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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 `Fail` trait in crate `failure`."><meta name="keywords" content="rust, rustlang, rust-lang, Fail"><title>failure::Fail - 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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script></head><body class="rustdoc trait"><!--[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><p class='location'>Trait Fail</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#provided-methods">Provided Methods</a><div class="sidebar-links"><a href="#method.cause">cause</a><a href="#method.backtrace">backtrace</a><a href="#method.context">context</a><a href="#method.compat">compat</a><a href="#method.causes">causes</a><a href="#method.root_cause">root_cause</a></div><a class="sidebar-title" href="#implementors">Implementors</a><a class="sidebar-title" href="#methods">Methods</a><div class="sidebar-links"><a href="#method.downcast_ref">downcast_ref</a><a href="#method.downcast_mut">downcast_mut</a><a href="#method.root_cause">root_cause</a><a href="#method.causes">causes</a></div></div><p class='location'><a href='index.html'>failure</a></p><script>window.sidebarCurrent = {name: 'Fail', ty: 'trait', 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 js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Trait <a href='index.html'>failure</a>::<wbr><a class="trait" href=''>Fail</a></span><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/failure/lib.rs.html#86-162' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust trait'>pub trait Fail: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static {
- fn <a href='#method.cause' class='fnname'>cause</a>(&self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><&<a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>> { ... }
- <div class='item-spacer'></div> fn <a href='#method.backtrace' class='fnname'>backtrace</a>(&self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><&<a class="struct" href="../failure/struct.Backtrace.html" title="struct failure::Backtrace">Backtrace</a>> { ... }
- <div class='item-spacer'></div> fn <a href='#method.context' class='fnname'>context</a><D>(self, context: D) -> <a class="struct" href="../failure/struct.Context.html" title="struct failure::Context">Context</a><D><br> <span class="where">where<br> D: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span>,
- { ... }
- <div class='item-spacer'></div> fn <a href='#method.compat' class='fnname'>compat</a>(self) -> <a class="struct" href="../failure/struct.Compat.html" title="struct failure::Compat">Compat</a><Self><br> <span class="where">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span>,
- { ... }
- <div class='item-spacer'></div> fn <a href='#method.causes' class='fnname'>causes</a>(&self) -> <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><br> <span class="where">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span>,
- { ... }
- <div class='item-spacer'></div> fn <a href='#method.root_cause' class='fnname'>root_cause</a>(&self) -> &<a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a><br> <span class="where">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span>,
- { ... }
- }</pre></div><div class='docblock'><p>The <code>Fail</code> trait.</p>
- <p>Implementors of this trait are called 'failures'.</p>
- <p>All error types should implement <code>Fail</code>, which provides a baseline of
- functionality that they all share.</p>
- <p><code>Fail</code> has no required methods, but it does require that your type
- implement several other traits:</p>
- <ul>
- <li><code>Display</code>: to print a user-friendly representation of the error.</li>
- <li><code>Debug</code>: to print a verbose, developer-focused representation of the
- error.</li>
- <li><code>Send + Sync</code>: Your error type is required to be safe to transfer to and
- reference from another thread</li>
- </ul>
- <p>Additionally, all failures must be <code>'static</code>. This enables downcasting.</p>
- <p><code>Fail</code> provides several methods with default implementations. Two of these
- may be appropriate to override depending on the definition of your
- particular failure: the <code>cause</code> and <code>backtrace</code> methods.</p>
- <p>The <code>failure_derive</code> crate provides a way to derive the <code>Fail</code> trait for
- your type. Additionally, all types that already implement
- <code>std::error::Error</code>, and are also <code>Send</code>, <code>Sync</code>, and <code>'static</code>, implement
- <code>Fail</code> by a blanket impl.</p>
- </div>
- <h2 id='provided-methods' class='small-section-header'>
- Provided Methods<a href='#provided-methods' class='anchor'></a>
- </h2>
- <div class='methods'>
- <h3 id='method.cause' class='method'><span id='cause.v' class='invisible'><code>fn <a href='#method.cause' class='fnname'>cause</a>(&self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><&<a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>></code></span></h3><div class='docblock'><p>Returns a reference to the underlying cause of this failure, if it
- is an error that wraps other errors.</p>
- <p>Returns <code>None</code> if this failure does not have another error as its
- underlying cause. By default, this returns <code>None</code>.</p>
- <p>This should <strong>never</strong> return a reference to <code>self</code>, but only return
- <code>Some</code> when it can return a <strong>different</strong> failure. Users may loop
- over the cause chain, and returning <code>self</code> would result in an infinite
- loop.</p>
- </div><h3 id='method.backtrace' class='method'><span id='backtrace.v' class='invisible'><code>fn <a href='#method.backtrace' class='fnname'>backtrace</a>(&self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><&<a class="struct" href="../failure/struct.Backtrace.html" title="struct failure::Backtrace">Backtrace</a>></code></span></h3><div class='docblock'><p>Returns a reference to the <code>Backtrace</code> carried by this failure, if it
- carries one.</p>
- <p>Returns <code>None</code> if this failure does not carry a backtrace. By
- default, this returns <code>None</code>.</p>
- </div><h3 id='method.context' class='method'><span id='context.v' class='invisible'><code>fn <a href='#method.context' class='fnname'>context</a><D>(self, context: D) -> <a class="struct" href="../failure/struct.Context.html" title="struct failure::Context">Context</a><D> <span class="where fmt-newline">where<br> D: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code></span></h3><div class='docblock'><p>Provides context for this failure.</p>
- <p>This can provide additional information about this error, appropriate
- to the semantics of the current layer. That is, if you have a
- lower-level error, such as an IO error, you can provide additional context
- about what that error means in the context of your function. This
- gives users of this function more information about what has gone
- wrong.</p>
- <p>This takes any type that implements <code>Display</code>, as well as
- <code>Send</code>/<code>Sync</code>/<code>'static</code>. In practice, this means it can take a <code>String</code>
- or a string literal, or another failure, or some other custom context-carrying
- type.</p>
- </div><h3 id='method.compat' class='method'><span id='compat.v' class='invisible'><code>fn <a href='#method.compat' class='fnname'>compat</a>(self) -> <a class="struct" href="../failure/struct.Compat.html" title="struct failure::Compat">Compat</a><Self> <span class="where fmt-newline">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code></span></h3><div class='docblock'><p>Wraps this failure in a compatibility wrapper that implements
- <code>std::error::Error</code>.</p>
- <p>This allows failures to be compatible with older crates that
- expect types that implement the <code>Error</code> trait from <code>std::error</code>.</p>
- </div><div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></h3><code class="content"><span class="where fmt-newline">impl<'f> <a class="trait" href="https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html" title="trait core::iter::iterator::Iterator">Iterator</a> for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></span><span class="where fmt-newline"> type <a href='https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = &'f <a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>;</span></code></div></div><h3 id='method.causes' class='method'><span id='causes.v' class='invisible'><code>fn <a href='#method.causes' class='fnname'>causes</a>(&self) -> <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a> <span class="where fmt-newline">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code></span></h3><div class='docblock'><p>Returns a iterator over the causes of this <code>Fail</code> with itself
- as the first item and the <code>root_cause</code> as the final item.</p>
- </div><h3 id='method.root_cause' class='method'><span id='root_cause.v' class='invisible'><code>fn <a href='#method.root_cause' class='fnname'>root_cause</a>(&self) -> &<a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a> <span class="where fmt-newline">where<br> Self: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>, </span></code></span></h3><div class='docblock'><p>Returns the "root cause" of this <code>Fail</code> - the last value in the
- cause chain which does not return an underlying <code>cause</code>.</p>
- <p>If this type does not have a cause, <code>self</code> is returned, because
- it is its own root cause.</p>
- </div></div>
- <h2 id='methods' class='small-section-header'>
- Methods<a href='#methods' class='anchor'></a>
- </h2>
- <h3 id='impl' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl <a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a></code><a href='#impl' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/failure/lib.rs.html#164-203' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.downcast_ref' class="method"><span id='downcast_ref.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.downcast_ref' class='fnname'>downcast_ref</a><T: <a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>>(&self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</a>T></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/failure/lib.rs.html#168-174' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Attempts to downcast this failure to a concrete type by reference.</p>
- <p>If the underlying error is not of type <code>T</code>, this will return <code>None</code>.</p>
- </div><h4 id='method.downcast_mut' class="method"><span id='downcast_mut.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.downcast_mut' class='fnname'>downcast_mut</a><T: <a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>>(&mut self) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>T></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/failure/lib.rs.html#180-186' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Attempts to downcast this failure to a concrete type by mutable
- reference.</p>
- <p>If the underlying error is not of type <code>T</code>, this will return <code>None</code>.</p>
- </div><h4 id='method.root_cause-1' class="method"><span id='root_cause.v-1' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.root_cause-1' class='fnname'>root_cause</a>(&self) -> &<a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/failure/lib.rs.html#194-196' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Returns the "root cause" of this <code>Fail</code> - the last value in the
- cause chain which does not return an underlying <code>cause</code>.</p>
- <p>If this type does not have a cause, <code>self</code> is returned, because
- it is its own root cause.</p>
- </div><h4 id='method.causes-1' class="method"><div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></h3><code class="content"><span class="where fmt-newline">impl<'f> <a class="trait" href="https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html" title="trait core::iter::iterator::Iterator">Iterator</a> for <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a><'f></span><span class="where fmt-newline"> type <a href='https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = &'f <a class="trait" href="../failure/trait.Fail.html" title="trait failure::Fail">Fail</a>;</span></code></div></div><span id='causes.v-1' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.causes-1' class='fnname'>causes</a>(&self) -> <a class="struct" href="../failure/struct.Causes.html" title="struct failure::Causes">Causes</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../src/failure/lib.rs.html#200-202' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Returns a iterator over the causes of this <code>Fail</code> with itself
- as the first item and the <code>root_cause</code> as the final item.</p>
- </div></div>
- <h2 id='implementors' class='small-section-header'>
- Implementors<a href='#implementors' class='anchor'></a>
- </h2>
- <ul class='item-list' id='implementors-list'>
- <li><table class='table-display'><tbody><tr><td><code>impl<D: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static> Fail for <a class="struct" href="../failure/struct.Context.html" title="struct failure::Context">Context</a><D></code><td><div class='out-of-band'><a class='srclink' href='../src/failure/context.rs.html#83-91' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
- <li><table class='table-display'><tbody><tr><td><code>impl<E: <a class="trait" href="https://doc.rust-lang.org/nightly/std/error/trait.Error.html" title="trait std::error::Error">Error</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static> Fail for <a class="struct" href="../failure/struct.SyncFailure.html" title="struct failure::SyncFailure">SyncFailure</a><E></code><td><div class='out-of-band'><a class='srclink' href='../src/failure/sync_failure.rs.html#97' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
- <li><table class='table-display'><tbody><tr><td><code>impl<E: <a class="trait" href="https://doc.rust-lang.org/nightly/std/error/trait.Error.html" title="trait std::error::Error">StdError</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static> Fail for E</code><td><div class='out-of-band'><a class='srclink' href='../src/failure/lib.rs.html#206' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
- </ul><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
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