# Selective Error Handling

What about functions returning different Error types?

In this example `func1()` can return either `Func1ErrorFunc2` or `Func1ErrorIO`.

We might want to `match` on `func1()` with something like:

    fn main() -> Result<(), Box<Error + Send + Sync>> {
        match func1() {
            Err(e) if let Some(s) = e.downcast_chain_ref::<Func1ErrorIO>() =>
            eprintln!("Func1ErrorIO:\n{:?}", s),

            Err(e) if let Some(s) = e.downcast_chain_ref::<Func1ErrorFunc2>() =>
            eprintln!("Func1ErrorFunc2:\n{:?}", s),

            Ok(_) => {},
        }
        Ok(())
    }

but this is not valid rust code, so we end up doing it the hard way. In the next chapter, we will see, how to solve this more elegantly.

    use chainerror::prelude::v1::*;
    use std::error::Error;
    use std::io;
    use std::result::Result;

    fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> {
        Err(io::Error::from(io::ErrorKind::NotFound))?;
        Ok(())
    }

    derive_str_context!(Func2Error);

    fn func2() -> Result<(), Box<dyn Error + Send + Sync>> {
        let filename = "foo.txt";
        do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?;
        Ok(())
    }

    derive_str_context!(Func1ErrorFunc2);
    derive_str_context!(Func1ErrorIO);

    fn func1() -> Result<(), Box<dyn Error + Send + Sync>> {
        func2().context(Func1ErrorFunc2(format!("func1 error calling func2")))?;
        let filename = "bar.txt";
        do_some_io().context(Func1ErrorIO(format!("Error reading '{}'", filename)))?;
        Ok(())
    }

    fn main() -> Result<(), Box<dyn Error + Send + Sync>> {
        if let Err(e) = func1() {
            if let Some(s) = e.downcast_ref::<ChainError<Func1ErrorIO>>() {
                eprintln!("Func1ErrorIO:\n{:?}", s);
            }

            if let Some(s) = e.downcast_chain_ref::<Func1ErrorFunc2>() {
                eprintln!("Func1ErrorFunc2:\n{:?}", s);
            }
            std::process::exit(1);
        }
        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 `#[track_caller]` and `Location` 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.
    //!
    //! Debug information is worth it!
    //!
    //! ## Features
    //!
    //! `display-cause`
    //! : turn on printing a backtrace of the errors in `Display`
    //!
    //! # Tutorial
    //!
    //! Read the [Tutorial](https://haraldh.github.io/chainerror/tutorial1.html)
    //!
    //! # Examples
    //!
    //! examples/example.rs:
    //! ```rust,ignore
    //! // […]
    //! fn main() {
    //!    if let Err(e) = func1() {
    //!        eprintln!("\nDebug Error {{:?}}:\n{:?}", e);
    //!        eprintln!("\nAlternative Debug Error {{:#?}}:\n{:#?}\n", e);
    //!        // […]
    //!   }
    //! }
    //! ```
    //!
    //! ```console
    //! $ cargo run -q --example example
    //! Debug Error {:?}:
    //! examples/example.rs:46:13: func1 error calling func2
    //! Caused by:
    //! examples/example.rs:21:13: Func2Error(func2 error: calling func3)
    //! Caused by:
    //! examples/example.rs:14:18: Error reading 'foo.txt'
    //! Caused by:
    //! Kind(NotFound)
    //!
    //! Alternative Debug Error {:#?}:
    //! ChainError<example::Func1Error> {
    //!     occurrence: Some(
    //!         "examples/example.rs:46:13",
    //!     ),
    //!     kind: func1 error calling func2,
    //!     source: Some(
    //!         ChainError<example::Func2Error> {
    //!             occurrence: Some(
    //!                 "examples/example.rs:21:13",
    //!             ),
    //!             kind: Func2Error(func2 error: calling func3),
    //!             source: Some(
    //!                 ChainError<alloc::string::String> {
    //!                     occurrence: Some(
    //!                         "examples/example.rs:14:18",
    //!                    ),
    //!                    kind: "Error reading \'foo.txt\'",
    //!                     source: Some(
    //!                         Kind(
    //!                             NotFound,
    //!                         ),
    //!                     ),
    //!                 },
    //!             ),
    //!         },
    //!     ),
    //! }
    //! ```
    //!
    //! ```rust
    //! use chainerror::prelude::v1::*;
    //! use std::error::Error;
    //! use std::io;
    //! use std::result::Result;
    //!
    //! 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>> {
    //!     func2().context("func1 error")?;
    //!     Ok(())
    //! }
    //!
    //! if let Err(e) = func1() {
    //!     #[cfg(not(windows))]
    //!     assert_eq!(
    //!         format!("\n{:?}\n", e),
    //!         r#"
    //! src/lib.rs:21:13: func1 error
    //! Caused by:
    //! src/lib.rs:16:18: Error reading 'foo.txt'
    //! Caused by:
    //! Kind(NotFound)
    //! "#
    //!     );
    //! }
    //! #    else {
    //! #        unreachable!();
    //! #    }
    //! ```
    //!
    //!
    //! ```rust
    //! use chainerror::prelude::v1::*;
    //! use std::error::Error;
    //! use std::io;
    //! use std::result::Result;
    //!
    //! fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> {
    //!     Err(io::Error::from(io::ErrorKind::NotFound))?;
    //!     Ok(())
    //! }
    //!
    //! fn func3() -> Result<(), Box<dyn Error + Send + Sync>> {
    //!     let filename = "foo.txt";
    //!     do_some_io().context(format!("Error reading '{}'", filename))?;
    //!     Ok(())
    //! }
    //!
    //! derive_str_context!(Func2Error);
    //!
    //! fn func2() -> ChainResult<(), Func2Error> {
    //!     func3().context(Func2Error("func2 error: calling func3".into()))?;
    //!     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().context(Func1Error::Func2)?;
    //!     let filename = String::from("bar.txt");
    //!     do_some_io().context(Func1Error::IO(filename))?;
    //!     Ok(())
    //! }
    //!
    //! 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 io_error = e.downcast_ref::<io::Error>().unwrap();
    //!         eprintln!("\nThe root cause was: std::io::Error: {:#?}", io_error);
    //!     }
    //!
    //!     #[cfg(not(windows))]
    //!     assert_eq!(
    //!         format!("\n{:?}\n", e),
    //!         r#"
    //! src/lib.rs:48:13: func1 error calling func2
    //! Caused by:
    //! src/lib.rs:23:13: Func2Error(func2 error: calling func3)
    //! Caused by:
    //! src/lib.rs:16:18: Error reading 'foo.txt'
    //! Caused by:
    //! Kind(NotFound)
    //! "#
    //!     );
    //! }
    //! #    else {
    //! #        unreachable!();
    //! #    }
    //! ```

    #![deny(clippy::all)]
    #![deny(clippy::integer_arithmetic)]
    #![deny(missing_docs)]

    use std::any::TypeId;
    use std::error::Error;
    use std::fmt::{Debug, Display, Formatter, Result};
    use std::panic::Location;

    pub mod prelude {
        //! convenience prelude
        pub mod v1 {
            //! convenience prelude
            pub use super::super::ChainErrorDown as _;
            pub use super::super::ResultTrait as _;
            pub use super::super::{ChainError, ChainResult};
            pub use crate::{derive_err_kind, derive_str_context};
        }
    }

    /// chains an inner error kind `T` with a causing error
    pub struct ChainError<T> {
        occurrence: Option<String>,
        kind: T,
        error_cause: Option<Box<dyn Error + 'static + Send + Sync>>,
    }

    /// convenience type alias
    pub type ChainResult<O, E> = std::result::Result<O, ChainError<E>>;

    impl<T: 'static + Display + Debug> ChainError<T> {
        /// Use the `context()` or `map_context()` Result methods instead of calling this directly
        #[inline]
        pub fn new(
            kind: T,
            error_cause: Option<Box<dyn Error + 'static + Send + Sync>>,
            occurrence: Option<String>,
        ) -> Self {
            Self {
                occurrence,
                kind,
                error_cause,
            }
        }

        /// return the root cause of the error chain, if any exists
        pub fn root_cause(&self) -> Option<&(dyn Error + 'static)> {
            self.iter().last()
        }

        /// Find the first error cause of type U, if any exists
        ///
        /// # Examples
        ///
        /// ```rust
        /// use chainerror::prelude::v1::*;
        /// 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(())
        /// }
        ///
        /// derive_str_context!(Func2Error);
        ///
        /// fn func2() -> Result<(), Box<dyn Error + Send + Sync>> {
        ///     let filename = "foo.txt";
        ///     do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?;
        ///     Ok(())
        /// }
        ///
        /// derive_str_context!(Func1Error);
        ///
        /// fn func1() -> Result<(), Box<dyn Error + Send + Sync>> {
        ///     func2().context(Func1Error("func1 error".into()))?;
        ///     Ok(())
        /// }
        ///
        /// 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!();
        /// #    }
        /// ```
        #[inline]
        pub fn find_cause<U: Error + 'static>(&self) -> Option<&U> {
            self.iter().filter_map(Error::downcast_ref::<U>).next()
        }

        /// 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
        /// # use chainerror::prelude::v1::*;
        /// # derive_str_context!(FooError);
        /// # let err = ChainError::new(String::new(), None, None);
        /// // Instead of writing
        /// err.find_cause::<ChainError<FooError>>();
        ///
        /// // leave out the ChainError<FooError> implementation detail
        /// err.find_chain_cause::<FooError>();
        /// ```
        #[inline]
        pub fn find_chain_cause<U: Error + 'static>(&self) -> Option<&ChainError<U>> {
            self.iter()
                .filter_map(Error::downcast_ref::<ChainError<U>>)
                .next()
        }

        /// Find the first error cause of type `ChainError<U>` or `U`, if any exists and return `U`
        ///
        /// Same as `find_cause` and `find_chain_cause`, but hides the `ChainError<U>` implementation internals
        ///
        /// # Examples
        ///
        /// ```rust
        /// # use chainerror::prelude::v1::*;
        /// # derive_str_context!(FooErrorKind);
        /// # let err = ChainError::new(String::new(), None, None);
        /// // Instead of writing
        /// err.find_cause::<ChainError<FooErrorKind>>();
        /// // and/or
        /// err.find_chain_cause::<FooErrorKind>();
        /// // and/or
        /// err.find_cause::<FooErrorKind>();
        ///
        /// // leave out the ChainError<FooErrorKind> implementation detail
        /// err.find_kind_or_cause::<FooErrorKind>();
        /// ```
        #[inline]
        pub fn find_kind_or_cause<U: Error + 'static>(&self) -> Option<&U> {
            self.iter()
                .filter_map(|e| {
                    e.downcast_ref::<ChainError<U>>()
                        .map(|e| e.kind())
                        .or_else(|| e.downcast_ref::<U>())
                })
                .next()
        }

        /// Return a reference to T of `ChainError<T>`
        ///
        /// # Examples
        ///
        /// ```rust
        /// use chainerror::prelude::v1::*;
        /// 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(())
        /// }
        ///
        /// derive_str_context!(Func2Error);
        ///
        /// fn func2() -> Result<(), Box<dyn Error + Send + Sync>> {
        ///     let filename = "foo.txt";
        ///     do_some_io().context(Func2Error(format!("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().context(Func1ErrorKind::Func2)?;
        ///     do_some_io().context(Func1ErrorKind::IO("bar.txt".into()))?;
        ///     Ok(())
        /// }
        ///
        /// if let Err(e) = func1() {
        ///     match e.kind() {
        ///         Func1ErrorKind::Func2 => {}
        ///         Func1ErrorKind::IO(filename) => panic!(),
        ///     }
        /// }
        /// #    else {
        /// #         unreachable!();
        /// #    }
        /// ```
        #[inline]
        pub fn kind(&self) -> &T {
            &self.kind
        }

        /// Returns an Iterator over all error causes/sources
        ///
        /// # Example
        #[inline]
        pub fn iter(&self) -> impl Iterator<Item = &(dyn Error + 'static)> {
            ErrorIter {
                current: Some(self),
            }
        }
    }

    /// Convenience methods for `Result<>` to turn the error into a decorated ChainError
    pub trait ResultTrait<O, E: Into<Box<dyn Error + 'static + Send + Sync>>> {
        /// Decorate the error with a `kind` of type `T` and the source `Location`
        fn context<T: 'static + Display + Debug>(
            self,
            kind: T,
        ) -> std::result::Result<O, ChainError<T>>;

        /// Decorate the `error` with a `kind` of type `T` produced with a `FnOnce(&error)` and the source `Location`
        fn map_context<T: 'static + Display + Debug, F: FnOnce(&E) -> T>(
            self,
            op: F,
        ) -> std::result::Result<O, ChainError<T>>;
    }

    impl<O, E: Into<Box<dyn Error + 'static + Send + Sync>>> ResultTrait<O, E>
        for std::result::Result<O, E>
    {
        #[track_caller]
        fn context<T: 'static + Display + Debug>(
            self,
            kind: T,
        ) -> std::result::Result<O, ChainError<T>> {
            match self {
                Ok(t) => Ok(t),
                Err(error_cause) => Err(ChainError::new(
                    kind,
                    Some(error_cause.into()),
                    Some(Location::caller().to_string()),
                )),
            }
        }

        #[track_caller]
        fn map_context<T: 'static + Display + Debug, F: FnOnce(&E) -> T>(
            self,
            op: F,
        ) -> std::result::Result<O, ChainError<T>> {
            match self {
                Ok(t) => Ok(t),
                Err(error_cause) => {
                    let kind = op(&error_cause);
                    Err(ChainError::new(
                        kind,
                        Some(error_cause.into()),
                        Some(Location::caller().to_string()),
                    ))
                }
            }
        }
    }

    /// An iterator over all error causes/sources
    pub struct ErrorIter<'a> {
        current: Option<&'a (dyn Error + 'static)>,
    }

    impl<'a> Iterator for ErrorIter<'a> {
        type Item = &'a (dyn Error + 'static);

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            let current = self.current;
            self.current = self.current.and_then(Error::source);
            current
        }
    }

    impl<T: 'static + Display + Debug> std::ops::Deref for ChainError<T> {
        type Target = T;

        #[inline]
        fn deref(&self) -> &Self::Target {
            &self.kind
        }
    }

    /// Convenience trait to hide the `ChainError<T>` implementation internals
    pub trait ChainErrorDown {
        /// Test if of type `ChainError<T>`
        fn is_chain<T: 'static + Display + Debug>(&self) -> bool;
        /// Downcast to a reference of `ChainError<T>`
        fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&ChainError<T>>;
        /// Downcast to a mutable reference of `ChainError<T>`
        fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut ChainError<T>>;
        /// Downcast to T of `ChainError<T>`
        fn downcast_inner_ref<T: 'static + Error>(&self) -> Option<&T>;
        /// Downcast to T mutable reference of `ChainError<T>`
        fn downcast_inner_mut<T: 'static + Error>(&mut self) -> Option<&mut T>;
    }

    impl<U: 'static + Display + Debug> ChainErrorDown for ChainError<U> {
        #[inline]
        fn is_chain<T: 'static + Display + Debug>(&self) -> bool {
            TypeId::of::<T>() == TypeId::of::<U>()
        }

        #[inline]
        fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&ChainError<T>> {
            if self.is_chain::<T>() {
                #[allow(clippy::cast_ptr_alignment)]
                unsafe {
                    #[allow(trivial_casts)]
                    Some(&*(self as *const dyn Error as *const &ChainError<T>))
                }
            } else {
                None
            }
        }

        #[inline]
        fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut ChainError<T>> {
            if self.is_chain::<T>() {
                #[allow(clippy::cast_ptr_alignment)]
                unsafe {
                    #[allow(trivial_casts)]
                    Some(&mut *(self as *mut dyn Error as *mut &mut ChainError<T>))
                }
            } else {
                None
            }
        }
        #[inline]
        fn downcast_inner_ref<T: 'static + Error>(&self) -> Option<&T> {
            if self.is_chain::<T>() {
                #[allow(clippy::cast_ptr_alignment)]
                unsafe {
                    #[allow(trivial_casts)]
                    Some(&(*(self as *const dyn Error as *const &ChainError<T>)).kind)
                }
            } else {
                None
            }
        }

        #[inline]
        fn downcast_inner_mut<T: 'static + Error>(&mut self) -> Option<&mut T> {
            if self.is_chain::<T>() {
                #[allow(clippy::cast_ptr_alignment)]
                unsafe {
                    #[allow(trivial_casts)]
                    Some(&mut (*(self as *mut dyn Error as *mut &mut ChainError<T>)).kind)
                }
            } else {
                None
            }
        }
    }

    impl ChainErrorDown for dyn Error + 'static {
        #[inline]
        fn is_chain<T: 'static + Display + Debug>(&self) -> bool {
            self.is::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&ChainError<T>> {
            self.downcast_ref::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut ChainError<T>> {
            self.downcast_mut::<ChainError<T>>()
        }

        #[inline]
        fn downcast_inner_ref<T: 'static + Error>(&self) -> Option<&T> {
            self.downcast_ref::<T>()
                .or_else(|| self.downcast_ref::<ChainError<T>>().map(|e| e.kind()))
        }

        #[inline]
        fn downcast_inner_mut<T: 'static + Error>(&mut self) -> Option<&mut T> {
            if self.is::<T>() {
                return self.downcast_mut::<T>();
            }

            self.downcast_mut::<ChainError<T>>()
                .and_then(|e| e.downcast_inner_mut::<T>())
        }
    }

    impl ChainErrorDown for dyn Error + 'static + Send {
        #[inline]
        fn is_chain<T: 'static + Display + Debug>(&self) -> bool {
            self.is::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&ChainError<T>> {
            self.downcast_ref::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut ChainError<T>> {
            self.downcast_mut::<ChainError<T>>()
        }

        #[inline]
        fn downcast_inner_ref<T: 'static + Error>(&self) -> Option<&T> {
            self.downcast_ref::<T>()
                .or_else(|| self.downcast_ref::<ChainError<T>>().map(|e| e.kind()))
        }

        #[inline]
        fn downcast_inner_mut<T: 'static + Error>(&mut self) -> Option<&mut T> {
            if self.is::<T>() {
                return self.downcast_mut::<T>();
            }

            self.downcast_mut::<ChainError<T>>()
                .and_then(|e| e.downcast_inner_mut::<T>())
        }
    }

    impl ChainErrorDown for dyn Error + 'static + Send + Sync {
        #[inline]
        fn is_chain<T: 'static + Display + Debug>(&self) -> bool {
            self.is::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_ref<T: 'static + Display + Debug>(&self) -> Option<&ChainError<T>> {
            self.downcast_ref::<ChainError<T>>()
        }

        #[inline]
        fn downcast_chain_mut<T: 'static + Display + Debug>(&mut self) -> Option<&mut ChainError<T>> {
            self.downcast_mut::<ChainError<T>>()
        }

        #[inline]
        fn downcast_inner_ref<T: 'static + Error>(&self) -> Option<&T> {
            self.downcast_ref::<T>()
                .or_else(|| self.downcast_ref::<ChainError<T>>().map(|e| e.kind()))
        }

        #[inline]
        fn downcast_inner_mut<T: 'static + Error>(&mut self) -> Option<&mut T> {
            if self.is::<T>() {
                return self.downcast_mut::<T>();
            }

            self.downcast_mut::<ChainError<T>>()
                .and_then(|e| e.downcast_inner_mut::<T>())
        }
    }

    impl<T: 'static + Display + Debug> Error for ChainError<T> {
        #[inline]
        fn source(&self) -> Option<&(dyn Error + 'static)> {
            self.error_cause
                .as_ref()
                .map(|e| e.as_ref() as &(dyn Error + 'static))
        }
    }

    impl<T: 'static + Display + Debug> Error for &ChainError<T> {
        #[inline]
        fn source(&self) -> Option<&(dyn Error + 'static)> {
            self.error_cause
                .as_ref()
                .map(|e| e.as_ref() as &(dyn Error + 'static))
        }
    }

    impl<T: 'static + Display + Debug> Error for &mut ChainError<T> {
        #[inline]
        fn source(&self) -> Option<&(dyn Error + 'static)> {
            self.error_cause
                .as_ref()
                .map(|e| e.as_ref() as &(dyn Error + 'static))
        }
    }

    impl<T: 'static + Display + Debug> Display for ChainError<T> {
        #[inline]
        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> {
        #[inline]
        fn fmt(&self, f: &mut Formatter<'_>) -> Result {
            if f.alternate() {
                let mut f = f.debug_struct(&format!("ChainError<{}>", std::any::type_name::<T>()));

                let f = f
                    .field("occurrence", &self.occurrence)
                    .field("kind", &self.kind)
                    .field("source", &self.source());

                f.finish()
            } else {
                if let Some(ref o) = self.occurrence {
                    write!(f, "{}: ", o)?;
                }

                if TypeId::of::<String>() == TypeId::of::<T>()
                    || TypeId::of::<&str>() == TypeId::of::<T>()
                {
                    Display::fmt(&self.kind, f)?;
                } else {
                    Debug::fmt(&self.kind, f)?;
                }

                if let Some(e) = self.source() {
                    writeln!(f, "\nCaused by:")?;
                    Debug::fmt(&e, f)?;
                }
                Ok(())
            }
        }
    }

    /// `ChainErrorFrom<T>` is similar to `From<T>`
    pub trait ChainErrorFrom<T>: Sized {
        /// similar to From<T>::from()
        fn chain_error_from(from: T, line_filename: Option<String>) -> ChainError<Self>;
    }

    /// `IntoChainError<T>` is similar to `Into<T>`
    pub trait IntoChainError<T>: Sized {
        /// similar to Into<T>::into()
        fn into_chain_error(self, line_filename: Option<String>) -> ChainError<T>;
    }

    impl<T, U> IntoChainError<U> for T
    where
        U: ChainErrorFrom<T>,
    {
        #[inline]
        fn into_chain_error(self, line_filename: Option<String>) -> ChainError<U> {
            U::chain_error_from(self, line_filename)
        }
    }

    impl<T, U> ChainErrorFrom<T> for U
    where
        T: Into<U>,
        U: 'static + Display + Debug,
    {
        #[inline]
        fn chain_error_from(t: T, line_filename: Option<String>) -> ChainError<Self> {
            let e: U = t.into();
            ChainError::new(e, None, line_filename)
        }
    }

    /// Convenience macro to create a "new type" T(String) and implement Display + Debug for T
    ///
    /// # Examples
    ///
    /// ```rust
    /// # use crate::chainerror::*;
    /// # use std::error::Error;
    /// # use std::io;
    /// # use std::result::Result;
    /// # fn do_some_io() -> Result<(), Box<dyn Error + Send + Sync>> {
    /// #     Err(io::Error::from(io::ErrorKind::NotFound))?;
    /// #     Ok(())
    /// # }
    /// derive_str_context!(Func2Error);
    ///
    /// fn func2() -> ChainResult<(), Func2Error> {
    ///     let filename = "foo.txt";
    ///     do_some_io().context(Func2Error(format!("Error reading '{}'", filename)))?;
    ///     Ok(())
    /// }
    ///
    /// derive_str_context!(Func1Error);
    ///
    /// fn func1() -> Result<(), Box<dyn Error>> {
    ///     func2().context(Func1Error("func1 error".into()))?;
    ///     Ok(())
    /// }
    /// #     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!();
    /// #     }
    /// ```
    #[macro_export]
    macro_rules! derive_str_context {
        ($e:ident) => {
            #[derive(Clone)]
            pub struct $e(pub 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 {}
        };
    }

    /// Derive an Error for an ErrorKind, which wraps a `ChainError` and implements a `kind()` method
    ///
    /// It basically hides `ChainError` to the outside and only exposes the `kind()`
    /// method.
    ///
    /// Error::kind() returns the ErrorKind
    /// Error::source() returns the parent error
    ///
    /// # Examples
    ///
    /// ```rust
    /// use chainerror::prelude::v1::*;
    /// use std::io;
    ///
    /// fn do_some_io(_f: &str) -> std::result::Result<(), io::Error> {
    ///     return Err(io::Error::from(io::ErrorKind::NotFound));
    /// }
    ///
    /// #[derive(Debug, Clone)]
    /// pub enum ErrorKind {
    ///     IO(String),
    ///     FatalError(String),
    ///     Unknown,
    /// }
    ///
    /// derive_err_kind!(Error, ErrorKind);
    ///
    /// impl std::fmt::Display for ErrorKind {
    ///     fn fmt(&self, f: &mut ::std::fmt::Formatter) -> std::fmt::Result {
    ///         match self {
    ///             ErrorKind::FatalError(e) => write!(f, "fatal error {}", e),
    ///             ErrorKind::Unknown => write!(f, "unknown error"),
    ///             ErrorKind::IO(filename) => write!(f, "Error reading '{}'", filename),
    ///         }
    ///     }
    /// }
    ///
    /// impl ErrorKind {
    ///     fn from_io_error(e: &io::Error, f: String) -> Self {
    ///         match e.kind() {
    ///             io::ErrorKind::BrokenPipe => panic!("Should not happen"),
    ///             io::ErrorKind::ConnectionReset => {
    ///                 ErrorKind::FatalError(format!("While reading `{}`: {}", f, e))
    ///             }
    ///             _ => ErrorKind::IO(f),
    ///         }
    ///     }
    /// }
    ///
    /// impl From<&io::Error> for ErrorKind {
    ///     fn from(e: &io::Error) -> Self {
    ///         ErrorKind::IO(format!("{}", e))
    ///     }
    /// }
    ///
    /// pub fn func1() -> std::result::Result<(), Error> {
    ///     let filename = "bar.txt";
    ///
    ///     do_some_io(filename).map_context(|e| ErrorKind::from_io_error(e, filename.into()))?;
    ///     do_some_io(filename).map_context(|e| ErrorKind::IO(filename.into()))?;
    ///     do_some_io(filename).map_context(|e| ErrorKind::from(e))?;
    ///     Ok(())
    /// }
    /// ```
    #[macro_export]
    macro_rules! derive_err_kind {
        ($e:ident, $k:ident) => {
            pub struct $e($crate::ChainError<$k>);

            impl $e {
                pub fn kind(&self) -> &$k {
                    self.0.kind()
                }
            }

            impl From<$k> for $e {
                fn from(e: $k) -> Self {
                    $e($crate::ChainError::new(e, None, None))
                }
            }

            impl From<ChainError<$k>> for $e {
                fn from(e: $crate::ChainError<$k>) -> Self {
                    $e(e)
                }
            }

            impl From<&$e> for $k
            where
                $k: Clone,
            {
                fn from(e: &$e) -> Self {
                    e.kind().clone()
                }
            }

            impl std::error::Error for $e {
                fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                    self.0.source()
                }
            }

            impl std::fmt::Display for $e {
                fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                    std::fmt::Display::fmt(&self.0, f)
                }
            }

            impl std::fmt::Debug for $e {
                fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                    std::fmt::Debug::fmt(&self.0, f)
                }
            }
        };
    }
    }