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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="github crates-io docs-rs"><title>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="../crates.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 mod crate"><!--[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"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#reexports">Re-exports</a></li><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li><li><a href="#types">Type Aliases</a></li></ul></section></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>Crate <a class="mod" href="#">anyhow</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/lib.rs.html#1-730">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p><a href="https://github.com/dtolnay/anyhow"><img src="https://img.shields.io/badge/github-8da0cb?style=for-the-badge&amp;labelColor=555555&amp;logo=github" alt="github" /></a> <a href="https://crates.io/crates/anyhow"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&amp;labelColor=555555&amp;logo=rust" alt="crates-io" /></a> <a href="https://docs.rs/anyhow"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&amp;labelColor=555555&amp;logo=docs.rs" alt="docs-rs" /></a></p>
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<br>
<p>This library provides <a href="struct.Error.html" title="struct anyhow::Error"><code>anyhow::Error</code></a>, a trait object based error
type for easy idiomatic error handling in Rust applications.</p>
<br>
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<h2 id="details"><a class="doc-anchor" href="#details">§</a>Details</h2>
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<ul>
<li>
<p>Use <code>Result&lt;T, anyhow::Error&gt;</code>, or equivalently <code>anyhow::Result&lt;T&gt;</code>, as
the return type of any fallible function.</p>
<p>Within the function, use <code>?</code> to easily propagate any error that implements
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the <a href="https://doc.rust-lang.org/1.80.0/core/error/trait.Error.html" title="trait core::error::Error"><code>std::error::Error</code></a> trait.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::Result;
<span class="kw">fn </span>get_cluster_info() -&gt; <span class="prelude-ty">Result</span>&lt;ClusterMap&gt; {
<span class="kw">let </span>config = std::fs::read_to_string(<span class="string">"cluster.json"</span>)<span class="question-mark">?</span>;
<span class="kw">let </span>map: ClusterMap = serde_json::from_str(<span class="kw-2">&amp;</span>config)<span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(map)
}</code></pre></div>
</li>
<li>
<p>Attach context to help the person troubleshooting the error understand
where things went wrong. A low-level error like “No such file or
directory” can be annoying to debug without more context about what higher
level step the application was in the middle of.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>};
<span class="kw">fn </span>main() -&gt; <span class="prelude-ty">Result</span>&lt;()&gt; {
...
it.detach().context(<span class="string">"Failed to detach the important thing"</span>)<span class="question-mark">?</span>;
<span class="kw">let </span>content = std::fs::read(path)
.with_context(|| <span class="macro">format!</span>(<span class="string">"Failed to read instrs from {}"</span>, path))<span class="question-mark">?</span>;
...
}</code></pre></div>
<div class="example-wrap"><pre class="language-console"><code>Error: Failed to read instrs from ./path/to/instrs.json
Caused by:
No such file or directory (os error 2)
</code></pre></div></li>
<li>
<p>Downcasting is supported and can be by value, by shared reference, or by
mutable reference as needed.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// If the error was caused by redaction, then return a
// tombstone instead of the content.
</span><span class="kw">match </span>root_cause.downcast_ref::&lt;DataStoreError&gt;() {
<span class="prelude-val">Some</span>(DataStoreError::Censored(<span class="kw">_</span>)) =&gt; <span class="prelude-val">Ok</span>(Poll::Ready(REDACTED_CONTENT)),
<span class="prelude-val">None </span>=&gt; <span class="prelude-val">Err</span>(error),
}</code></pre></div>
</li>
<li>
<p>If using Rust ≥ 1.65, a backtrace is captured and printed with the
error if the underlying error type does not already provide its own. In
order to see backtraces, they must be enabled through the environment
variables described in <a href="https://doc.rust-lang.org/std/backtrace/index.html#environment-variables"><code>std::backtrace</code></a>:</p>
<ul>
<li>If you want panics and errors to both have backtraces, set
<code>RUST_BACKTRACE=1</code>;</li>
<li>If you want only errors to have backtraces, set <code>RUST_LIB_BACKTRACE=1</code>;</li>
<li>If you want only panics to have backtraces, set <code>RUST_BACKTRACE=1</code> and
<code>RUST_LIB_BACKTRACE=0</code>.</li>
</ul>
</li>
<li>
<p>Anyhow works with any error type that has an impl of <code>std::error::Error</code>,
including ones defined in your crate. We do not bundle a <code>derive(Error)</code>
macro but you can write the impls yourself or use a standalone macro like
<a href="https://github.com/dtolnay/thiserror">thiserror</a>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>thiserror::Error;
<span class="attr">#[derive(Error, Debug)]
</span><span class="kw">pub enum </span>FormatError {
<span class="attr">#[error(<span class="string">"Invalid header (expected {expected:?}, got {found:?})"</span>)]
</span>InvalidHeader {
expected: String,
found: String,
},
<span class="attr">#[error(<span class="string">"Missing attribute: {0}"</span>)]
</span>MissingAttribute(String),
}</code></pre></div>
</li>
<li>
<p>One-off error messages can be constructed using the <code>anyhow!</code> macro, which
supports string interpolation and produces an <code>anyhow::Error</code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">return </span><span class="prelude-val">Err</span>(<span class="macro">anyhow!</span>(<span class="string">"Missing attribute: {}"</span>, missing));</code></pre></div>
<p>A <code>bail!</code> macro is provided as a shorthand for the same early return.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">bail!</span>(<span class="string">"Missing attribute: {}"</span>, missing);</code></pre></div>
</li>
</ul>
<br>
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<h2 id="no-std-support"><a class="doc-anchor" href="#no-std-support">§</a>No-std support</h2>
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<p>In no_std mode, almost all of the same API is available and works the same
way. To depend on Anyhow in no_std mode, disable our default enabled “std”
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feature in Cargo.toml. A global allocator is required.</p>
<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
anyhow = { version = &quot;1.0&quot;, default-features = false }
</code></pre></div>
<p>Since the <code>?</code>-based error conversions would normally rely on the
<code>std::error::Error</code> trait which is only available through std, no_std mode
will require an explicit <code>.map_err(Error::msg)</code> when working with a
non-Anyhow error type inside a function that returns Anyhow’s error type.</p>
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</div></details><h2 id="reexports" class="section-header">Re-exports<a href="#reexports" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name" id="reexport.format_err"><code>pub use <a class="macro" href="macro.anyhow.html" title="macro anyhow::anyhow">anyhow</a> as format_err;</code></div></li></ul><h2 id="macros" class="section-header">Macros<a href="#macros" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="macro" href="macro.anyhow.html" title="macro anyhow::anyhow">anyhow</a></div><div class="desc docblock-short">Construct an ad-hoc error from a string or existing non-<code>anyhow</code> error
value.</div></li><li><div class="item-name"><a class="macro" href="macro.bail.html" title="macro anyhow::bail">bail</a></div><div class="desc docblock-short">Return early with an error.</div></li><li><div class="item-name"><a class="macro" href="macro.ensure.html" title="macro anyhow::ensure">ensure</a></div><div class="desc docblock-short">Return early with an error if a condition is not satisfied.</div></li></ul><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Chain.html" title="struct anyhow::Chain">Chain</a></div><div class="desc docblock-short">Iterator of a chain of source errors.</div></li><li><div class="item-name"><a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a></div><div class="desc docblock-short">The <code>Error</code> type, a wrapper around a dynamic error type.</div></li></ul><h2 id="traits" class="section-header">Traits<a href="#traits" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Context.html" title="trait anyhow::Context">Context</a></div><div class="desc docblock-short">Provides the <code>context</code> method for <code>Result</code>.</div></li></ul><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.Ok.html" title="fn anyhow::Ok">Ok</a></div><div class="desc docblock-short">Equivalent to Ok::&lt;_, anyhow::Error&gt;(value).</div></li></ul><h2 id="types" class="section-header">Type Aliases<a href="#types" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.Result.html" title="type anyhow::Result">Result</a></div><div class="desc docblock-short"><code>Result&lt;T, Error&gt;</code></div></li></ul></section></div></main></body></html>
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