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<ol class="chapter"><li class="affix"><a href="index.html">chainerror</a></li><li><a href="tutorial1.html"><strong aria-hidden="true">1.</strong> Simple String Errors</a></li><li><a href="tutorial2.html"><strong aria-hidden="true">2.</strong> Simple Chained String Errors</a></li><li><a href="tutorial3.html"><strong aria-hidden="true">3.</strong> Mapping Errors</a></li><li><a href="tutorial4.html"><strong aria-hidden="true">4.</strong> Saving coding chars</a></li><li><a href="tutorial5.html"><strong aria-hidden="true">5.</strong> The source() of Errors</a></li><li><a href="tutorial6.html" class="active"><strong aria-hidden="true">6.</strong> Downcast the Errors</a></li><li><a href="tutorial7.html"><strong aria-hidden="true">7.</strong> The root cause of all Errors</a></li><li><a href="tutorial8.html"><strong aria-hidden="true">8.</strong> Finding an Error cause</a></li><li><a href="tutorial9.html"><strong aria-hidden="true">9.</strong> Selective Error Handling</a></li><li><a href="tutorial10.html"><strong aria-hidden="true">10.</strong> ErrorKind to the rescue</a></li><li><a href="tutorial11.html"><strong aria-hidden="true">11.</strong> Debug for the ErrorKind</a></li><li class="affix"><a href="end.html">The End</a></li></ol>
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<a class="header" href="#downcast-the-errors" id="downcast-the-errors"><h1>Downcast the Errors</h1></a>
<p><code>std::error::Error</code> comes with some helper methods to get to the original object of the
<code>&amp;(dyn Error + 'static)</code> returned by <code>.source()</code>.</p>
<pre><code class="language-rust ignore">pub fn downcast_ref&lt;T: Error + 'static&gt;(&amp;self) -&gt; Option&lt;&amp;T&gt;
pub fn downcast_mut&lt;T: Error + 'static&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut T&gt;
</code></pre>
<p>This is how it looks like, when using those:</p>
<pre><pre class="playpen"><code class="language-rust">use crate::chainerror::*;
use std::error::Error;
use std::io;
use std::result::Result;
fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
let filename = &quot;foo.txt&quot;;
do_some_io().map_err(mstrerr!(&quot;Error reading '{}'&quot;, filename))?;
Ok(())
}
fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
func2().map_err(mstrerr!(&quot;func1 error&quot;))?;
Ok(())
}
fn main() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
if let Err(e) = func1() {
eprintln!(&quot;Error: {}&quot;, e);
let mut s = e.as_ref();
while let Some(c) = s.source() {
if let Some(ioerror) = c.downcast_ref::&lt;io::Error&gt;() {
eprintln!(&quot;caused by: std::io::Error: {}&quot;, ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound =&gt; eprintln!(&quot;of kind: std::io::ErrorKind::NotFound&quot;),
_ =&gt; {}
}
} else {
eprintln!(&quot;caused by: {}&quot;, c);
}
s = c;
}
}
Ok(())
}
# #[allow(dead_code)]
# mod chainerror {
# /*!
#
# `chainerror` provides an error backtrace without doing a real backtrace, so even after you `strip` your
# binaries, you still have the error backtrace.
#
# `chainerror` has no dependencies!
#
# `chainerror` uses `.source()` of `std::error::Error` along with `line()!` and `file()!` to provide a nice debug error backtrace.
# It encapsulates all types, which have `Display + Debug` and can store the error cause internally.
#
# Along with the `ChainError&lt;T&gt;` struct, `chainerror` comes with some useful helper macros to save a lot of typing.
#
# ## Features
#
# `no-fileline`
# : completely turn off storing filename and line
#
# `display-cause`
# : turn on printing a backtrace of the errors in `Display`
#
# `no-debug-cause`
# : turn off printing a backtrace of the errors in `Debug`
#
#
# # Tutorial
#
# Read the [Tutorial](https://haraldh.github.io/chainerror/tutorial1.html)
#
# # Examples
#
# ~~~rust
# use chainerror::*;
# use std::error::Error;
# use std::io;
# use std::result::Result;
#
# fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(&quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# func2().map_err(mstrerr!(&quot;func1 error&quot;))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# assert_eq!(
# format!(&quot;\n{:?}\n&quot;, e), r#&quot;
# src/lib.rs:20: func1 error
# Caused by:
# src/lib.rs:15: Error reading 'foo.txt'
# Caused by:
# Kind(NotFound)
# &quot;#
# );
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
#
# ~~~rust
# use chainerror::*;
# use std::error::Error;
# use std::io;
# use std::result::Result;
#
# fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func3() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(&quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -&gt; ChainResult&lt;(), Func2Error&gt; {
# func3().map_err(mstrerr!(Func2Error, &quot;func2 error: calling func3&quot;))?;
# Ok(())
# }
#
# enum Func1Error {
# Func2,
# IO(String),
# }
#
# impl ::std::fmt::Display for Func1Error {
# fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# match self {
# Func1Error::Func2 =&gt; write!(f, &quot;func1 error calling func2&quot;),
# Func1Error::IO(filename) =&gt; write!(f, &quot;Error reading '{}'&quot;, filename),
# }
# }
# }
#
# impl ::std::fmt::Debug for Func1Error {
# fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# write!(f, &quot;{}&quot;, self)
# }
# }
#
# fn func1() -&gt; ChainResult&lt;(), Func1Error&gt; {
# func2().map_err(|e| cherr!(e, Func1Error::Func2))?;
# let filename = String::from(&quot;bar.txt&quot;);
# do_some_io().map_err(|e| cherr!(e, Func1Error::IO(filename)))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# assert!(
# match e.kind() {
# Func1Error::Func2 =&gt; {
# eprintln!(&quot;Main Error Report: func1 error calling func2&quot;);
# true
# }
# Func1Error::IO(filename) =&gt; {
# eprintln!(&quot;Main Error Report: func1 error reading '{}'&quot;, filename);
# false
# }
# }
# );
#
# assert!(e.find_chain_cause::&lt;Func2Error&gt;().is_some());
#
# if let Some(e) = e.find_chain_cause::&lt;Func2Error&gt;() {
# eprintln!(&quot;\nError reported by Func2Error: {}&quot;, e)
# }
#
#
# assert!(e.root_cause().is_some());
#
# if let Some(e) = e.root_cause() {
# let ioerror = e.downcast_ref::&lt;io::Error&gt;().unwrap();
# eprintln!(&quot;\nThe root cause was: std::io::Error: {:#?}&quot;, ioerror);
# }
#
# assert_eq!(
# format!(&quot;\n{:?}\n&quot;, e), r#&quot;
# src/lib.rs:47: func1 error calling func2
# Caused by:
# src/lib.rs:22: Func2Error(func2 error: calling func3)
# Caused by:
# src/lib.rs:15: Error reading 'foo.txt'
# Caused by:
# Kind(NotFound)
# &quot;#
# );
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
# !*/
#
# use std::any::TypeId;
# use std::error::Error;
# use std::fmt::{Debug, Display, Formatter, Result};
#
# /** chains an inner error kind `T` with a causing error
# **/
# pub struct ChainError&lt;T&gt; {
# #[cfg(not(feature = &quot;no-fileline&quot;))]
# occurrence: Option&lt;(u32, &amp;'static str)&gt;,
# kind: T,
# error_cause: Option&lt;Box&lt;dyn Error + 'static&gt;&gt;,
# }
#
# /// convenience type alias
# pub type ChainResult&lt;O, E&gt; = std::result::Result&lt;O, ChainError&lt;E&gt;&gt;;
#
# impl&lt;T: 'static + Display + Debug&gt; ChainError&lt;T&gt; {
# #[cfg(not(feature = &quot;no-fileline&quot;))]
# /// Use the `cherr!()` or `mstrerr!()` macro instead of calling this directly
# pub fn new(
# kind: T,
# error_cause: Option&lt;Box&lt;dyn Error + 'static&gt;&gt;,
# occurrence: Option&lt;(u32, &amp;'static str)&gt;,
# ) -&gt; Self {
# Self {
# occurrence,
# kind,
# error_cause,
# }
# }
#
# #[cfg(feature = &quot;no-fileline&quot;)]
# /// Use the `cherr!()` or `mstrerr!()` macro instead of calling this directly
# pub fn new(
# kind: T,
# error_cause: Option&lt;Box&lt;dyn Error + 'static&gt;&gt;,
# _occurrence: Option&lt;(u32, &amp;'static str)&gt;,
# ) -&gt; Self {
# Self { kind, error_cause }
# }
#
# /// return the root cause of the error chain, if any exists
# pub fn root_cause(&amp;self) -&gt; Option&lt;&amp;(dyn Error + 'static)&gt; {
# let mut cause = self as &amp;(dyn Error + 'static);
# while let Some(c) = cause.source() {
# cause = c;
# }
# Some(cause)
# }
#
# /** find the first error cause of type U, if any exists
#
# # Examples
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(Func2Error, &quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# func2().map_err(mstrerr!(Func1Error, &quot;func1 error&quot;))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# if let Some(f1err) = e.downcast_chain_ref::&lt;Func1Error&gt;() {
#
# assert!(f1err.find_cause::&lt;io::Error&gt;().is_some());
#
# assert!(f1err.find_chain_cause::&lt;Func2Error&gt;().is_some());
# }
# # else {
# # panic!();
# # }
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
# **/
# pub fn find_cause&lt;U: Error + 'static&gt;(&amp;self) -&gt; Option&lt;&amp;U&gt; {
# let mut cause = self as &amp;(dyn Error + 'static);
# loop {
# if cause.is::&lt;U&gt;() {
# return cause.downcast_ref::&lt;U&gt;();
# }
#
# match cause.source() {
# Some(c) =&gt; cause = c,
# None =&gt; return None,
# }
# }
# }
#
# /** find the first error cause of type ChainError&lt;U&gt;, if any exists
#
# Same as `find_cause`, but hides the `ChainError&lt;U&gt;` implementation internals
#
# # Examples
#
# ~~~rust,ignore
# /// Instead of writing
# err.find_cause::&lt;ChainError&lt;FooError&gt;&gt;();
#
# /// leave out the ChainError&lt;T&gt; implementation detail
# err.find_chain_cause::&lt;FooError&gt;();
# ~~~
#
# **/
# pub fn find_chain_cause&lt;U: Error + 'static&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;U&gt;&gt; {
# let mut cause = self as &amp;(dyn Error + 'static);
# loop {
# if cause.is::&lt;ChainError&lt;U&gt;&gt;() {
# return cause.downcast_ref::&lt;ChainError&lt;U&gt;&gt;();
# }
#
# match cause.source() {
# Some(c) =&gt; cause = c,
# None =&gt; return None,
# }
# }
# }
#
# /** return a reference to T of `ChainError&lt;T&gt;`
#
# # Examples
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(Func2Error, &quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# #[derive(Debug)]
# enum Func1ErrorKind {
# Func2,
# IO(String),
# }
#
# // impl ::std::fmt::Display for Func1ErrorKind {…}
# # impl ::std::fmt::Display for Func1ErrorKind {
# # fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# # match self {
# # Func1ErrorKind::Func2 =&gt; write!(f, &quot;func1 error calling func2&quot;),
# # Func1ErrorKind::IO(filename) =&gt; write!(f, &quot;Error reading '{}'&quot;, filename),
# # }
# # }
# # }
#
# fn func1() -&gt; ChainResult&lt;(), Func1ErrorKind&gt; {
# func2().map_err(|e| cherr!(e, Func1ErrorKind::Func2))?;
# do_some_io().map_err(|e| cherr!(e, Func1ErrorKind::IO(&quot;bar.txt&quot;.into())))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# match e.kind() {
# Func1ErrorKind::Func2 =&gt; {},
# Func1ErrorKind::IO(filename) =&gt; panic!(),
# }
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
# **/
# pub fn kind&lt;'a&gt;(&amp;'a self) -&gt; &amp;'a T {
# &amp;self.kind
# }
# }
#
# /** convenience trait to hide the `ChainError&lt;T&gt;` implementation internals
# **/
# pub trait ChainErrorDown {
# /** test if of type `ChainError&lt;T&gt;`
# **/
# fn is_chain&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; bool;
# /** downcast to a reference of `ChainError&lt;T&gt;`
# **/
# fn downcast_chain_ref&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;T&gt;&gt;;
# /** downcast to a mutable reference of `ChainError&lt;T&gt;`
# **/
# fn downcast_chain_mut&lt;T: 'static + Display + Debug&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut ChainError&lt;T&gt;&gt;;
# }
#
# impl&lt;U: 'static + Display + Debug&gt; ChainErrorDown for ChainError&lt;U&gt; {
# fn is_chain&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; bool {
# TypeId::of::&lt;T&gt;() == TypeId::of::&lt;U&gt;()
# }
#
# fn downcast_chain_ref&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;T&gt;&gt; {
# if self.is_chain::&lt;T&gt;() {
# unsafe { Some(&amp;*(self as *const dyn Error as *const &amp;ChainError&lt;T&gt;)) }
# } else {
# None
# }
# }
#
# fn downcast_chain_mut&lt;T: 'static + Display + Debug&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut ChainError&lt;T&gt;&gt; {
# if self.is_chain::&lt;T&gt;() {
# unsafe { Some(&amp;mut *(self as *mut dyn Error as *mut &amp;mut ChainError&lt;T&gt;)) }
# } else {
# None
# }
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static {
# fn is_chain&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; bool {
# self.is::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_ref&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;T&gt;&gt; {
# self.downcast_ref::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_mut&lt;T: 'static + Display + Debug&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut ChainError&lt;T&gt;&gt; {
# self.downcast_mut::&lt;ChainError&lt;T&gt;&gt;()
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static + Send {
# fn is_chain&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; bool {
# self.is::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_ref&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;T&gt;&gt; {
# self.downcast_ref::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_mut&lt;T: 'static + Display + Debug&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut ChainError&lt;T&gt;&gt; {
# self.downcast_mut::&lt;ChainError&lt;T&gt;&gt;()
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static + Send + Sync {
# fn is_chain&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; bool {
# self.is::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_ref&lt;T: 'static + Display + Debug&gt;(&amp;self) -&gt; Option&lt;&amp;ChainError&lt;T&gt;&gt; {
# self.downcast_ref::&lt;ChainError&lt;T&gt;&gt;()
# }
#
# fn downcast_chain_mut&lt;T: 'static + Display + Debug&gt;(&amp;mut self) -&gt; Option&lt;&amp;mut ChainError&lt;T&gt;&gt; {
# self.downcast_mut::&lt;ChainError&lt;T&gt;&gt;()
# }
# }
#
# impl&lt;T: 'static + Display + Debug&gt; Error for ChainError&lt;T&gt; {
# fn source(&amp;self) -&gt; Option&lt;&amp;(dyn Error + 'static)&gt; {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl&lt;T: 'static + Display + Debug&gt; Error for &amp;ChainError&lt;T&gt; {
# fn source(&amp;self) -&gt; Option&lt;&amp;(dyn Error + 'static)&gt; {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl&lt;T: 'static + Display + Debug&gt; Error for &amp;mut ChainError&lt;T&gt; {
# fn source(&amp;self) -&gt; Option&lt;&amp;(dyn Error + 'static)&gt; {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl&lt;T: 'static + Display + Debug&gt; Display for ChainError&lt;T&gt; {
# fn fmt(&amp;self, f: &amp;mut Formatter) -&gt; Result {
# write!(f, &quot;{}&quot;, self.kind)?;
#
# #[cfg(feature = &quot;display-cause&quot;)]
# {
# if let Some(e) = self.source() {
# writeln!(f, &quot;\nCaused by:&quot;)?;
# Display::fmt(&amp;e, f)?;
# }
# }
# Ok(())
# }
# }
#
# impl&lt;T: 'static + Display + Debug&gt; Debug for ChainError&lt;T&gt; {
# fn fmt(&amp;self, f: &amp;mut Formatter) -&gt; Result {
# #[cfg(not(feature = &quot;no-fileline&quot;))]
# {
# if let Some(o) = self.occurrence {
# write!(f, &quot;{}:{}: &quot;, o.1, o.0)?;
# }
# }
#
# if self.is_chain::&lt;String&gt;() {
# Display::fmt(&amp;self.kind, f)?;
# } else {
# Debug::fmt(&amp;self.kind, f)?;
# }
#
# #[cfg(not(feature = &quot;no-debug-cause&quot;))]
# {
# if let Some(e) = self.source() {
# writeln!(f, &quot;\nCaused by:&quot;)?;
# Debug::fmt(&amp;e, f)?;
# }
# }
# Ok(())
# }
# }
#
# /** creates a new `ChainError&lt;T&gt;`
#
# # Examples
#
# Create a new ChainError&lt;FooError&gt;, where `FooError` must implement `Display` and `Debug`.
# ~~~rust
# # use chainerror::*;
# #
# # #[derive(Debug)]
# enum FooError {
# Bar,
# Baz(&amp;'static str),
# }
# #
# # impl ::std::fmt::Display for FooError {
# # fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# # match self {
# # FooError::Bar =&gt; write!(f, &quot;Bar Error&quot;),
# # FooError::Baz(s) =&gt; write!(f, &quot;Baz Error: '{}'&quot;, s),
# # }
# # }
# # }
#
# // impl ::std::fmt::Display for FooError
#
# fn do_some_stuff() -&gt; bool {
# false
# }
#
# fn func() -&gt; ChainResult&lt;(), FooError&gt; {
# if ! do_some_stuff() {
# Err(cherr!(FooError::Baz(&quot;Error&quot;)))?;
# }
# Ok(())
# }
# #
# # pub fn main() {
# # match func().unwrap_err().kind() {
# # FooError::Baz(s) if s == &amp;&quot;Error&quot; =&gt; {},
# # _ =&gt; panic!(),
# # }
# # }
# ~~~
#
# Additionally an error cause can be added.
#
# ~~~rust
# # use chainerror::*;
# # use std::io;
# # use std::error::Error;
# #
# # #[derive(Debug)]
# # enum FooError {
# # Bar,
# # Baz(&amp;'static str),
# # }
# #
# # impl ::std::fmt::Display for FooError {
# # fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# # match self {
# # FooError::Bar =&gt; write!(f, &quot;Bar Error&quot;),
# # FooError::Baz(s) =&gt; write!(f, &quot;Baz Error: '{}'&quot;, s),
# # }
# # }
# # }
# #
# fn do_some_stuff() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func() -&gt; ChainResult&lt;(), FooError&gt; {
# do_some_stuff().map_err(
# |e| cherr!(e, FooError::Baz(&quot;Error&quot;))
# )?;
# Ok(())
# }
# #
# # pub fn main() {
# # match func().unwrap_err().kind() {
# # FooError::Baz(s) if s == &amp;&quot;Error&quot; =&gt; {},
# # _ =&gt; panic!(),
# # }
# # }
# ~~~
#
# **/
# #[macro_export]
# macro_rules! cherr {
# ( $k:expr ) =&gt; {
# ChainError::&lt;_&gt;::new($k, None, Some((line!(), file!())))
# };
# ( $e:expr, $k:expr ) =&gt; {
# ChainError::&lt;_&gt;::new($k, Some(Box::from($e)), Some((line!(), file!())))
# };
# }
#
# /** convenience macro for |e| cherr!(e, format!(…))
#
# # Examples
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# # fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(&quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# func2().map_err(mstrerr!(&quot;func1 error&quot;))?;
# Ok(())
# }
#
# # fn main() {
# # if let Err(e) = func1() {
# # assert_eq!(
# # format!(&quot;\n{:?}\n&quot;, e), r#&quot;
# # src/lib.rs:20: func1 error
# # Caused by:
# # src/lib.rs:15: Error reading 'foo.txt'
# # Caused by:
# # Kind(NotFound)
# # &quot;#
# # );
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
#
# `mstrerr!()` can also be used to map a new `ChainError&lt;T&gt;`, where T was defined with
# `derive_str_cherr!(T)`
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# # fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# derive_str_cherr!(Func2Error);
#
# fn func2() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(Func2Error, &quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# func2().map_err(mstrerr!(Func1Error, &quot;func1 error&quot;))?;
# Ok(())
# }
# #
# # fn main() {
# # if let Err(e) = func1() {
# # if let Some(f1err) = e.downcast_chain_ref::&lt;Func1Error&gt;() {
# # assert!(f1err.find_cause::&lt;ChainError&lt;Func2Error&gt;&gt;().is_some());
# # assert!(f1err.find_chain_cause::&lt;Func2Error&gt;().is_some());
# # } else {
# # panic!();
# # }
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
# **/
# #[macro_export]
# macro_rules! mstrerr {
# ( $t:ident, $v:expr $(, $more:expr)* ) =&gt; {
# |e| cherr!(e, $t (format!($v, $( $more , )* )))
# };
# ( $t:path, $v:expr $(, $more:expr)* ) =&gt; {
# |e| cherr!(e, $t (format!($v, $( $more , )* )))
# };
# ( $v:expr $(, $more:expr)* ) =&gt; {
# |e| cherr!(e, format!($v, $( $more , )* ))
# };
# }
#
# /** convenience macro to create a &quot;new type&quot; T(String) and implement Display + Debug for T
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# # fn do_some_io() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# derive_str_cherr!(Func2Error);
#
# fn func2() -&gt; ChainResult&lt;(), Func2Error&gt; {
# let filename = &quot;foo.txt&quot;;
# do_some_io().map_err(mstrerr!(Func2Error, &quot;Error reading '{}'&quot;, filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -&gt; Result&lt;(), Box&lt;Error&gt;&gt; {
# func2().map_err(mstrerr!(Func1Error, &quot;func1 error&quot;))?;
# Ok(())
# }
# #
# # fn main() {
# # if let Err(e) = func1() {
# # if let Some(f1err) = e.downcast_chain_ref::&lt;Func1Error&gt;() {
# # assert!(f1err.find_cause::&lt;ChainError&lt;Func2Error&gt;&gt;().is_some());
# # assert!(f1err.find_chain_cause::&lt;Func2Error&gt;().is_some());
# # } else {
# # panic!();
# # }
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
#
# **/
# #[macro_export]
# macro_rules! derive_str_cherr {
# ($e:ident) =&gt; {
# struct $e(String);
# impl ::std::fmt::Display for $e {
# fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# write!(f, &quot;{}&quot;, self.0)
# }
# }
# impl ::std::fmt::Debug for $e {
# fn fmt(&amp;self, f: &amp;mut ::std::fmt::Formatter) -&gt; ::std::fmt::Result {
# write!(f, &quot;{}({})&quot;, stringify!($e), self.0)
# }
# }
# impl ::std::error::Error for $e {}
# };
# }
# }
</code></pre></pre>
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