<!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="Source to the Rust file `libcore/hint.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>hint.rs.html -- source</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 source"><!--[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></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"><pre class="line-numbers"><span id="1"> 1</span> <span id="2"> 2</span> <span id="3"> 3</span> <span id="4"> 4</span> <span id="5"> 5</span> <span id="6"> 6</span> <span id="7"> 7</span> <span id="8"> 8</span> <span id="9"> 9</span> <span id="10">10</span> <span id="11">11</span> <span id="12">12</span> <span id="13">13</span> <span id="14">14</span> <span id="15">15</span> <span id="16">16</span> <span id="17">17</span> <span id="18">18</span> <span id="19">19</span> <span id="20">20</span> <span id="21">21</span> <span id="22">22</span> <span id="23">23</span> <span id="24">24</span> <span id="25">25</span> <span id="26">26</span> <span id="27">27</span> <span id="28">28</span> <span id="29">29</span> <span id="30">30</span> <span id="31">31</span> <span id="32">32</span> <span id="33">33</span> <span id="34">34</span> <span id="35">35</span> <span id="36">36</span> <span id="37">37</span> <span id="38">38</span> <span id="39">39</span> <span id="40">40</span> <span id="41">41</span> <span id="42">42</span> <span id="43">43</span> <span id="44">44</span> <span id="45">45</span> <span id="46">46</span> <span id="47">47</span> <span id="48">48</span> <span id="49">49</span> <span id="50">50</span> <span id="51">51</span> <span id="52">52</span> <span id="53">53</span> <span id="54">54</span> <span id="55">55</span> <span id="56">56</span> <span id="57">57</span> <span id="58">58</span> <span id="59">59</span> <span id="60">60</span> <span id="61">61</span> </pre><pre class="rust "> <span class="comment">// Copyright 2018 The Rust Project Developers. See the COPYRIGHT</span> <span class="comment">// file at the top-level directory of this distribution and at</span> <span class="comment">// http://rust-lang.org/COPYRIGHT.</span> <span class="comment">//</span> <span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span> <span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span> <span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span> <span class="comment">// option. This file may not be copied, modified, or distributed</span> <span class="comment">// except according to those terms.</span> <span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"core_hint"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span> <span class="doccomment">//! Hints to compiler that affects how code should be emitted or optimized.</span> <span class="kw">use</span> <span class="ident">intrinsics</span>; <span class="doccomment">/// Informs the compiler that this point in the code is not reachable, enabling</span> <span class="doccomment">/// further optimizations.</span> <span class="doccomment">///</span> <span class="doccomment">/// # Safety</span> <span class="doccomment">///</span> <span class="doccomment">/// Reaching this function is completely *undefined behavior* (UB). In</span> <span class="doccomment">/// particular, the compiler assumes that all UB must never happen, and</span> <span class="doccomment">/// therefore will eliminate all branches that reach to a call to</span> <span class="doccomment">/// `unreachable_unchecked()`.</span> <span class="doccomment">///</span> <span class="doccomment">/// Like all instances of UB, if this assumption turns out to be wrong, i.e. the</span> <span class="doccomment">/// `unreachable_unchecked()` call is actually reachable among all possible</span> <span class="doccomment">/// control flow, the compiler will apply the wrong optimization strategy, and</span> <span class="doccomment">/// may sometimes even corrupt seemingly unrelated code, causing</span> <span class="doccomment">/// difficult-to-debug problems.</span> <span class="doccomment">///</span> <span class="doccomment">/// Use this function only when you can prove that the code will never call it.</span> <span class="doccomment">///</span> <span class="doccomment">/// The [`unreachable!()`] macro is the safe counterpart of this function, which</span> <span class="doccomment">/// will panic instead when executed.</span> <span class="doccomment">///</span> <span class="doccomment">/// [`unreachable!()`]: ../macro.unreachable.html</span> <span class="doccomment">///</span> <span class="doccomment">/// # Example</span> <span class="doccomment">///</span> <span class="doccomment">/// ```</span> <span class="doccomment">/// fn div_1(a: u32, b: u32) -> u32 {</span> <span class="doccomment">/// use std::hint::unreachable_unchecked;</span> <span class="doccomment">///</span> <span class="doccomment">/// // `b.saturating_add(1)` is always positive (not zero),</span> <span class="doccomment">/// // hence `checked_div` will never return None.</span> <span class="doccomment">/// // Therefore, the else branch is unreachable.</span> <span class="doccomment">/// a.checked_div(b.saturating_add(1))</span> <span class="doccomment">/// .unwrap_or_else(|| unsafe { unreachable_unchecked() })</span> <span class="doccomment">/// }</span> <span class="doccomment">///</span> <span class="doccomment">/// assert_eq!(div_1(7, 0), 7);</span> <span class="doccomment">/// assert_eq!(div_1(9, 1), 4);</span> <span class="doccomment">/// assert_eq!(div_1(11, std::u32::MAX), 0);</span> <span class="doccomment">/// ```</span> <span class="attribute">#[<span class="ident">inline</span>]</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"unreachable"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">unreachable_unchecked</span>() <span class="op">-></span> <span class="op">!</span> { <span class="ident">intrinsics</span>::<span class="ident">unreachable</span>() } </pre> </section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "core";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>