> implements the actual <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html" title="trait core::ops::drop::Drop"><code>Drop</code></a> trait via PinnedDrop you implemented. To
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explicitly drop a type that implements PinnedDrop, use the <a href="https://doc.rust-lang.org/1.82.0/core/mem/fn.drop.html" title="fn core::mem::drop">drop</a> function just like dropping a
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type that directly implements <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html" title="trait core::ops::drop::Drop"><code>Drop</code></a>.</p>
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<p><code>PinnedDrop::drop</code> will never be called more than once, just like <a href="https://doc.rust-lang.org/1.82.0/core/ops/drop/trait.Drop.html#tymethod.drop" title="method core::ops::drop::Drop::drop"><code>Drop::drop</code></a>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>pin_project_lite::pin_project;
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<span class="macro">pin_project!</span> {
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<span class="kw">pub struct </span>Struct<<span class="lifetime">'a</span>> {
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was_dropped: <span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>bool,
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<span class="attr">#[pin]
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</span>field: u8,
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}
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<span class="kw">impl </span>PinnedDrop <span class="kw">for </span>Struct<<span class="lifetime">'_</span>> {
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<span class="kw">fn </span>drop(this: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) { <span class="comment">// <----- NOTE: this is not `self`
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</span><span class="kw-2">**</span>this.project().was_dropped = <span class="bool-val">true</span>;
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}
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}
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}
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<span class="kw">let </span><span class="kw-2">mut </span>was_dropped = <span class="bool-val">false</span>;
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drop(Struct { was_dropped: <span class="kw-2">&mut </span>was_dropped, field: <span class="number">42 </span>});
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<span class="macro">assert!</span>(was_dropped);</code></pre></div>
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</div></details></section></div></main></body></html>
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