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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>thiserror - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="thiserror" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><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">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../thiserror/index.html">thiserror</a><span class="version">1.0.57</span></h2></div><div class="sidebar-elems"><ul class="block">
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<li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#derives">Derive Macros</a></li></ul></section></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../thiserror/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">thiserror</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/thiserror/lib.rs.html#1-260">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</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/thiserror"><img src="https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github" alt="github" /></a> <a href="https://crates.io/crates/thiserror"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt="crates-io" /></a> <a href="https://docs.rs/thiserror"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" alt="docs-rs" /></a></p>
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<br>
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<p>This library provides a convenient derive macro for the standard library’s
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<a href="https://doc.rust-lang.org/std/error/trait.Error.html"><code>std::error::Error</code></a> trait.</p>
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<br>
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<h2 id="example"><a href="#example">Example</a></h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>thiserror::Error;
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<span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>DataStoreError {
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<span class="attr">#[error(<span class="string">"data store disconnected"</span>)]
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</span>Disconnect(<span class="attr">#[from] </span>io::Error),
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<span class="attr">#[error(<span class="string">"the data for key `{0}` is not available"</span>)]
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</span>Redaction(String),
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<span class="attr">#[error(<span class="string">"invalid header (expected {expected:?}, found {found:?})"</span>)]
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</span>InvalidHeader {
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expected: String,
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found: String,
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},
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<span class="attr">#[error(<span class="string">"unknown data store error"</span>)]
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</span>Unknown,
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}</code></pre></div>
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<br>
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<h2 id="details"><a href="#details">Details</a></h2>
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<ul>
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<li>
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<p>Thiserror deliberately does not appear in your public API. You get the
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same thing as if you had written an implementation of <code>std::error::Error</code>
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by hand, and switching from handwritten impls to thiserror or vice versa
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is not a breaking change.</p>
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</li>
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<li>
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<p>Errors may be enums, structs with named fields, tuple structs, or unit
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structs.</p>
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</li>
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<li>
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<p>A <code>Display</code> impl is generated for your error if you provide
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<code>#[error("...")]</code> messages on the struct or each variant of your enum, as
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shown above in the example.</p>
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<p>The messages support a shorthand for interpolating fields from the error.</p>
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<ul>
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<li><code>#[error("{var}")]</code> ⟶ <code>write!("{}", self.var)</code></li>
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<li><code>#[error("{0}")]</code> ⟶ <code>write!("{}", self.0)</code></li>
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<li><code>#[error("{var:?}")]</code> ⟶ <code>write!("{:?}", self.var)</code></li>
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<li><code>#[error("{0:?}")]</code> ⟶ <code>write!("{:?}", self.0)</code></li>
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</ul>
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<p>These shorthands can be used together with any additional format args,
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which may be arbitrary expressions. For example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>Error {
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<span class="attr">#[error(<span class="string">"invalid rdo_lookahead_frames {0} (expected < {})"</span>, i32::MAX)]
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</span>InvalidLookahead(u32),
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}</code></pre></div>
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<p>If one of the additional expression arguments needs to refer to a field of
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the struct or enum, then refer to named fields as <code>.var</code> and tuple fields
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as <code>.0</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>Error {
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<span class="attr">#[error(<span class="string">"first letter must be lowercase but was {:?}"</span>, first_char(.<span class="number">0</span>))]
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</span>WrongCase(String),
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<span class="attr">#[error(<span class="string">"invalid index {idx}, expected at least {} and at most {}"</span>, .limits.lo, .limits.hi)]
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</span>OutOfBounds { idx: usize, limits: Limits },
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}</code></pre></div>
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</li>
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<li>
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<p>A <code>From</code> impl is generated for each variant containing a <code>#[from]</code>
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attribute.</p>
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<p>Note that the variant must not contain any other fields beyond the source
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error and possibly a backtrace. A backtrace is captured from within the
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<code>From</code> impl if there is a field for it.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>MyError {
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Io {
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<span class="attr">#[from]
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</span>source: io::Error,
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backtrace: Backtrace,
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},
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}</code></pre></div>
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</li>
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<li>
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<p>The Error trait’s <code>source()</code> method is implemented to return whichever
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field has a <code>#[source]</code> attribute or is named <code>source</code>, if any. This is
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for identifying the underlying lower level error that caused your error.</p>
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<p>The <code>#[from]</code> attribute always implies that the same field is <code>#[source]</code>,
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so you don’t ever need to specify both attributes.</p>
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<p>Any error type that implements <code>std::error::Error</code> or dereferences to <code>dyn std::error::Error</code> will work as a source.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub struct </span>MyError {
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msg: String,
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<span class="attr">#[source] </span><span class="comment">// optional if field name is `source`
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</span>source: anyhow::Error,
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}</code></pre></div>
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</li>
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<li>
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<p>The Error trait’s <code>provide()</code> method is implemented to provide whichever
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field has a type named <code>Backtrace</code>, if any, as a
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<code>std::backtrace::Backtrace</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::backtrace::Backtrace;
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<span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub struct </span>MyError {
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msg: String,
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backtrace: Backtrace, <span class="comment">// automatically detected
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</span>}</code></pre></div>
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</li>
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<li>
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<p>If a field is both a source (named <code>source</code>, or has <code>#[source]</code> or
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<code>#[from]</code> attribute) <em>and</em> is marked <code>#[backtrace]</code>, then the Error
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trait’s <code>provide()</code> method is forwarded to the source’s <code>provide</code> so that
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both layers of the error share the same backtrace.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>MyError {
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Io {
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<span class="attr">#[backtrace]
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</span>source: io::Error,
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},
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}</code></pre></div>
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</li>
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<li>
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<p>Errors may use <code>error(transparent)</code> to forward the source and Display
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methods straight through to an underlying error without adding an
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additional message. This would be appropriate for enums that need an
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“anything else” variant.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">pub enum </span>MyError {
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...
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<span class="attr">#[error(transparent)]
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</span>Other(<span class="attr">#[from] </span>anyhow::Error), <span class="comment">// source and Display delegate to anyhow::Error
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</span>}</code></pre></div>
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<p>Another use case is hiding implementation details of an error
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representation behind an opaque error type, so that the representation is
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able to evolve without breaking the crate’s public API.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// PublicError is public, but opaque and easy to keep compatible.
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</span><span class="attr">#[derive(Error, Debug)]
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#[error(transparent)]
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</span><span class="kw">pub struct </span>PublicError(<span class="attr">#[from] </span>ErrorRepr);
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<span class="kw">impl </span>PublicError {
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<span class="comment">// Accessors for anything we do want to expose publicly.
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</span>}
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<span class="comment">// Private and free to change across minor version of the crate.
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</span><span class="attr">#[derive(Error, Debug)]
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</span><span class="kw">enum </span>ErrorRepr {
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...
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}</code></pre></div>
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</li>
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<li>
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<p>See also the <a href="https://github.com/dtolnay/anyhow"><code>anyhow</code></a> library for a convenient single error type to use
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in application code.</p>
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</li>
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</ul>
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</div></details><h2 id="derives" class="section-header"><a href="#derives">Derive Macros</a></h2><ul class="item-table"><li><div class="item-name"><a class="derive" href="derive.Error.html" title="derive thiserror::Error">Error</a></div></li></ul></section></div></main></body></html> |