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70 lines
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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="Analyzes whether a type is [`FromZeros`]."><title>FromZeros 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="#">FromZeros</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="#">From<wbr>Zeros</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#analysis" title="Analysis">Analysis</a><ul><li><a href="#why-isnt-an-explicit-representation-required-for-structs" title="Why isn’t an explicit representation required for structs?">Why isn’t an explicit representation required for structs?</a></li></ul></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">From<wbr>Zeros</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(FromZeros)]
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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>Analyzes whether a type is <a href="trait.FromZeros.html" title="trait zerocopy::FromZeros"><code>FromZeros</code></a>.</p>
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<p>This derive analyzes, at compile time, whether the annotated type satisfies
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the <a href="trait.FromZeros.html#safety" title="trait zerocopy::FromZeros">safety conditions</a> of <code>FromZeros</code> and implements <code>FromZeros</code> and its
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supertraits if it is sound to do so. This derive can be applied to structs,
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enums, and unions; e.g.:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(FromZeros)]
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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(FromZeros)]
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#[repr(u8)]
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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(FromZeros, Immutable)]
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</span><span class="kw">union </span>MyUnion {
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...
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}</code></pre></div><h2 id="analysis"><a class="doc-anchor" href="#analysis">§</a>Analysis</h2>
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<p><em>This section describes, roughly, the analysis performed by this derive to
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determine whether it is sound to implement <code>FromZeros</code> for a given type.
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Unless you are modifying the implementation of this derive, or attempting to
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manually implement <code>FromZeros</code> for a type yourself, you don’t need to read
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this section.</em></p>
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<p>If a type has the following properties, then this derive can implement
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<code>FromZeros</code> for that type:</p>
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<ul>
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<li>If the type is a struct, all of its fields must be <code>FromZeros</code>.</li>
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<li>If the type is an enum:
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<ul>
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<li>It must have a defined representation (<code>repr</code>s <code>C</code>, <code>u8</code>, <code>u16</code>, <code>u32</code>,
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<code>u64</code>, <code>usize</code>, <code>i8</code>, <code>i16</code>, <code>i32</code>, <code>i64</code>, or <code>isize</code>).</li>
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<li>It must have a variant with a discriminant/tag of <code>0</code>, and its fields
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must be <code>FromZeros</code>. See <a href="https://doc.rust-lang.org/reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations">the reference</a> for a description of
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discriminant values are specified.</li>
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<li>The fields of that variant must be <code>FromZeros</code>.</li>
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</ul>
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</li>
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</ul>
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<p>This analysis is subject to change. Unsafe code may <em>only</em> rely on the
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documented <a href="trait.FromZeros.html#safety" title="trait zerocopy::FromZeros">safety conditions</a> of <code>FromZeros</code>, and must <em>not</em> rely on the
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implementation details of this derive.</p>
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<h3 id="why-isnt-an-explicit-representation-required-for-structs"><a class="doc-anchor" href="#why-isnt-an-explicit-representation-required-for-structs">§</a>Why isn’t an explicit representation required for structs?</h3>
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<p>Neither this derive, nor the <a href="trait.FromZeros.html#safety" title="trait zerocopy::FromZeros">safety conditions</a> of <code>FromZeros</code>, requires
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that structs are marked with <code>#[repr(C)]</code>.</p>
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<p>Per the <a href="https://doc.rust-lang.org/1.93.1/core/primitive.reference.html" title="primitive reference">Rust reference</a>,</p>
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<blockquote>
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<p>The representation of a type can change the padding between fields, but
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does not change the layout of the fields themselves.</p>
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</blockquote>
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<p>Since the layout of structs only consists of padding bytes and field bytes,
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a struct is soundly <code>FromZeros</code> if:</p>
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<ol>
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<li>its padding is soundly <code>FromZeros</code>, and</li>
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<li>its fields are soundly <code>FromZeros</code>.</li>
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</ol>
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<p>The answer to the first question is always yes: padding bytes do not have
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any validity constraints. A <a href="https://github.com/rust-lang/unsafe-code-guidelines/issues/174">discussion</a> of this question in the Unsafe Code
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Guidelines Working Group concluded that it would be virtually unimaginable
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for future versions of rustc to add validity constraints to padding bytes.</p>
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<p>Whether a struct is soundly <code>FromZeros</code> therefore solely depends on whether
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its fields are <code>FromZeros</code>.</p>
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</div></details></section></div></main></body></html> |