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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="This function is a best-effort attempt to prevent the compiler from knowing anything about the value of the returned `u8`, other than its type."><title>black_box in subtle - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.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" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../static.files/rustdoc-46132b98.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="subtle" data-themes="" data-resource-suffix="" data-rustdoc-version="1.85.0 (4d91de4e4 2025-02-17)" data-channel="1.85.0" data-search-js="search-75f5ac3e.js" data-settings-js="settings-0f613d39.js" ><script src="../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-044be391.svg"></head><body class="rustdoc fn"><!--[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><a class="logo-container" href="../subtle/index.html"><img src="https://doc.dalek.rs/assets/dalek-logo-clear.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../subtle/index.html"><img src="https://doc.dalek.rs/assets/dalek-logo-clear.png" alt="logo"></a><h2><a href="../subtle/index.html">subtle</a><span class="version">2.6.1</span></h2></div><div class="sidebar-elems"><div id="rustdoc-modnav"></div></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"><span class="rustdoc-breadcrumbs"><a href="index.html">subtle</a></span><h1>Function <span class="fn">black_box</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/subtle/lib.rs.html#224-234">Source</a> </span></div><pre class="rust item-decl"><code>pub(crate) fn black_box<T: <a class="trait" href="https://doc.rust-lang.org/1.85.0/core/marker/trait.Copy.html" title="trait core::marker::Copy">Copy</a>>(input: T) -> T</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>This function is a best-effort attempt to prevent the compiler from knowing
anything about the value of the returned <code>u8</code>, other than its type.</p>
<p>Because we want to support stable Rust, we don’t have access to inline
assembly or test::black_box, so we use the fact that volatile values will
never be elided to register values.</p>
<p>Note: Rust’s notion of “volatile” is subject to change over time. While this
code may break in a non-destructive way in the future, “constant-time” code
is a continually moving target, and this is better than doing nothing.</p>
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