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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="Return early with an error if a condition is not satisfied."><title>ensure in anyhow - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.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-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-dd39b87e5fcfba68.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="anyhow" data-themes="" data-resource-suffix="" data-rustdoc-version="1.80.0 (051478957 2024-07-21)" data-channel="1.80.0" data-search-js="search-d52510db62a78183.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-20a3ad099b048cf2.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-df360f571f6edeae.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc macro"><!--[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></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../anyhow/index.html">anyhow</a><span class="version">1.0.86</span></h2></div><div class="sidebar-elems"></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"><h1>Macro <a href="index.html">anyhow</a>::<wbr><a class="macro" href="#">ensure</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/anyhow/macros.rs.html#127-150">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules!</span> ensure {
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(<span class="macro-nonterminal">$cond</span>:expr $(,)<span class="question-mark">?</span>) =&gt; { ... };
(<span class="macro-nonterminal">$cond</span>:expr, <span class="macro-nonterminal">$msg</span>:literal $(,)<span class="question-mark">?</span>) =&gt; { ... };
(<span class="macro-nonterminal">$cond</span>:expr, <span class="macro-nonterminal">$err</span>:expr $(,)<span class="question-mark">?</span>) =&gt; { ... };
(<span class="macro-nonterminal">$cond</span>:expr, <span class="macro-nonterminal">$fmt</span>:expr, $(<span class="macro-nonterminal">$arg</span>:tt)<span class="kw-2">*</span>) =&gt; { ... };
}</pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Return early with an error if a condition is not satisfied.</p>
<p>This macro is equivalent to <code>if !$cond { return Err(</code><a href="macro.anyhow.html" title="macro anyhow::anyhow"><code>anyhow!($args...)</code></a><code>); }</code>.</p>
<p>The surrounding function’s or closure’s return value is required to be
<code>Result&lt;_,</code><a href="struct.Error.html" title="struct anyhow::Error"><code>anyhow::Error</code></a><code>&gt;</code>.</p>
<p>Analogously to <code>assert!</code>, <code>ensure!</code> takes a condition and exits the function
if the condition fails. Unlike <code>assert!</code>, <code>ensure!</code> returns an <code>Error</code>
rather than panicking.</p>
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<h2 id="example"><a class="doc-anchor" href="#example">§</a>Example</h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">ensure!</span>(user == <span class="number">0</span>, <span class="string">"only user 0 is allowed"</span>);</code></pre></div>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
</span><span class="kw">enum </span>ScienceError {
<span class="attr">#[error(<span class="string">"recursion limit exceeded"</span>)]
</span>RecursionLimitExceeded,
...
}
<span class="macro">ensure!</span>(depth &lt;= MAX_DEPTH, ScienceError::RecursionLimitExceeded);</code></pre></div>
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