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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="Provides the `context` method for `Result`."><title>Context 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 trait"><!--[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><h2 class="location"><a href="#">Context</a></h2><div class="sidebar-elems"><section><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.context">context</a></li><li><a href="#tymethod.with_context">with_context</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-Context%3CT,+Infallible%3E-for-Option%3CT%3E">Option<T></a></li><li><a href="#impl-Context%3CT,+E%3E-for-Result%3CT,+E%3E">Result<T, E></a></li></ul><h3><a href="#object-safety">Object Safety</a></h3><h3><a href="#implementors">Implementors</a></h3></section><h2><a href="index.html">In crate anyhow</a></h2></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>Trait <a href="index.html">anyhow</a>::<wbr><a class="trait" href="#">Context</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#614-626">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub trait Context<T, E>: Sealed {
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// Required methods
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fn <a href="#tymethod.context" class="fn">context</a><C>(self, context: C) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>>
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<span class="where">where C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static</span>;
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<span class="item-spacer"></span> fn <a href="#tymethod.with_context" class="fn">with_context</a><C, F>(self, f: F) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>>
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<span class="where">where C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,
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F: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C</span>;
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}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Provides the <code>context</code> method for <code>Result</code>.</p>
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<p>This trait is sealed and cannot be implemented for types outside of
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<code>anyhow</code>.</p>
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<br>
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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="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>};
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<span class="kw">use </span>std::fs;
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<span class="kw">use </span>std::path::PathBuf;
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<span class="kw">pub struct </span>ImportantThing {
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path: PathBuf,
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}
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<span class="kw">impl </span>ImportantThing {
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<span class="kw">pub fn </span>detach(<span class="kw-2">&mut </span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><()> {...}
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}
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<span class="kw">pub fn </span>do_it(<span class="kw-2">mut </span>it: ImportantThing) -> <span class="prelude-ty">Result</span><Vec<u8>> {
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it.detach().context(<span class="string">"Failed to detach the important thing"</span>)<span class="question-mark">?</span>;
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<span class="kw">let </span>path = <span class="kw-2">&</span>it.path;
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<span class="kw">let </span>content = fs::read(path)
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.with_context(|| <span class="macro">format!</span>(<span class="string">"Failed to read instrs from {}"</span>, path.display()))<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(content)
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}</code></pre></div>
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<p>When printed, the outermost context would be printed first and the lower
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level underlying causes would be enumerated below.</p>
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<div class="example-wrap"><pre class="language-console"><code>Error: Failed to read instrs from ./path/to/instrs.json
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Caused by:
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No such file or directory (os error 2)
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</code></pre></div>
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<p>Refer to the <a href="struct.Error.html#display-representations" title="struct anyhow::Error">Display representations</a> documentation for other forms in
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which this context chain can be rendered.</p>
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<br>
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<h2 id="effect-on-downcasting"><a class="doc-anchor" href="#effect-on-downcasting">§</a>Effect on downcasting</h2>
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<p>After attaching context of type <code>C</code> onto an error of type <code>E</code>, the resulting
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<code>anyhow::Error</code> may be downcast to <code>C</code> <strong>or</strong> to <code>E</code>.</p>
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<p>That is, in codebases that rely on downcasting, Anyhow’s context supports
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both of the following use cases:</p>
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<ul>
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<li>
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<p><strong>Attaching context whose type is insignificant onto errors whose type
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is used in downcasts.</strong></p>
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<p>In other error libraries whose context is not designed this way, it can
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be risky to introduce context to existing code because new context might
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break existing working downcasts. In Anyhow, any downcast that worked
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before adding context will continue to work after you add a context, so
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you should freely add human-readable context to errors wherever it would
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be helpful.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>};
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<span class="kw">fn </span>do_it() -> <span class="prelude-ty">Result</span><()> {
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helper().context(<span class="string">"Failed to complete the work"</span>)<span class="question-mark">?</span>;
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...
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>err = do_it().unwrap_err();
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<span class="kw">if let </span><span class="prelude-val">Some</span>(e) = err.downcast_ref::<SuspiciousError>() {
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<span class="comment">// If helper() returned SuspiciousError, this downcast will
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// correctly succeed even with the context in between.
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</span>}
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}</code></pre></div>
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</li>
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<li>
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<p><strong>Attaching context whose type is used in downcasts onto errors whose
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type is insignificant.</strong></p>
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<p>Some codebases prefer to use machine-readable context to categorize
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lower level errors in a way that will be actionable to higher levels of
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the application.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>};
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<span class="kw">fn </span>do_it() -> <span class="prelude-ty">Result</span><()> {
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helper().context(HelperFailed)<span class="question-mark">?</span>;
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...
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>err = do_it().unwrap_err();
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<span class="kw">if let </span><span class="prelude-val">Some</span>(e) = err.downcast_ref::<HelperFailed>() {
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<span class="comment">// If helper failed, this downcast will succeed because
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// HelperFailed is the context that has been attached to
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// that error.
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</span>}
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}</code></pre></div>
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</li>
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</ul>
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</div></details><h2 id="required-methods" class="section-header">Required Methods<a href="#required-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="tymethod.context" class="method"><a class="src rightside" href="../src/anyhow/lib.rs.html#616-618">source</a><h4 class="code-header">fn <a href="#tymethod.context" class="fn">context</a><C>(self, context: C) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,</div></h4></section></summary><div class="docblock"><p>Wrap the error value with additional context.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="tymethod.with_context" class="method"><a class="src rightside" href="../src/anyhow/lib.rs.html#622-625">source</a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fn">with_context</a><C, F>(self, f: F) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,
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F: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C,</div></h4></section></summary><div class="docblock"><p>Wrap the error value with additional context that is evaluated lazily
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only once an error does occur.</p>
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</div></details></div><h2 id="object-safety" class="section-header">Object Safety<a href="#object-safety" class="anchor">§</a></h2><div class="object-safety-info">This trait is <b>not</b> <a href="https://doc.rust-lang.org/1.80.0/reference/items/traits.html#object-safety">object safe</a>.</div><h2 id="foreign-impls" class="section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor">§</a></h2><details class="toggle implementors-toggle"><summary><section id="impl-Context%3CT,+Infallible%3E-for-Option%3CT%3E" class="impl"><a class="src rightside" href="../src/anyhow/context.rs.html#90-113">source</a><a href="#impl-Context%3CT,+Infallible%3E-for-Option%3CT%3E" class="anchor">§</a><h3 class="code-header">impl<T> <a class="trait" href="trait.Context.html" title="trait anyhow::Context">Context</a><T, <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/convert/enum.Infallible.html" title="enum core::convert::Infallible">Infallible</a>> for <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a><T></h3></section></summary><div class="docblock">
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>};
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<span class="kw">fn </span>maybe_get() -> <span class="prelude-ty">Option</span><T> {
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...
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}
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<span class="kw">fn </span>demo() -> <span class="prelude-ty">Result</span><()> {
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<span class="kw">let </span>t = maybe_get().context(<span class="string">"there is no T"</span>)<span class="question-mark">?</span>;
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...
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}</code></pre></div>
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</div><div class="impl-items"><section id="method.context" class="method trait-impl"><a class="src rightside" href="../src/anyhow/context.rs.html#91-101">source</a><a href="#method.context" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.context" class="fn">context</a><C>(self, context: C) -> <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,</div></h4></section><section id="method.with_context" class="method trait-impl"><a class="src rightside" href="../src/anyhow/context.rs.html#103-112">source</a><a href="#method.with_context" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fn">with_context</a><C, F>(self, context: F) -> <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,
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F: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C,</div></h4></section></div></details><details class="toggle implementors-toggle"><summary><section id="impl-Context%3CT,+E%3E-for-Result%3CT,+E%3E" class="impl"><a class="src rightside" href="../src/anyhow/context.rs.html#42-68">source</a><a href="#impl-Context%3CT,+E%3E-for-Result%3CT,+E%3E" class="anchor">§</a><h3 class="code-header">impl<T, E> <a class="trait" href="trait.Context.html" title="trait anyhow::Context">Context</a><T, E> for <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, E><div class="where">where
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E: StdError + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,</div></h3></section></summary><div class="impl-items"><section id="method.context-1" class="method trait-impl"><a class="src rightside" href="../src/anyhow/context.rs.html#46-56">source</a><a href="#method.context-1" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.context" class="fn">context</a><C>(self, context: C) -> <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,</div></h4></section><section id="method.with_context-1" class="method trait-impl"><a class="src rightside" href="../src/anyhow/context.rs.html#58-67">source</a><a href="#method.with_context-1" class="anchor">§</a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fn">with_context</a><C, F>(self, context: F) -> <a class="enum" href="https://doc.rust-lang.org/1.80.0/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><div class="where">where
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C: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,
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F: <a class="trait" href="https://doc.rust-lang.org/1.80.0/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C,</div></h4></section></div></details><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"></div><script src="../trait.impl/anyhow/trait.Context.js" data-ignore-extern-crates="core" async></script></section></div></main></body></html> |