<!DOCTYPE HTML> <html lang="en" class="sidebar-visible no-js light"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>The source() of Errors - chainerror</title> <!-- Custom HTML head --> <meta name="description" content="A tutorial for the chainerror rust crate."> <meta name="viewport" content="width=device-width, initial-scale=1"> <meta name="theme-color" content="#ffffff" /> <link rel="icon" href="favicon.svg"> <link rel="shortcut icon" href="favicon.png"> <link rel="stylesheet" href="css/variables.css"> <link rel="stylesheet" href="css/general.css"> <link rel="stylesheet" href="css/chrome.css"> <link rel="stylesheet" href="css/print.css" media="print"> <!-- Fonts --> <link rel="stylesheet" href="FontAwesome/css/font-awesome.css"> <link rel="stylesheet" href="fonts/fonts.css"> <!-- Highlight.js Stylesheets --> <link rel="stylesheet" href="highlight.css"> <link rel="stylesheet" href="tomorrow-night.css"> <link rel="stylesheet" href="ayu-highlight.css"> <!-- Custom theme stylesheets --> </head> <body> <div id="body-container"> <!-- Provide site root to javascript --> <script> var path_to_root = ""; var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light"; </script> <!-- Work around some values being stored in localStorage wrapped in quotes --> <script> try { var theme = localStorage.getItem('mdbook-theme'); var sidebar = localStorage.getItem('mdbook-sidebar'); if (theme.startsWith('"') && theme.endsWith('"')) { localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1)); } if (sidebar.startsWith('"') && sidebar.endsWith('"')) { localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1)); } } catch (e) { } </script> <!-- Set the theme before any content is loaded, prevents flash --> <script> var theme; try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { } if (theme === null || theme === undefined) { theme = default_theme; } var html = document.querySelector('html'); html.classList.remove('no-js') html.classList.remove('light') html.classList.add(theme); html.classList.add('js'); </script> <!-- Hide / unhide sidebar before it is displayed --> <script> var html = document.querySelector('html'); var sidebar = null; if (document.body.clientWidth >= 1080) { try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } sidebar = sidebar || 'visible'; } else { sidebar = 'hidden'; } html.classList.remove('sidebar-visible'); html.classList.add("sidebar-" + sidebar); </script> <nav id="sidebar" class="sidebar" aria-label="Table of contents"> <div class="sidebar-scrollbox"> <ol class="chapter"><li class="chapter-item expanded affix "><a href="index.html">chainerror</a></li><li class="chapter-item expanded "><a href="tutorial1.html"><strong aria-hidden="true">1.</strong> Simple String Errors</a></li><li class="chapter-item expanded "><a href="tutorial2.html"><strong aria-hidden="true">2.</strong> Simple Chained String Errors</a></li><li class="chapter-item expanded "><a href="tutorial3.html"><strong aria-hidden="true">3.</strong> Mapping Errors</a></li><li class="chapter-item expanded "><a href="tutorial4.html"><strong aria-hidden="true">4.</strong> More Information</a></li><li class="chapter-item expanded "><a href="tutorial5.html" class="active"><strong aria-hidden="true">5.</strong> The source() of Errors</a></li><li class="chapter-item expanded "><a href="tutorial6.html"><strong aria-hidden="true">6.</strong> Downcast the Errors</a></li><li class="chapter-item expanded "><a href="tutorial7.html"><strong aria-hidden="true">7.</strong> The root cause of all Errors</a></li><li class="chapter-item expanded "><a href="tutorial8.html"><strong aria-hidden="true">8.</strong> Finding an Error cause</a></li><li class="chapter-item expanded "><a href="tutorial9.html"><strong aria-hidden="true">9.</strong> Selective Error Handling</a></li><li class="chapter-item expanded "><a href="tutorial10.html"><strong aria-hidden="true">10.</strong> ErrorKind to the rescue</a></li><li class="chapter-item expanded "><a href="tutorial11.html"><strong aria-hidden="true">11.</strong> Debug for the ErrorKind</a></li><li class="chapter-item expanded "><a href="tutorial12.html"><strong aria-hidden="true">12.</strong> Deref for the ErrorKind</a></li><li class="chapter-item expanded "><a href="tutorial13.html"><strong aria-hidden="true">13.</strong> Writing a library</a></li><li class="chapter-item expanded "><a href="tutorial14.html"><strong aria-hidden="true">14.</strong> Going back to std</a></li><li class="chapter-item expanded affix "><a href="end.html">The End</a></li></ol> </div> <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div> </nav> <!-- Track and set sidebar scroll position --> <script> var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox'); sidebarScrollbox.addEventListener('click', function(e) { if (e.target.tagName === 'A') { sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop); } }, { passive: true }); var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll'); sessionStorage.removeItem('sidebar-scroll'); if (sidebarScrollTop) { // preserve sidebar scroll position when navigating via links within sidebar sidebarScrollbox.scrollTop = sidebarScrollTop; } else { // scroll sidebar to current active section when navigating via "next/previous chapter" buttons var activeSection = document.querySelector('#sidebar .active'); if (activeSection) { activeSection.scrollIntoView({ block: 'center' }); } } </script> <div id="page-wrapper" class="page-wrapper"> <div class="page"> <div id="menu-bar-hover-placeholder"></div> <div id="menu-bar" class="menu-bar sticky"> <div class="left-buttons"> <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> <i class="fa fa-bars"></i> </button> <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list"> <i class="fa fa-paint-brush"></i> </button> <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu"> <li role="none"><button role="menuitem" class="theme" id="light">Light</button></li> <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li> <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li> <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li> <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li> </ul> <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> <i class="fa fa-search"></i> </button> </div> <h1 class="menu-title">chainerror</h1> <div class="right-buttons"> <a href="print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> </div> </div> <div id="search-wrapper" class="hidden"> <form id="searchbar-outer" class="searchbar-outer"> <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> </form> <div id="searchresults-outer" class="searchresults-outer hidden"> <div id="searchresults-header" class="searchresults-header"></div> <ul id="searchresults"> </ul> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <h1 id="the-source-of-errors"><a class="header" href="#the-source-of-errors">The source() of Errors</a></h1> <p>Sometimes you want to inspect the <code>source()</code> of an <code>Error</code>. <code>chainerror</code> implements <code>std::error::Error::source()</code>, so you can get the cause of an error.</p> <pre><pre class="playground"><code class="language-rust">use chainerror::Context as _; use std::error::Error; use std::io; fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> { Err(io::Error::from(io::ErrorKind::NotFound))?; Ok(()) } fn func2() -> Result<(), Box<dyn Error + Send + Sync>> { let filename = "foo.txt"; do_some_io().context(format!("Error reading '{}'", filename))?; Ok(()) } fn func1() -> Result<(), Box<dyn Error + Send + Sync>> { if let Err(e) = func2() { if let Some(s) = e.source() { eprintln!("func2 failed because of '{}'", s); Err(e).context("func1 error")?; } } Ok(()) } fn main() -> Result<(), Box<dyn Error + Send + Sync>> { if let Err(e) = func1() { eprintln!("{}", e); std::process::exit(1); } Ok(()) } <span class="boring">#[allow(dead_code)] </span><span class="boring">mod chainerror { </span><span class="boring">#![doc = include_str!("../README.md")] </span><span class="boring">#![deny(clippy::all)] </span><span class="boring">#![allow(clippy::needless_doctest_main)] </span><span class="boring">#![deny(missing_docs)] </span><span class="boring"> </span><span class="boring">use std::any::TypeId; </span><span class="boring">use std::error::Error as StdError; </span><span class="boring">use std::fmt::{Debug, Display, Formatter}; </span><span class="boring">use std::panic::Location; </span><span class="boring"> </span><span class="boring">/// chains an inner error kind `T` with a causing error </span><span class="boring">pub struct Error<T> { </span><span class="boring"> occurrence: Option<String>, </span><span class="boring"> kind: T, </span><span class="boring"> error_cause: Option<Box<dyn StdError + 'static + Send + Sync>>, </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// convenience type alias </span><span class="boring">pub type Result<O, E> = std::result::Result<O, Error<E>>; </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> Error<T> { </span><span class="boring"> /// Use the `context()` or `map_context()` Result methods instead of calling this directly </span><span class="boring"> #[inline] </span><span class="boring"> pub fn new( </span><span class="boring"> kind: T, </span><span class="boring"> error_cause: Option<Box<dyn StdError + 'static + Send + Sync>>, </span><span class="boring"> occurrence: Option<String>, </span><span class="boring"> ) -> Self { </span><span class="boring"> Self { </span><span class="boring"> occurrence, </span><span class="boring"> kind, </span><span class="boring"> error_cause, </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// return the root cause of the error chain, if any exists </span><span class="boring"> pub fn root_cause(&self) -> Option<&(dyn StdError + 'static)> { </span><span class="boring"> self.iter().last() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// Find the first error cause of type U, if any exists </span><span class="boring"> /// </span><span class="boring"> /// # Examples </span><span class="boring"> /// </span><span class="boring"> /// ```rust </span><span class="boring"> /// use chainerror::Context as _; </span><span class="boring"> /// use chainerror::ErrorDown as _; </span><span class="boring"> /// use std::error::Error; </span><span class="boring"> /// use std::io; </span><span class="boring"> /// </span><span class="boring"> /// fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring"> /// Err(io::Error::from(io::ErrorKind::NotFound))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// chainerror::str_context!(Func2Error); </span><span class="boring"> /// </span><span class="boring"> /// fn func2() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring"> /// let filename = "foo.txt"; </span><span class="boring"> /// do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// chainerror::str_context!(Func1Error); </span><span class="boring"> /// </span><span class="boring"> /// fn func1() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring"> /// func2().context(Func1Error::new("func1 error"))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// if let Err(e) = func1() { </span><span class="boring"> /// if let Some(f1err) = e.downcast_chain_ref::<Func1Error>() { </span><span class="boring"> /// assert!(f1err.find_cause::<io::Error>().is_some()); </span><span class="boring"> /// </span><span class="boring"> /// assert!(f1err.find_chain_cause::<Func2Error>().is_some()); </span><span class="boring"> /// } </span><span class="boring"> /// # else { </span><span class="boring"> /// # panic!(); </span><span class="boring"> /// # } </span><span class="boring"> /// } </span><span class="boring"> /// # else { </span><span class="boring"> /// # unreachable!(); </span><span class="boring"> /// # } </span><span class="boring"> /// ``` </span><span class="boring"> #[inline] </span><span class="boring"> pub fn find_cause<U: StdError + 'static>(&self) -> Option<&U> { </span><span class="boring"> self.iter() </span><span class="boring"> .filter_map(<dyn StdError>::downcast_ref::<U>) </span><span class="boring"> .next() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// Find the first error cause of type [`Error<U>`](Error), if any exists </span><span class="boring"> /// </span><span class="boring"> /// Same as `find_cause`, but hides the [`Error<U>`](Error) implementation internals </span><span class="boring"> /// </span><span class="boring"> /// # Examples </span><span class="boring"> /// </span><span class="boring"> /// ```rust </span><span class="boring"> /// # chainerror::str_context!(FooError); </span><span class="boring"> /// # let err = chainerror::Error::new(String::new(), None, None); </span><span class="boring"> /// // Instead of writing </span><span class="boring"> /// err.find_cause::<chainerror::Error<FooError>>(); </span><span class="boring"> /// </span><span class="boring"> /// // leave out the chainerror::Error<FooError> implementation detail </span><span class="boring"> /// err.find_chain_cause::<FooError>(); </span><span class="boring"> /// ``` </span><span class="boring"> #[inline] </span><span class="boring"> pub fn find_chain_cause<U: StdError + 'static>(&self) -> Option<&Error<U>> { </span><span class="boring"> self.iter() </span><span class="boring"> .filter_map(<dyn StdError>::downcast_ref::<Error<U>>) </span><span class="boring"> .next() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// Find the first error cause of type [`Error<U>`](Error) or `U`, if any exists and return `U` </span><span class="boring"> /// </span><span class="boring"> /// Same as `find_cause` and `find_chain_cause`, but hides the [`Error<U>`](Error) implementation internals </span><span class="boring"> /// </span><span class="boring"> /// # Examples </span><span class="boring"> /// </span><span class="boring"> /// ```rust </span><span class="boring"> /// # chainerror::str_context!(FooErrorKind); </span><span class="boring"> /// # let err = chainerror::Error::new(String::new(), None, None); </span><span class="boring"> /// // Instead of writing </span><span class="boring"> /// err.find_cause::<chainerror::Error<FooErrorKind>>(); </span><span class="boring"> /// // and/or </span><span class="boring"> /// err.find_chain_cause::<FooErrorKind>(); </span><span class="boring"> /// // and/or </span><span class="boring"> /// err.find_cause::<FooErrorKind>(); </span><span class="boring"> /// </span><span class="boring"> /// // leave out the chainerror::Error<FooErrorKind> implementation detail </span><span class="boring"> /// err.find_kind_or_cause::<FooErrorKind>(); </span><span class="boring"> /// ``` </span><span class="boring"> #[inline] </span><span class="boring"> pub fn find_kind_or_cause<U: StdError + 'static>(&self) -> Option<&U> { </span><span class="boring"> self.iter() </span><span class="boring"> .filter_map(|e| { </span><span class="boring"> e.downcast_ref::<Error<U>>() </span><span class="boring"> .map(|e| e.kind()) </span><span class="boring"> .or_else(|| e.downcast_ref::<U>()) </span><span class="boring"> }) </span><span class="boring"> .next() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// Return a reference to T of [`Error<T>`](Error) </span><span class="boring"> /// </span><span class="boring"> /// # Examples </span><span class="boring"> /// </span><span class="boring"> /// ```rust </span><span class="boring"> /// use chainerror::Context as _; </span><span class="boring"> /// use std::error::Error; </span><span class="boring"> /// use std::io; </span><span class="boring"> /// </span><span class="boring"> /// fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring"> /// Err(io::Error::from(io::ErrorKind::NotFound))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// chainerror::str_context!(Func2Error); </span><span class="boring"> /// </span><span class="boring"> /// fn func2() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring"> /// let filename = "foo.txt"; </span><span class="boring"> /// do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// #[derive(Debug)] </span><span class="boring"> /// enum Func1ErrorKind { </span><span class="boring"> /// Func2, </span><span class="boring"> /// IO(String), </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// /// impl ::std::fmt::Display for Func1ErrorKind {…} </span><span class="boring"> /// # impl ::std::fmt::Display for Func1ErrorKind { </span><span class="boring"> /// # fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { </span><span class="boring"> /// # match self { </span><span class="boring"> /// # Func1ErrorKind::Func2 => write!(f, "func1 error calling func2"), </span><span class="boring"> /// # Func1ErrorKind::IO(filename) => write!(f, "Error reading '{}'", filename), </span><span class="boring"> /// # } </span><span class="boring"> /// # } </span><span class="boring"> /// # } </span><span class="boring"> /// </span><span class="boring"> /// fn func1() -> chainerror::Result<(), Func1ErrorKind> { </span><span class="boring"> /// func2().context(Func1ErrorKind::Func2)?; </span><span class="boring"> /// do_some_io().context(Func1ErrorKind::IO("bar.txt".into()))?; </span><span class="boring"> /// Ok(()) </span><span class="boring"> /// } </span><span class="boring"> /// </span><span class="boring"> /// if let Err(e) = func1() { </span><span class="boring"> /// match e.kind() { </span><span class="boring"> /// Func1ErrorKind::Func2 => {} </span><span class="boring"> /// Func1ErrorKind::IO(filename) => panic!(), </span><span class="boring"> /// } </span><span class="boring"> /// } </span><span class="boring"> /// # else { </span><span class="boring"> /// # unreachable!(); </span><span class="boring"> /// # } </span><span class="boring"> /// ``` </span><span class="boring"> #[inline] </span><span class="boring"> pub fn kind(&self) -> &T { </span><span class="boring"> &self.kind </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> /// Returns an Iterator over all error causes/sources </span><span class="boring"> /// </span><span class="boring"> /// # Example </span><span class="boring"> #[inline] </span><span class="boring"> pub fn iter(&self) -> impl Iterator<Item = &(dyn StdError + 'static)> { </span><span class="boring"> ErrorIter { </span><span class="boring"> current: Some(self), </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// Convenience methods for `Result<>` to turn the error into a decorated [`Error`](Error) </span><span class="boring">pub trait Context<O, E: Into<Box<dyn StdError + 'static + Send + Sync>>> { </span><span class="boring"> /// Decorate the error with a `kind` of type `T` and the source `Location` </span><span class="boring"> fn context<T: 'static + Display + Debug>(self, kind: T) -> std::result::Result<O, Error<T>>; </span><span class="boring"> </span><span class="boring"> /// Decorate the error just with the source `Location` </span><span class="boring"> fn annotate(self) -> std::result::Result<O, Error<AnnotatedError>>; </span><span class="boring"> </span><span class="boring"> /// Decorate the `error` with a `kind` of type `T` produced with a `FnOnce(&error)` and the source `Location` </span><span class="boring"> fn map_context<T: 'static + Display + Debug, F: FnOnce(&E) -> T>( </span><span class="boring"> self, </span><span class="boring"> op: F, </span><span class="boring"> ) -> std::result::Result<O, Error<T>>; </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// Convenience type to just decorate the error with the source `Location` </span><span class="boring">pub struct AnnotatedError(()); </span><span class="boring"> </span><span class="boring">impl Display for AnnotatedError { </span><span class="boring"> fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { </span><span class="boring"> write!(f, "(passed error)") </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl Debug for AnnotatedError { </span><span class="boring"> fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { </span><span class="boring"> write!(f, "(passed error)") </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<O, E: Into<Box<dyn StdError + 'static + Send + Sync>>> Context<O, E> </span><span class="boring"> for std::result::Result<O, E> </span><span class="boring">{ </span><span class="boring"> #[track_caller] </span><span class="boring"> #[inline] </span><span class="boring"> fn context<T: 'static + Display + Debug>(self, kind: T) -> std::result::Result<O, Error<T>> { </span><span class="boring"> match self { </span><span class="boring"> Ok(t) => Ok(t), </span><span class="boring"> Err(error_cause) => Err(Error::new( </span><span class="boring"> kind, </span><span class="boring"> Some(error_cause.into()), </span><span class="boring"> Some(Location::caller().to_string()), </span><span class="boring"> )), </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[track_caller] </span><span class="boring"> #[inline] </span><span class="boring"> fn annotate(self) -> std::result::Result<O, Error<AnnotatedError>> { </span><span class="boring"> match self { </span><span class="boring"> Ok(t) => Ok(t), </span><span class="boring"> Err(error_cause) => Err(Error::new( </span><span class="boring"> AnnotatedError(()), </span><span class="boring"> Some(error_cause.into()), </span><span class="boring"> Some(Location::caller().to_string()), </span><span class="boring"> )), </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[track_caller] </span><span class="boring"> #[inline] </span><span class="boring"> fn map_context<T: 'static + Display + Debug, F: FnOnce(&E) -> T>( </span><span class="boring"> self, </span><span class="boring"> op: F, </span><span class="boring"> ) -> std::result::Result<O, Error<T>> { </span><span class="boring"> match self { </span><span class="boring"> Ok(t) => Ok(t), </span><span class="boring"> Err(error_cause) => { </span><span class="boring"> let kind = op(&error_cause); </span><span class="boring"> Err(Error::new( </span><span class="boring"> kind, </span><span class="boring"> Some(error_cause.into()), </span><span class="boring"> Some(Location::caller().to_string()), </span><span class="boring"> )) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// An iterator over all error causes/sources </span><span class="boring">pub struct ErrorIter<'a> { </span><span class="boring"> current: Option<&'a (dyn StdError + 'static)>, </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<'a> Iterator for ErrorIter<'a> { </span><span class="boring"> type Item = &'a (dyn StdError + 'static); </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn next(&mut self) -> Option<Self::Item> { </span><span class="boring"> let current = self.current; </span><span class="boring"> self.current = self.current.and_then(StdError::source); </span><span class="boring"> current </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> std::ops::Deref for Error<T> { </span><span class="boring"> type Target = T; </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn deref(&self) -> &Self::Target { </span><span class="boring"> &self.kind </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// Convenience trait to hide the [`Error<T>`](Error) implementation internals </span><span class="boring">pub trait ErrorDown { </span><span class="boring"> /// Test if of type `Error<T>` </span><span class="boring"> fn is_chain<T: 'static + Display + Debug>(&self) -> bool; </span><span class="boring"> /// Downcast to a reference of `Error<T>` </span><span class="boring"> fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>>; </span><span class="boring"> /// Downcast to a mutable reference of `Error<T>` </span><span class="boring"> fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>>; </span><span class="boring"> /// Downcast to T of `Error<T>` </span><span class="boring"> fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T>; </span><span class="boring"> /// Downcast to T mutable reference of `Error<T>` </span><span class="boring"> fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T>; </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<U: 'static + Display + Debug> ErrorDown for Error<U> { </span><span class="boring"> #[inline] </span><span class="boring"> fn is_chain<T: 'static + Display + Debug>(&self) -> bool { </span><span class="boring"> TypeId::of::<T>() == TypeId::of::<U>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>> { </span><span class="boring"> if self.is_chain::<T>() { </span><span class="boring"> #[allow(clippy::cast_ptr_alignment)] </span><span class="boring"> unsafe { </span><span class="boring"> #[allow(trivial_casts)] </span><span class="boring"> Some(*(self as *const dyn StdError as *const &Error<T>)) </span><span class="boring"> } </span><span class="boring"> } else { </span><span class="boring"> None </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>> { </span><span class="boring"> if self.is_chain::<T>() { </span><span class="boring"> #[allow(clippy::cast_ptr_alignment)] </span><span class="boring"> unsafe { </span><span class="boring"> #[allow(trivial_casts)] </span><span class="boring"> Some(&mut *(self as *mut dyn StdError as *mut &mut Error<T>)) </span><span class="boring"> } </span><span class="boring"> } else { </span><span class="boring"> None </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T> { </span><span class="boring"> if self.is_chain::<T>() { </span><span class="boring"> #[allow(clippy::cast_ptr_alignment)] </span><span class="boring"> unsafe { </span><span class="boring"> #[allow(trivial_casts)] </span><span class="boring"> Some(&(*(self as *const dyn StdError as *const &Error<T>)).kind) </span><span class="boring"> } </span><span class="boring"> } else { </span><span class="boring"> None </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T> { </span><span class="boring"> if self.is_chain::<T>() { </span><span class="boring"> #[allow(clippy::cast_ptr_alignment)] </span><span class="boring"> unsafe { </span><span class="boring"> #[allow(trivial_casts)] </span><span class="boring"> Some(&mut (*(self as *mut dyn StdError as *mut &mut Error<T>)).kind) </span><span class="boring"> } </span><span class="boring"> } else { </span><span class="boring"> None </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl ErrorDown for dyn StdError + 'static { </span><span class="boring"> #[inline] </span><span class="boring"> fn is_chain<T: 'static + Display + Debug>(&self) -> bool { </span><span class="boring"> self.is::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>> { </span><span class="boring"> self.downcast_ref::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>> { </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T> { </span><span class="boring"> self.downcast_ref::<T>() </span><span class="boring"> .or_else(|| self.downcast_ref::<Error<T>>().map(|e| e.kind())) </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T> { </span><span class="boring"> if self.is::<T>() { </span><span class="boring"> return self.downcast_mut::<T>(); </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> .and_then(|e| e.downcast_inner_mut::<T>()) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl ErrorDown for dyn StdError + 'static + Send { </span><span class="boring"> #[inline] </span><span class="boring"> fn is_chain<T: 'static + Display + Debug>(&self) -> bool { </span><span class="boring"> self.is::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>> { </span><span class="boring"> self.downcast_ref::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>> { </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T> { </span><span class="boring"> self.downcast_ref::<T>() </span><span class="boring"> .or_else(|| self.downcast_ref::<Error<T>>().map(|e| e.kind())) </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T> { </span><span class="boring"> if self.is::<T>() { </span><span class="boring"> return self.downcast_mut::<T>(); </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> .and_then(|e| e.downcast_inner_mut::<T>()) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl ErrorDown for dyn StdError + 'static + Send + Sync { </span><span class="boring"> #[inline] </span><span class="boring"> fn is_chain<T: 'static + Display + Debug>(&self) -> bool { </span><span class="boring"> self.is::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&Error<T>> { </span><span class="boring"> self.downcast_ref::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut Error<T>> { </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_ref<T: 'static + StdError>(&self) -> Option<&T> { </span><span class="boring"> self.downcast_ref::<T>() </span><span class="boring"> .or_else(|| self.downcast_ref::<Error<T>>().map(|e| e.kind())) </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> #[inline] </span><span class="boring"> fn downcast_inner_mut<T: 'static + StdError>(&mut self) -> Option<&mut T> { </span><span class="boring"> if self.is::<T>() { </span><span class="boring"> return self.downcast_mut::<T>(); </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> self.downcast_mut::<Error<T>>() </span><span class="boring"> .and_then(|e| e.downcast_inner_mut::<T>()) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> StdError for Error<T> { </span><span class="boring"> #[inline] </span><span class="boring"> fn source(&self) -> Option<&(dyn StdError + 'static)> { </span><span class="boring"> self.error_cause </span><span class="boring"> .as_ref() </span><span class="boring"> .map(|e| e.as_ref() as &(dyn StdError + 'static)) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> StdError for &mut Error<T> { </span><span class="boring"> #[inline] </span><span class="boring"> fn source(&self) -> Option<&(dyn StdError + 'static)> { </span><span class="boring"> self.error_cause </span><span class="boring"> .as_ref() </span><span class="boring"> .map(|e| e.as_ref() as &(dyn StdError + 'static)) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> Display for Error<T> { </span><span class="boring"> #[inline] </span><span class="boring"> fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { </span><span class="boring"> write!(f, "{}", self.kind)?; </span><span class="boring"> </span><span class="boring"> if f.alternate() { </span><span class="boring"> if let Some(e) = self.source() { </span><span class="boring"> write!(f, "\nCaused by:\n {:#}", &e)?; </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> Ok(()) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T: 'static + Display + Debug> Debug for Error<T> { </span><span class="boring"> #[inline] </span><span class="boring"> fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { </span><span class="boring"> if f.alternate() { </span><span class="boring"> let mut f = f.debug_struct(&format!("Error<{}>", std::any::type_name::<T>())); </span><span class="boring"> </span><span class="boring"> let f = f </span><span class="boring"> .field("occurrence", &self.occurrence) </span><span class="boring"> .field("kind", &self.kind) </span><span class="boring"> .field("source", &self.source()); </span><span class="boring"> </span><span class="boring"> f.finish() </span><span class="boring"> } else { </span><span class="boring"> if let Some(ref o) = self.occurrence { </span><span class="boring"> write!(f, "{}: ", o)?; </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> if TypeId::of::<String>() == TypeId::of::<T>() </span><span class="boring"> || TypeId::of::<&str>() == TypeId::of::<T>() </span><span class="boring"> { </span><span class="boring"> Display::fmt(&self.kind, f)?; </span><span class="boring"> } else { </span><span class="boring"> Debug::fmt(&self.kind, f)?; </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> if let Some(e) = self.source() { </span><span class="boring"> write!(f, "\nCaused by:\n{:?}", &e)?; </span><span class="boring"> } </span><span class="boring"> Ok(()) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring">} </span><span class="boring"> </span><span class="boring">impl<T> From<T> for Error<T> </span><span class="boring">where </span><span class="boring"> T: 'static + Display + Debug, </span><span class="boring">{ </span><span class="boring"> #[track_caller] </span><span class="boring"> #[inline] </span><span class="boring"> fn from(e: T) -> Error<T> { </span><span class="boring"> Error::new(e, None, Some(Location::caller().to_string())) </span><span class="boring"> } </span><span class="boring">} </span><span class="boring">/// Convenience macro to create a "new type" T(String) and implement Display + Debug for T </span><span class="boring">/// </span><span class="boring">/// # Examples </span><span class="boring">/// </span><span class="boring">/// ```rust </span><span class="boring">/// # use chainerror::Context as _; </span><span class="boring">/// # use chainerror::ErrorDown as _; </span><span class="boring">/// # use std::error::Error; </span><span class="boring">/// # use std::io; </span><span class="boring">/// # use std::result::Result; </span><span class="boring">/// # fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> { </span><span class="boring">/// # Err(io::Error::from(io::ErrorKind::NotFound))?; </span><span class="boring">/// # Ok(()) </span><span class="boring">/// # } </span><span class="boring">/// chainerror::str_context!(Func2Error); </span><span class="boring">/// </span><span class="boring">/// fn func2() -> chainerror::Result<(), Func2Error> { </span><span class="boring">/// let filename = "foo.txt"; </span><span class="boring">/// do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?; </span><span class="boring">/// Ok(()) </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// chainerror::str_context!(Func1Error); </span><span class="boring">/// </span><span class="boring">/// fn func1() -> Result<(), Box<dyn Error>> { </span><span class="boring">/// func2().context(Func1Error::new("func1 error"))?; </span><span class="boring">/// Ok(()) </span><span class="boring">/// } </span><span class="boring">/// # if let Err(e) = func1() { </span><span class="boring">/// # if let Some(f1err) = e.downcast_chain_ref::<Func1Error>() { </span><span class="boring">/// # assert!(f1err.find_cause::<chainerror::Error<Func2Error>>().is_some()); </span><span class="boring">/// # assert!(f1err.find_chain_cause::<Func2Error>().is_some()); </span><span class="boring">/// # } else { </span><span class="boring">/// # panic!(); </span><span class="boring">/// # } </span><span class="boring">/// # } else { </span><span class="boring">/// # unreachable!(); </span><span class="boring">/// # } </span><span class="boring">/// ``` </span><span class="boring">#[macro_export] </span><span class="boring">macro_rules! str_context { </span><span class="boring"> ($e:ident) => { </span><span class="boring"> #[derive(Clone)] </span><span class="boring"> pub struct $e(pub String); </span><span class="boring"> impl $e { </span><span class="boring"> pub fn new<S: Into<String>>(s: S) -> Self { </span><span class="boring"> $e(s.into()) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> impl ::std::fmt::Display for $e { </span><span class="boring"> fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { </span><span class="boring"> write!(f, "{}", self.0) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> impl ::std::fmt::Debug for $e { </span><span class="boring"> fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result { </span><span class="boring"> write!(f, "{}({})", stringify!($e), self.0) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> impl ::std::error::Error for $e {} </span><span class="boring"> }; </span><span class="boring">} </span><span class="boring"> </span><span class="boring">/// Derive an Error for an ErrorKind, which wraps a [`Error`](Error) and implements a `kind()` method </span><span class="boring">/// </span><span class="boring">/// It basically hides [`Error`](Error) to the outside and only exposes the [`kind()`](Error::kind) </span><span class="boring">/// method. </span><span class="boring">/// </span><span class="boring">/// Error::kind() returns the ErrorKind </span><span class="boring">/// Error::source() returns the parent error </span><span class="boring">/// </span><span class="boring">/// # Examples </span><span class="boring">/// </span><span class="boring">/// ```rust </span><span class="boring">/// use chainerror::Context as _; </span><span class="boring">/// use std::io; </span><span class="boring">/// </span><span class="boring">/// fn do_some_io(_f: &str) -> std::result::Result<(), io::Error> { </span><span class="boring">/// return Err(io::Error::from(io::ErrorKind::NotFound)); </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// #[derive(Debug, Clone)] </span><span class="boring">/// pub enum ErrorKind { </span><span class="boring">/// IO(String), </span><span class="boring">/// FatalError(String), </span><span class="boring">/// Unknown, </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// chainerror::err_kind!(Error, ErrorKind); </span><span class="boring">/// </span><span class="boring">/// impl std::fmt::Display for ErrorKind { </span><span class="boring">/// fn fmt(&self, f: &mut ::std::fmt::Formatter) -> std::fmt::Result { </span><span class="boring">/// match self { </span><span class="boring">/// ErrorKind::FatalError(e) => write!(f, "fatal error {}", e), </span><span class="boring">/// ErrorKind::Unknown => write!(f, "unknown error"), </span><span class="boring">/// ErrorKind::IO(filename) => write!(f, "Error reading '{}'", filename), </span><span class="boring">/// } </span><span class="boring">/// } </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// impl ErrorKind { </span><span class="boring">/// fn from_io_error(e: &io::Error, f: String) -> Self { </span><span class="boring">/// match e.kind() { </span><span class="boring">/// io::ErrorKind::BrokenPipe => panic!("Should not happen"), </span><span class="boring">/// io::ErrorKind::ConnectionReset => { </span><span class="boring">/// ErrorKind::FatalError(format!("While reading `{}`: {}", f, e)) </span><span class="boring">/// } </span><span class="boring">/// _ => ErrorKind::IO(f), </span><span class="boring">/// } </span><span class="boring">/// } </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// impl From<&io::Error> for ErrorKind { </span><span class="boring">/// fn from(e: &io::Error) -> Self { </span><span class="boring">/// ErrorKind::IO(format!("{}", e)) </span><span class="boring">/// } </span><span class="boring">/// } </span><span class="boring">/// </span><span class="boring">/// pub fn func1() -> std::result::Result<(), Error> { </span><span class="boring">/// let filename = "bar.txt"; </span><span class="boring">/// </span><span class="boring">/// do_some_io(filename).map_context(|e| ErrorKind::from_io_error(e, filename.into()))?; </span><span class="boring">/// do_some_io(filename).map_context(|e| ErrorKind::IO(filename.into()))?; </span><span class="boring">/// do_some_io(filename).map_context(|e| ErrorKind::from(e))?; </span><span class="boring">/// Ok(()) </span><span class="boring">/// } </span><span class="boring">/// ``` </span><span class="boring">#[macro_export] </span><span class="boring">macro_rules! err_kind { </span><span class="boring"> ($e:ident, $k:ident) => { </span><span class="boring"> pub struct $e($crate::Error<$k>); </span><span class="boring"> </span><span class="boring"> impl $e { </span><span class="boring"> pub fn kind(&self) -> &$k { </span><span class="boring"> self.0.kind() </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl From<$k> for $e { </span><span class="boring"> fn from(e: $k) -> Self { </span><span class="boring"> $e($crate::Error::new(e, None, None)) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl From<$crate::Error<$k>> for $e { </span><span class="boring"> fn from(e: $crate::Error<$k>) -> Self { </span><span class="boring"> $e(e) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl From<&$e> for $k </span><span class="boring"> where </span><span class="boring"> $k: Clone, </span><span class="boring"> { </span><span class="boring"> fn from(e: &$e) -> Self { </span><span class="boring"> e.kind().clone() </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl std::error::Error for $e { </span><span class="boring"> fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { </span><span class="boring"> self.0.source() </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl std::fmt::Display for $e { </span><span class="boring"> fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { </span><span class="boring"> std::fmt::Display::fmt(&self.0, f) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> </span><span class="boring"> impl std::fmt::Debug for $e { </span><span class="boring"> fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { </span><span class="boring"> std::fmt::Debug::fmt(&self.0, f) </span><span class="boring"> } </span><span class="boring"> } </span><span class="boring"> }; </span><span class="boring">} </span><span class="boring">}</span></code></pre></pre> <p>Note, that because we changed the output of the error in <code>main()</code> from <code>Debug</code> to <code>Display</code>, we don't see the error backtrace with filename and line number.</p> <p>To use the <code>Display</code> backtrace, you have to use the alternative display format output <code>{:#}</code>.</p> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="tutorial4.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="tutorial6.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a rel="prev" href="tutorial4.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="tutorial6.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <script> window.playground_copyable = true; </script> <script src="elasticlunr.min.js"></script> <script src="mark.min.js"></script> <script src="searcher.js"></script> <script src="clipboard.min.js"></script> <script src="highlight.js"></script> <script src="book.js"></script> <!-- Custom JS scripts --> </div> </body> </html>