<!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="API documentation for the Rust `unreachable_unchecked` fn in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, unreachable_unchecked"><title>core::hint::unreachable_unchecked - 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="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>hint</a></p><script>window.sidebarCurrent = {name: 'unreachable_unchecked', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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'>Function <a href='../index.html'>core</a>::<wbr><a href='index.html'>hint</a>::<wbr><a class="fn" href=''>unreachable_unchecked</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.27.0'>1.27.0</span><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/core/hint.rs.html#59-61' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub unsafe fn unreachable_unchecked() -> !</pre><div class='docblock'><p>Informs the compiler that this point in the code is not reachable, enabling
further optimizations.</p>
<h1 id="safety" class="section-header"><a href="#safety">Safety</a></h1>
<p>Reaching this function is completely <em>undefined behavior</em> (UB). In
particular, the compiler assumes that all UB must never happen, and
therefore will eliminate all branches that reach to a call to
<code>unreachable_unchecked()</code>.</p>
<p>Like all instances of UB, if this assumption turns out to be wrong, i.e. the
<code>unreachable_unchecked()</code> call is actually reachable among all possible
control flow, the compiler will apply the wrong optimization strategy, and
may sometimes even corrupt seemingly unrelated code, causing
difficult-to-debug problems.</p>
<p>Use this function only when you can prove that the code will never call it.</p>
<p>The <a href="../macro.unreachable.html"><code>unreachable!()</code></a> macro is the safe counterpart of this function, which
will panic instead when executed.</p>
<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">div_1</span>(<span class="ident">a</span>: <span class="ident">u32</span>, <span class="ident">b</span>: <span class="ident">u32</span>) <span class="op">-></span> <span class="ident">u32</span> {
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">hint</span>::<span class="ident">unreachable_unchecked</span>;
<span class="comment">// `b.saturating_add(1)` is always positive (not zero),</span>
<span class="comment">// hence `checked_div` will never return None.</span>
<span class="comment">// Therefore, the else branch is unreachable.</span>
<span class="ident">a</span>.<span class="ident">checked_div</span>(<span class="ident">b</span>.<span class="ident">saturating_add</span>(<span class="number">1</span>))
.<span class="ident">unwrap_or_else</span>(<span class="op">||</span> <span class="kw">unsafe</span> { <span class="ident">unreachable_unchecked</span>() })
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">div_1</span>(<span class="number">7</span>, <span class="number">0</span>), <span class="number">7</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">div_1</span>(<span class="number">9</span>, <span class="number">1</span>), <span class="number">4</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">div_1</span>(<span class="number">11</span>, <span class="ident">std</span>::<span class="ident">u32</span>::<span class="ident">MAX</span>), <span class="number">0</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20div_1(a%3A%20u32%2C%20b%3A%20u32)%20-%3E%20u32%20%7B%0A%20%20%20%20use%20std%3A%3Ahint%3A%3Aunreachable_unchecked%3B%0A%0A%20%20%20%20%2F%2F%20%60b.saturating_add(1)%60%20is%20always%20positive%20(not%20zero)%2C%0A%20%20%20%20%2F%2F%20hence%20%60checked_div%60%20will%20never%20return%20None.%0A%20%20%20%20%2F%2F%20Therefore%2C%20the%20else%20branch%20is%20unreachable.%0A%20%20%20%20a.checked_div(b.saturating_add(1))%0A%20%20%20%20%20%20%20%20.unwrap_or_else(%7C%7C%20unsafe%20%7B%20unreachable_unchecked()%20%7D)%0A%7D%0A%0Aassert_eq!(div_1(7%2C%200)%2C%207)%3B%0Aassert_eq!(div_1(9%2C%201)%2C%204)%3B%0Aassert_eq!(div_1(11%2C%20std%3A%3Au32%3A%3AMAX)%2C%200)%3B%0A%7D">Run</a></pre>
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