Uttarayan Mondal
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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="An attribute used for custom implementations of [`Drop`]."><meta name="keywords" content="rust, rustlang, rust-lang, pinned_drop"><title>pin_project_internal::pinned_drop - 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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<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">Attribute Macro <a href="index.html">pin_project_internal</a>::<wbr><a class="attr" href="">pinned_drop</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_internal/lib.rs.html#568-571" title="goto source code">[src]</a></span></h1><pre class="rust attr">#[pinned_drop]</pre><div class="docblock"><p>An attribute used for custom implementations of <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a>.</p>
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<p>This attribute is used in conjunction with the <code>PinnedDrop</code> argument to
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the <a href="../pin_project_internal/attr.pin_project.html" title="pin_project"><code>#[pin_project]</code></a> attribute.</p>
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<p>The impl block annotated with this attribute acts just like a normal
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<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>
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<ul>
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<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>
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<li>Name of the trait is <code>PinnedDrop</code>.</li>
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</ul>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">PinnedDrop</span> {
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<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>);
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}</pre></div>
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<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
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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>
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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>
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<p>In particular, it will never be called more than once, just like
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<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>
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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</span>::{<span class="ident">pin_project</span>, <span class="ident">pinned_drop</span>};
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<span class="attribute">#[<span class="ident">pin_project</span>(<span class="ident">PinnedDrop</span>)]</span>
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<span class="kw">struct</span> <span class="ident">PrintOnDrop</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">u8</span>,
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}
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<span class="attribute">#[<span class="ident">pinned_drop</span>]</span>
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<span class="kw">impl</span> <span class="ident">PinnedDrop</span> <span class="kw">for</span> <span class="ident">PrintOnDrop</span> {
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<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>) {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Dropping: {}"</span>, <span class="self">self</span>.<span class="ident">field</span>);
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}
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">_x</span> <span class="op">=</span> <span class="ident">PrintOnDrop</span> { <span class="ident">field</span>: <span class="number">50</span> };
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}</pre></div>
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<p>See also <a href="../pin_project_internal/attr.pin_project.html#pinned_drop">“pinned-drop” section of <code>#[pin_project]</code> attribute</a>.</p>
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<h1 id="why-pinned_drop-attribute-is-needed" class="section-header"><a href="#why-pinned_drop-attribute-is-needed">Why <code>#[pinned_drop]</code> attribute is needed?</a></h1>
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<p>Implementing <code>PinnedDrop::drop</code> is safe, but calling it is not safe.
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This is because destructors can be called multiple times in safe code and
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<a href="https://github.com/rust-lang/rust/pull/62360">double dropping is unsound</a>.</p>
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<p>Ideally, it would be desirable to be able to forbid manual calls in
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the same way as <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html#tymethod.drop" title="Drop::drop"><code>Drop::drop</code></a>, but the library cannot do it. So, by using
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macros and replacing them with private traits like the following,
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this crate prevent users from calling <code>PinnedDrop::drop</code> in safe code.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">PinnedDrop</span> {
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<span class="kw">unsafe</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>);
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}</pre></div>
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<p>This allows implementing <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a> safely using <code>#[pinned_drop]</code>.
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Also by using 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
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implements <a href="https://doc.rust-lang.org/nightly/core/ops/drop/trait.Drop.html" title="Drop"><code>Drop</code></a>, can drop safely a type that implements <code>PinnedDrop</code>.</p>
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