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

    use Fail;

    without_std! {
        /// An error with context around it.
        ///
        /// The context is intended to be a human-readable, user-facing explanation for the
        /// error that has occurred. The underlying error is not assumed to be end-user-relevant
        /// information.
        ///
        /// The `Display` impl for `Context` only prints the human-readable context, while the
        /// `Debug` impl also prints the underlying error.
        pub struct Context<D: Display + Send + Sync + 'static> {
            context: D,
        }

        impl<D: Display + Send + Sync + 'static> Context<D> {
            /// Creates a new context without an underlying error message.
            pub fn new(context: D) -> Context<D> {
                Context { context }
            }

            /// Returns a reference to the context provided with this error.
            pub fn get_context(&self) -> &D {
                &self.context
            }

            pub(crate) fn with_err<E: Fail>(context: D, _: E) -> Context<D> {
                Context { context }
            }
        }

        impl<D: Display + Send + Sync + 'static> Fail for Context<D> { }

        impl<D: Display + Send + Sync + 'static> Debug for Context<D> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "{}", self.context)
            }
        }

        impl<D: Display + Send + Sync + 'static> Display for Context<D> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "{}", self.context)
            }
        }
    }

    with_std! {
        use {Error, Backtrace};

        /// An error with context around it.
        ///
        /// The context is intended to be a human-readable, user-facing explanation for the
        /// error that has occurred. The underlying error is not assumed to be end-user-relevant
        /// information.
        ///
        /// The `Display` impl for `Context` only prints the human-readable context, while the
        /// `Debug` impl also prints the underlying error.
        pub struct Context<D: Display + Send + Sync + 'static> {
            context: D,
            failure: Either<Backtrace, Error>,
        }

        impl<D: Display + Send + Sync + 'static> Context<D> {
            /// Creates a new context without an underlying error message.
            pub fn new(context: D) -> Context<D> {
                let failure = Either::This(Backtrace::new());
                Context { context, failure }
            }

            /// Returns a reference to the context provided with this error.
            pub fn get_context(&self) -> &D {
                &self.context
            }

            pub(crate) fn with_err<E: Into<Error>>(context: D, error: E) -> Context<D> {
                let failure = Either::That(error.into());
                Context { context, failure }
            }
        }

        impl<D: Display + Send + Sync + 'static> Fail for Context<D> {
            fn cause(&self) -> Option<&Fail> {
                self.failure.cause()
            }

            fn backtrace(&self) -> Option<&Backtrace> {
                Some(self.failure.backtrace())
            }
        }

        impl<D: Display + Send + Sync + 'static> Debug for Context<D> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "{:?}\n\n{}", self.failure, self.context)
            }
        }

        impl<D: Display + Send + Sync + 'static> Display for Context<D> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "{}", self.context)
            }
        }

        enum Either<A, B> {
            This(A),
            That(B),
        }

        impl Either<Backtrace, Error> {
            fn backtrace(&self) -> &Backtrace {
                match *self {
                    Either::This(ref backtrace) => backtrace,
                    Either::That(ref error)     => error.backtrace(),
                }
            }

            fn cause(&self) -> Option<&Fail> {
                match *self {
                    Either::This(_)         => None,
                    Either::That(ref error) => Some(error.cause())
                }
            }
        }

        impl Debug for Either<Backtrace, Error> {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                match *self {
                    Either::This(ref backtrace) => write!(f, "{:?}", backtrace),
                    Either::That(ref error)     => write!(f, "{:?}", error),
                }
            }
        }
    }

    impl<D> From<D> for Context<D> where
        D: Display + Send + Sync + 'static
    {
        fn from(display: D) -> Context<D> {
            Context::new(display)
        }
    }

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