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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="API documentation for the Rust `ensure` macro in crate `anyhow`."><meta name="keywords" content="rust, rustlang, rust-lang, ensure"><title>anyhow::ensure - 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 src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc macro"><!--[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='../anyhow/index.html'><div class='logo-container rust-logo'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>anyhow</a></p><script>window.sidebarCurrent = {name: 'ensure', ty: 'macro', 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"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<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='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/anyhow/macros.rs.html#108-124' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>anyhow</a>::<wbr><a class="macro" href=''>ensure</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">ensure</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">msg</span>:<span class="ident">literal</span> $(,)<span class="question-mark">?</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">err</span>:<span class="ident">expr</span> $(,)<span class="question-mark">?</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">cond</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">arg</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> { ... };
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}</pre></div>
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</div><div class='docblock'><p>Return early with an error if a condition is not satisfied.</p>
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<p>This macro is equivalent to <code>if !$cond { return Err(From::from($err)); }</code>.</p>
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<p>Analogously to <code>assert!</code>, <code>ensure!</code> takes a condition and exits the function
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if the condition fails. Unlike <code>assert!</code>, <code>ensure!</code> returns an <code>Error</code>
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rather than panicking.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">ensure</span><span class="macro">!</span>(<span class="ident">user</span> <span class="op">==</span> <span class="number">0</span>, <span class="string">"only user 0 is allowed"</span>);</pre></div>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Error</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">enum</span> <span class="ident">ScienceError</span> {
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<span class="attribute">#[<span class="ident">error</span>(<span class="string">"recursion limit exceeded"</span>)]</span>
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<span class="ident">RecursionLimitExceeded</span>,
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...
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}
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<span class="macro">ensure</span><span class="macro">!</span>(<span class="ident">depth</span> <span class="op"><=</span> <span class="ident">MAX_DEPTH</span>, <span class="ident">ScienceError</span>::<span class="ident">RecursionLimitExceeded</span>);</pre></div>
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