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    use core::fmt::{self, Display, Debug};

    use core::mem;
    use core::ptr;

    use {Causes, Fail};
    use backtrace::Backtrace;
    use context::Context;
    use compat::Compat;

    /// The `Error` type, which can contain any failure.
    ///
    /// Functions which accumulate many kinds of errors should return this type.
    /// All failures can be converted into it, so functions which catch those
    /// errors can be tried with `?` inside of a function that returns this kind
    /// of error.
    ///
    /// In addition to implementing `Debug` and `Display`, this type carries `Backtrace`
    /// information, and can be downcast into the failure that underlies it for
    /// more detailed inspection.
    pub struct Error {
        pub(crate) inner: Box<Inner<Fail>>,
    }

    pub(crate) struct Inner<F: ?Sized + Fail> {
        backtrace: Backtrace,
        pub(crate) failure: F,
    }

    impl<F: Fail> From<F> for Error {
        fn from(failure: F) -> Error {
            let inner: Inner<F> = {
                let backtrace = if failure.backtrace().is_none() {
                    Backtrace::new()
                } else { Backtrace::none() };
                Inner { failure, backtrace }
            };
            Error { inner: Box::new(inner) }
        }
    }

    impl Error {
        /// Returns a reference to the underlying cause of this `Error`. Unlike the
        /// method on `Fail`, this does not return an `Option`. The `Error` type
        /// always has an underlying failure.
        pub fn cause(&self) -> &Fail {
            &self.inner.failure
        }

        /// Gets a reference to the `Backtrace` for this `Error`.
        ///
        /// If the failure this wrapped carried a backtrace, that backtrace will
        /// be returned. Otherwise, the backtrace will have been constructed at
        /// the point that failure was cast into the `Error` type.
        pub fn backtrace(&self) -> &Backtrace {
            self.inner.failure.backtrace().unwrap_or(&self.inner.backtrace)
        }

        /// Provides context for this `Error`.
        ///
        /// This can provide additional information about this error, appropriate
        /// to the semantics of the current layer. That is, if you have a
        /// lower-level error, such as an IO error, you can provide additional context
        /// about what that error means in the context of your function. This
        /// gives users of this function more information about what has gone
        /// wrong.
        ///
        /// This takes any type that implements `Display`, as well as
        /// `Send`/`Sync`/`'static`. In practice, this means it can take a `String`
        /// or a string literal, or a failure, or some other custom context-carrying
        /// type.
        pub fn context<D: Display + Send + Sync + 'static>(self, context: D) -> Context<D> {
            Context::with_err(context, self)
        }

        /// Wraps `Error` in a compatibility type.
        ///
        /// This type implements the `Error` trait from `std::error`. If you need
        /// to pass failure's `Error` to an interface that takes any `Error`, you
        /// can use this method to get a compatible type.
        pub fn compat(self) -> Compat<Error> {
            Compat { error: self }
        }

        /// Attempts to downcast this `Error` to a particular `Fail` type.
        ///
        /// This downcasts by value, returning an owned `T` if the underlying
        /// failure is of the type `T`. For this reason it returns a `Result` - in
        /// the case that the underlying error is of a different type, the
        /// original `Error` is returned.
        pub fn downcast<T: Fail>(self) -> Result<T, Error> {
            let ret: Option<T> = self.downcast_ref().map(|fail| {
                unsafe {
                    // drop the backtrace
                    let _ = ptr::read(&self.inner.backtrace as *const Backtrace);
                    // read out the fail type
                    ptr::read(fail as *const T)
                }
            });
            match ret {
                Some(ret) => {
                    // forget self (backtrace is dropped, failure is moved
                    mem::forget(self);
                    Ok(ret)
                }
                _ => Err(self)
            }
        }

        /// Returns the "root cause" of this error - the last value in the
        /// cause chain which does not return an underlying `cause`.
        pub fn root_cause(&self) -> &Fail {
            ::find_root_cause(self.cause())
        }

        /// Attempts to downcast this `Error` to a particular `Fail` type by
        /// reference.
        ///
        /// If the underlying error is not of type `T`, this will return `None`.
        pub fn downcast_ref<T: Fail>(&self) -> Option<&T> {
            self.inner.failure.downcast_ref()
        }

        /// Attempts to downcast this `Error` to a particular `Fail` type by
        /// mutable reference.
        ///
        /// If the underlying error is not of type `T`, this will return `None`.
        pub fn downcast_mut<T: Fail>(&mut self) -> Option<&mut T> {
            self.inner.failure.downcast_mut()
        }

        /// Returns a iterator over the causes of the `Error`, beginning with
        /// the failure returned by the `cause` method and ending with the failure
        /// returned by `root_cause`.
        pub fn causes(&self) -> Causes {
            Causes { fail: Some(self.cause()) }
        }
    }

    impl Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            Display::fmt(&self.inner.failure, f)
        }
    }

    impl Debug for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            if self.inner.backtrace.is_none() {
                Debug::fmt(&self.inner.failure, f)
            } else {
                write!(f, "{:?}\n\n{:?}", &self.inner.failure, self.inner.backtrace)
            }
        }
    }

    #[cfg(test)]
    mod test {
        fn assert_just_data<T: Send + Sync + 'static>() { }

        #[test]
        fn assert_error_is_just_data() {
            assert_just_data::<super::Error>();
        }
    }

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