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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="Implements [`KnownLayout`]."><title>KnownLayout in zerocopy - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="zerocopy" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../static.files/storage-e2aeef58.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-044be391.svg"></head><body class="rustdoc derive"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">KnownLayout</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../zerocopy/index.html">zerocopy</a><span class="version">0.8.39</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Known<wbr>Layout</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#limitations" title="Limitations">Limitations</a></li></ul></section><div id="rustdoc-modnav"><h2 class="in-crate"><a href="index.html">In crate zerocopy</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="index.html">zerocopy</a></div><h1>Derive Macro <span class="derive">Known<wbr>Layout</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../src/zerocopy_derive/lib.rs.html#82">Source</a> </span></div><pre class="rust item-decl"><code>#[derive(KnownLayout)]
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{
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<span class="comment">// Attributes available to this derive:</span>
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#[zerocopy]
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}
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</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Implements <a href="trait.KnownLayout.html" title="trait zerocopy::KnownLayout"><code>KnownLayout</code></a>.</p>
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<p>This derive analyzes various aspects of a type’s layout that are needed for
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some of zerocopy’s APIs. It can be applied to structs, enums, and unions;
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e.g.:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">struct </span>MyStruct {
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...
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}
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<span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">enum </span>MyEnum {
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...
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}
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<span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">union </span>MyUnion {
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...
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}</code></pre></div><h2 id="limitations"><a class="doc-anchor" href="#limitations">§</a>Limitations</h2>
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<p>This derive cannot currently be applied to unsized structs without an
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explicit <code>repr</code> attribute.</p>
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<p>Some invocations of this derive run afoul of a <a href="https://github.com/rust-lang/rust/issues/45713">known bug</a> in Rust’s type
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privacy checker. For example, this code:</p>
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<div class="example-wrap compile_fail"><a href="#" class="tooltip" title="This example deliberately fails to compile">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="kw">use </span>zerocopy::<span class="kw-2">*</span>;
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<span class="attr">#[derive(KnownLayout)]
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#[repr(C)]
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</span><span class="kw">pub struct </span>PublicType {
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leading: Foo,
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trailing: Bar,
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}
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<span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">struct </span>Foo;
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<span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">struct </span>Bar;</code></pre></div>
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<p>…results in a compilation error:</p>
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<div class="example-wrap"><pre class="language-text"><code>error[E0446]: private type `Bar` in public interface
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--> examples/bug.rs:3:10
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3 | #[derive(KnownLayout)]
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| ^^^^^^^^^^^ can't leak private type
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...
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14 | struct Bar;
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| ---------- `Bar` declared as private
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= note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)</code></pre></div>
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<p>This issue arises when <code>#[derive(KnownLayout)]</code> is applied to <code>repr(C)</code>
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structs whose trailing field type is less public than the enclosing struct.</p>
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<p>To work around this, mark the trailing field type <code>pub</code> and annotate it with
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<code>#[doc(hidden)]</code>; e.g.:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>zerocopy::<span class="kw-2">*</span>;
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<span class="attr">#[derive(KnownLayout)]
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#[repr(C)]
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</span><span class="kw">pub struct </span>PublicType {
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leading: Foo,
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trailing: Bar,
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}
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<span class="attr">#[derive(KnownLayout)]
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</span><span class="kw">struct </span>Foo;
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<span class="attr">#[doc(hidden)]
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#[derive(KnownLayout)]
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</span><span class="kw">pub struct </span>Bar; <span class="comment">// <- `Bar` is now also `pub`</span></code></pre></div></div></details></section></div></main></body></html> |