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macro.assert_eq_size.html
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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="Asserts that types are equal in size."><meta name="keywords" content="rust, rustlang, rust-lang, assert_eq_size"><title>static_assertions::assert_eq_size - 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><script src="../crates.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" height="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Macro <a href="index.html">static_assertions</a>::<wbr><a class="macro" href="">assert_eq_size</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/static_assertions/assert_eq_size.rs.html#34-40" title="goto source code">[src]</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">assert_eq_size</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">x</span>:<span class="ident">ty</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">xs</span>:<span class="ident">ty</span>),<span class="op">+</span> $(,)<span class="question-mark">?</span>) <span class="op">=</span><span class="op">></span> { ... };
}</pre></div>
</div><div class="docblock"><p>Asserts that types are equal in size.</p>
<p>When performing operations such as pointer casts or dealing with <a href="https://doc.rust-lang.org/std/primitive.usize.html"><code>usize</code></a>
versus <a href="https://doc.rust-lang.org/std/primitive.u64.html"><code>u64</code></a> versus <a href="https://doc.rust-lang.org/std/primitive.u32.html"><code>u32</code></a>, the size of your types matter. That is where
this macro comes into play.</p>
<h1 id="alternatives" class="section-header"><a href="#alternatives">Alternatives</a></h1>
<p>There also exists <a href="macro.assert_eq_size_val.html"><code>assert_eq_size_val</code></a> and
<a href="macro.assert_eq_size_ptr.html"><code>assert_eq_size_ptr</code></a>. Instead of specifying
types to compare, values’ sizes can be directly compared against each other.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>These three types, despite being very different, all have the same size:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="macro">assert_eq_size</span><span class="macro">!</span>([<span class="ident">u8</span>; <span class="number">4</span>], (<span class="ident">u16</span>, <span class="ident">u16</span>), <span class="ident">u32</span>);</pre></div>
<p>The following example fails to compile because <code>u32</code> has 4 times the size of
<code>u8</code>:</p>
<div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
<span class="macro">assert_eq_size</span><span class="macro">!</span>(<span class="ident">u32</span>, <span class="ident">u8</span>);</pre></div>
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