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< title > Downcast the Errors - chainerror< / title >
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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 >
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< p > < code > std::error::Error< / code > comes with some helper methods to get to the original object of the
< code > & (dyn Error + 'static)< / code > returned by < code > .source()< / code > .< / p >
< pre > < code class = "language-rust ignore" > pub fn downcast_ref< T: Error + 'static> (& self) -> Option< & T>
pub fn downcast_mut< T: Error + 'static> (& mut self) -> Option< & mut T>
< / code > < / pre >
< p > This is how it looks like, when using those:< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > use crate::chainerror::*;
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use std::error::Error;
use std::io;
use std::result::Result;
fn do_some_io() -> Result< (), Box< Error> > {
Err(io::Error::from(io::ErrorKind::NotFound))?;
Ok(())
}
fn func2() -> Result< (), Box< Error> > {
let filename = " foo.txt" ;
do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
Ok(())
}
fn func1() -> Result< (), Box< Error> > {
func2().map_err(mstrerr!(" func1 error" ))?;
Ok(())
}
fn main() -> Result< (), Box< Error> > {
if let Err(e) = func1() {
eprintln!(" Error: {}" , e);
let mut s = e.as_ref();
while let Some(c) = s.source() {
if let Some(ioerror) = c.downcast_ref::< io::Error> () {
eprintln!(" caused by: std::io::Error: {}" , ioerror);
match ioerror.kind() {
io::ErrorKind::NotFound => eprintln!(" of kind: std::io::ErrorKind::NotFound" ),
_ => {}
}
} else {
eprintln!(" caused by: {}" , c);
}
s = c;
}
}
Ok(())
}
# #[allow(dead_code)]
# mod chainerror {
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# /*!
#
# `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< T> ` struct, `chainerror` comes with some useful helper macros to save a lot of typing.
#
# # Examples
#
# ~~~rust
# use chainerror::*;
# use std::error::Error;
# use std::io;
# use std::result::Result;
#
# fn do_some_io() -> Result< (), Box< Error> > {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func2() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
# Ok(())
# }
#
# fn func1() -> Result< (), Box< Error> > {
# func2().map_err(mstrerr!(" func1 error" ))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# assert_eq!(
# format!(" \n{:?}\n" , e), r#"
# src/lib.rs:20: func1 error
# Caused by:
# src/lib.rs:15: Error reading 'foo.txt'
# Caused by:
# Kind(NotFound)
# " #
# );
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
#
# ~~~rust
# use chainerror::*;
# use std::error::Error;
# use std::io;
# use std::result::Result;
#
# fn do_some_io() -> Result< (), Box< Error> > {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func3() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -> ChainResult< (), Func2Error> {
# func3().map_err(mstrerr!(Func2Error, " func2 error: calling func3" ))?;
# Ok(())
# }
#
# enum Func1Error {
# Func2,
# IO(String),
# }
#
# impl ::std::fmt::Display for Func1Error {
# fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# match self {
# Func1Error::Func2 => write!(f, " func1 error calling func2" ),
# Func1Error::IO(filename) => write!(f, " Error reading '{}'" , filename),
# }
# }
# }
#
# impl ::std::fmt::Debug for Func1Error {
# fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# write!(f, " {}" , self)
# }
# }
#
# fn func1() -> ChainResult< (), Func1Error> {
# func2().map_err(|e| cherr!(e, Func1Error::Func2))?;
# let filename = String::from(" bar.txt" );
# 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 => {
# eprintln!(" Main Error Report: func1 error calling func2" );
# true
# }
# Func1Error::IO(filename) => {
# eprintln!(" Main Error Report: func1 error reading '{}'" , filename);
# false
# }
# }
# );
#
# assert!(e.find_chain_cause::< Func2Error> ().is_some());
#
# if let Some(e) = e.find_chain_cause::< Func2Error> () {
# eprintln!(" \nError reported by Func2Error: {}" , e)
# }
#
#
# assert!(e.root_cause().is_some());
#
# if let Some(e) = e.root_cause() {
# let ioerror = e.downcast_ref::< io::Error> ().unwrap();
# eprintln!(" \nThe root cause was: std::io::Error: {:#?}" , ioerror);
# }
#
# assert_eq!(
# format!(" \n{:?}\n" , e), r#"
# 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)
# " #
# );
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
# !*/
#
# use std::any::TypeId;
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# use std::error::Error;
# use std::fmt::{Debug, Display, Formatter, Result};
#
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# /** chains an inner error kind `T` with a causing error
# **/
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# pub struct ChainError< T> {
# #[cfg(not(feature = " no-fileline" ))]
# occurrence: Option< (u32, & 'static str)> ,
# kind: T,
# error_cause: Option< Box< dyn Error + 'static> > ,
# }
#
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# /// convenience type alias
# pub type ChainResult< O, E> = std::result::Result< O, ChainError< E> > ;
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#
# impl< T: 'static + Display + Debug> ChainError< T> {
# #[cfg(not(feature = " no-fileline" ))]
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# /// Use the `cherr!()` or `mstrerr!()` macro instead of calling this directly
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# pub fn new(
# kind: T,
# error_cause: Option< Box< dyn Error + 'static> > ,
# occurrence: Option< (u32, & 'static str)> ,
# ) -> Self {
# Self {
# occurrence,
# kind,
# error_cause,
# }
# }
#
# #[cfg(feature = " no-fileline" )]
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# /// Use the `cherr!()` or `mstrerr!()` macro instead of calling this directly
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# pub fn new(
# kind: T,
# error_cause: Option< Box< dyn Error + 'static> > ,
# _occurrence: Option< (u32, & 'static str)> ,
# ) -> Self {
# Self { kind, error_cause }
# }
#
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# /// return the root cause of the error chain, if any exists
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# pub fn root_cause(& self) -> Option< & (dyn Error + 'static)> {
# let mut cause = self as & (dyn Error + 'static);
# while let Some(c) = cause.source() {
# cause = c;
# }
# Some(cause)
# }
#
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# /** 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() -> Result< (), Box< Error> > {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -> Result< (), Box< Error> > {
# func2().map_err(mstrerr!(Func1Error, " func1 error" ))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# if let Some(f1err) = e.downcast_chain_ref::< Func1Error> () {
#
# assert!(f1err.find_cause::< io::Error> ().is_some());
#
# assert!(f1err.find_chain_cause::< Func2Error> ().is_some());
# }
# # else {
# # panic!();
# # }
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
# **/
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# pub fn find_cause< U: Error + 'static> (& self) -> Option< & U> {
# let mut cause = self as & (dyn Error + 'static);
# loop {
# if cause.is::< U> () {
# return cause.downcast_ref::< U> ();
# }
#
# match cause.source() {
# Some(c) => cause = c,
# None => return None,
# }
# }
# }
#
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# /** find the first error cause of type ChainError< U> , if any exists
#
# Same as `find_cause`, but hides the `ChainError< U> ` implementation internals
#
# # Examples
#
# ~~~rust,ignore
# /// Instead of writing
# err.find_cause::< ChainError< FooError> > ();
#
# /// leave out the ChainError< T> implementation detail
# err.find_chain_cause::< FooError> ();
# ~~~
#
# **/
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# pub fn find_chain_cause< U: Error + 'static> (& self) -> Option< & ChainError< U> > {
# let mut cause = self as & (dyn Error + 'static);
# loop {
# if cause.is::< ChainError< U> > () {
# return cause.downcast_ref::< ChainError< U> > ();
# }
#
# match cause.source() {
# Some(c) => cause = c,
# None => return None,
# }
# }
# }
#
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# /** return a reference to T of `ChainError< T> `
#
# # Examples
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# fn do_some_io() -> Result< (), Box< Error> > {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# derive_str_cherr!(Func2Error);
#
# fn func2() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
# Ok(())
# }
#
# #[derive(Debug)]
# enum Func1ErrorKind {
# Func2,
# IO(String),
# }
#
# // impl ::std::fmt::Display for Func1ErrorKind {…}
# # impl ::std::fmt::Display for Func1ErrorKind {
# # fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# # match self {
# # Func1ErrorKind::Func2 => write!(f, " func1 error calling func2" ),
# # Func1ErrorKind::IO(filename) => write!(f, " Error reading '{}'" , filename),
# # }
# # }
# # }
#
# fn func1() -> ChainResult< (), Func1ErrorKind> {
# func2().map_err(|e| cherr!(e, Func1ErrorKind::Func2))?;
# do_some_io().map_err(|e| cherr!(e, Func1ErrorKind::IO(" bar.txt" .into())))?;
# Ok(())
# }
#
# fn main() {
# if let Err(e) = func1() {
# match e.kind() {
# Func1ErrorKind::Func2 => {},
# Func1ErrorKind::IO(filename) => panic!(),
# }
# }
# # else {
# # unreachable!();
# # }
# }
# ~~~
#
# **/
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# pub fn kind< 'a> (& 'a self) -> & 'a T {
# & self.kind
# }
# }
#
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# /** convenience trait to hide the `ChainError< T> ` implementation internals
# **/
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# pub trait ChainErrorDown {
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# /** test if of type `ChainError< T> `
# **/
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# fn is_chain< T: 'static + Display + Debug> (& self) -> bool;
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# /** downcast to a reference of `ChainError< T> `
# **/
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# fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> > ;
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# /** downcast to a mutable reference of `ChainError< T> `
# **/
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# fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> > ;
# }
#
# impl< U: 'static + Display + Debug> ChainErrorDown for ChainError< U> {
# fn is_chain< T: 'static + Display + Debug> (& self) -> bool {
# TypeId::of::< T> () == TypeId::of::< U> ()
# }
#
# fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> > {
# if self.is_chain::< T> () {
# unsafe { Some(& *(self as *const dyn Error as *const & ChainError< T> )) }
# } else {
# None
# }
# }
#
# fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> > {
# if self.is_chain::< T> () {
# unsafe { Some(& mut *(self as *mut dyn Error as *mut & mut ChainError< T> )) }
# } else {
# None
# }
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static {
# fn is_chain< T: 'static + Display + Debug> (& self) -> bool {
# self.is::< ChainError< T> > ()
# }
#
# fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> > {
# self.downcast_ref::< ChainError< T> > ()
# }
#
# fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> > {
# self.downcast_mut::< ChainError< T> > ()
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static + Send {
# fn is_chain< T: 'static + Display + Debug> (& self) -> bool {
# self.is::< ChainError< T> > ()
# }
#
# fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> > {
# self.downcast_ref::< ChainError< T> > ()
# }
#
# fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> > {
# self.downcast_mut::< ChainError< T> > ()
# }
# }
#
# impl ChainErrorDown for dyn Error + 'static + Send + Sync {
# fn is_chain< T: 'static + Display + Debug> (& self) -> bool {
# self.is::< ChainError< T> > ()
# }
#
# fn downcast_chain_ref< T: 'static + Display + Debug> (& self) -> Option< & ChainError< T> > {
# self.downcast_ref::< ChainError< T> > ()
# }
#
# fn downcast_chain_mut< T: 'static + Display + Debug> (& mut self) -> Option< & mut ChainError< T> > {
# self.downcast_mut::< ChainError< T> > ()
# }
# }
#
# impl< T: 'static + Display + Debug> Error for ChainError< T> {
# fn source(& self) -> Option< & (dyn Error + 'static)> {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl< T: 'static + Display + Debug> Error for & ChainError< T> {
# fn source(& self) -> Option< & (dyn Error + 'static)> {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl< T: 'static + Display + Debug> Error for & mut ChainError< T> {
# fn source(& self) -> Option< & (dyn Error + 'static)> {
# if let Some(ref e) = self.error_cause {
# Some(e.as_ref())
# } else {
# None
# }
# }
# }
#
# impl< T: 'static + Display + Debug> Display for ChainError< T> {
# fn fmt(& self, f: & mut Formatter) -> Result {
# write!(f, " {}" , self.kind)?;
#
# #[cfg(feature = " display-cause" )]
# {
# if let Some(e) = self.source() {
# writeln!(f, " \nCaused by:" )?;
# Display::fmt(& e, f)?;
# }
# }
# Ok(())
# }
# }
#
# impl< T: 'static + Display + Debug> Debug for ChainError< T> {
# fn fmt(& self, f: & mut Formatter) -> Result {
# #[cfg(not(feature = " no-fileline" ))]
# {
# if let Some(o) = self.occurrence {
# write!(f, " {}:{}: " , o.1, o.0)?;
# }
# }
#
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# if self.is_chain::< String> () {
# Display::fmt(& self.kind, f)?;
# } else {
# Debug::fmt(& self.kind, f)?;
# }
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#
# #[cfg(not(feature = " no-debug-cause" ))]
# {
# if let Some(e) = self.source() {
# writeln!(f, " \nCaused by:" )?;
# Debug::fmt(& e, f)?;
# }
# }
# Ok(())
# }
# }
#
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# /** creates a new `ChainError< T> `
#
# # Examples
#
# Create a new ChainError< FooError> , where `FooError` must implement `Display` and `Debug`.
# ~~~rust
# # use chainerror::*;
# #
# # #[derive(Debug)]
# enum FooError {
# Bar,
# Baz(& 'static str),
# }
# #
# # impl ::std::fmt::Display for FooError {
# # fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# # match self {
# # FooError::Bar => write!(f, " Bar Error" ),
# # FooError::Baz(s) => write!(f, " Baz Error: '{}'" , s),
# # }
# # }
# # }
#
# // impl ::std::fmt::Display for FooError
#
# fn do_some_stuff() -> bool {
# false
# }
#
# fn func() -> ChainResult< (), FooError> {
# if ! do_some_stuff() {
# Err(cherr!(FooError::Baz(" Error" )))?;
# }
# Ok(())
# }
# #
# # pub fn main() {
# # match func().unwrap_err().kind() {
# # FooError::Baz(s) if s == & " Error" => {},
# # _ => panic!(),
# # }
# # }
# ~~~
#
# Additionally an error cause can be added.
#
# ~~~rust
# # use chainerror::*;
# # use std::io;
# # use std::error::Error;
# #
# # #[derive(Debug)]
# # enum FooError {
# # Bar,
# # Baz(& 'static str),
# # }
# #
# # impl ::std::fmt::Display for FooError {
# # fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# # match self {
# # FooError::Bar => write!(f, " Bar Error" ),
# # FooError::Baz(s) => write!(f, " Baz Error: '{}'" , s),
# # }
# # }
# # }
# #
# fn do_some_stuff() -> Result< (), Box< Error> > {
# Err(io::Error::from(io::ErrorKind::NotFound))?;
# Ok(())
# }
#
# fn func() -> ChainResult< (), FooError> {
# do_some_stuff().map_err(
# |e| cherr!(e, FooError::Baz(" Error" ))
# )?;
# Ok(())
# }
# #
# # pub fn main() {
# # match func().unwrap_err().kind() {
# # FooError::Baz(s) if s == & " Error" => {},
# # _ => panic!(),
# # }
# # }
# ~~~
#
# **/
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# #[macro_export]
# macro_rules! cherr {
# ( $k:expr ) => {
# ChainError::< _> ::new($k, None, Some((line!(), file!())))
# };
# ( $e:expr, $k:expr ) => {
# ChainError::< _> ::new($k, Some(Box::from($e)), Some((line!(), file!())))
# };
# }
#
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# /** 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() -> Result< (), Box< Error> > {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# fn func2() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(" Error reading '{}'" , filename))?;
# Ok(())
# }
#
# fn func1() -> Result< (), Box< Error> > {
# func2().map_err(mstrerr!(" func1 error" ))?;
# Ok(())
# }
#
# # fn main() {
# # if let Err(e) = func1() {
# # assert_eq!(
# # format!(" \n{:?}\n" , e), r#"
# # src/lib.rs:20: func1 error
# # Caused by:
# # src/lib.rs:15: Error reading 'foo.txt'
# # Caused by:
# # Kind(NotFound)
# # " #
# # );
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
#
# `mstrerr!()` can also be used to map a new `ChainError< T> `, 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() -> Result< (), Box< Error> > {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# derive_str_cherr!(Func2Error);
#
# fn func2() -> Result< (), Box< Error> > {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -> Result< (), Box< Error> > {
# func2().map_err(mstrerr!(Func1Error, " func1 error" ))?;
# Ok(())
# }
# #
# # fn main() {
# # if let Err(e) = func1() {
# # if let Some(f1err) = e.downcast_chain_ref::< Func1Error> () {
# # assert!(f1err.find_cause::< ChainError< Func2Error> > ().is_some());
# # assert!(f1err.find_chain_cause::< Func2Error> ().is_some());
# # } else {
# # panic!();
# # }
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
# **/
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# #[macro_export]
# macro_rules! mstrerr {
# ( $t:ident, $v:expr $(, $more:expr)* ) => {
# |e| cherr!(e, $t (format!($v, $( $more , )* )))
# };
# ( $t:path, $v:expr $(, $more:expr)* ) => {
# |e| cherr!(e, $t (format!($v, $( $more , )* )))
# };
# ( $v:expr $(, $more:expr)* ) => {
# |e| cherr!(e, format!($v, $( $more , )* ))
# };
# }
#
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# /** convenience macro to create a " new type" T(String) and implement Display + Debug for T
#
# ~~~rust
# # use crate::chainerror::*;
# # use std::error::Error;
# # use std::io;
# # use std::result::Result;
# #
# # fn do_some_io() -> Result< (), Box< Error> > {
# # Err(io::Error::from(io::ErrorKind::NotFound))?;
# # Ok(())
# # }
# #
# derive_str_cherr!(Func2Error);
#
# fn func2() -> ChainResult< (), Func2Error> {
# let filename = " foo.txt" ;
# do_some_io().map_err(mstrerr!(Func2Error, " Error reading '{}'" , filename))?;
# Ok(())
# }
#
# derive_str_cherr!(Func1Error);
#
# fn func1() -> Result< (), Box< Error> > {
# func2().map_err(mstrerr!(Func1Error, " func1 error" ))?;
# Ok(())
# }
# #
# # fn main() {
# # if let Err(e) = func1() {
# # if let Some(f1err) = e.downcast_chain_ref::< Func1Error> () {
# # assert!(f1err.find_cause::< ChainError< Func2Error> > ().is_some());
# # assert!(f1err.find_chain_cause::< Func2Error> ().is_some());
# # } else {
# # panic!();
# # }
# # } else {
# # unreachable!();
# # }
# # }
# ~~~
#
# **/
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# #[macro_export]
# macro_rules! derive_str_cherr {
# ($e:ident) => {
# struct $e(String);
# impl ::std::fmt::Display for $e {
# fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# write!(f, " {}" , self.0)
# }
# }
# impl ::std::fmt::Debug for $e {
# fn fmt(& self, f: & mut ::std::fmt::Formatter) -> ::std::fmt::Result {
# write!(f, " {}({})" , stringify!($e), self.0)
# }
# }
# impl ::std::error::Error for $e {}
# };
# }
# }
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