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    //! An experimental new error-handling library.
    //!
    //! The primary items exported by this library are:
    //!
    //! - `Fail`: a new trait for custom error types in Rust.
    //! - `Error`: a wrapper around `Fail` types to make it easy to coalesce them
    //!   at higher levels.
    //!
    //! As a general rule, library authors should create their own error types and
    //! implement `Fail` for them, whereas application authors should primarily
    //! deal with the `Error` type. There are exceptions to this rule, though, in
    //! both directions, and users should do whatever seems most appropriate to
    //! their situation.
    #![cfg_attr(not(feature = "std"), no_std)]
    #![deny(missing_docs)]

    macro_rules! with_std { ($($i:item)*) => ($(#[cfg(feature = "std")]$i)*) }
    macro_rules! without_std { ($($i:item)*) => ($(#[cfg(not(feature = "std"))]$i)*) }

    mod backtrace;
    mod compat;
    mod context;
    mod result_ext;

    use core::any::TypeId;
    use core::fmt::{Debug, Display};

    pub use backtrace::Backtrace;
    pub use compat::Compat;
    pub use context::Context;
    pub use result_ext::ResultExt;

    #[cfg(feature = "derive")]
    #[allow(unused_imports)]
    #[macro_use]
    extern crate failure_derive;

    #[cfg(feature = "derive")]
    #[doc(hidden)]
    pub use failure_derive::*;

    with_std! {
        extern crate core;

        mod sync_failure;
        pub use sync_failure::SyncFailure;

        mod error;

        use std::error::Error as StdError;

        pub use error::Error;

        mod macros;
        mod error_message;
        pub use error_message::err_msg;
    }

    /// The `Fail` trait.
    ///
    /// Implementors of this trait are called 'failures'.
    ///
    /// All error types should implement `Fail`, which provides a baseline of
    /// functionality that they all share.
    ///
    /// `Fail` has no required methods, but it does require that your type
    /// implement several other traits:
    ///
    /// - `Display`: to print a user-friendly representation of the error.
    /// - `Debug`: to print a verbose, developer-focused representation of the
    ///   error.
    /// - `Send + Sync`: Your error type is required to be safe to transfer to and
    ///   reference from another thread
    ///
    /// Additionally, all failures must be `'static`. This enables downcasting.
    ///
    /// `Fail` provides several methods with default implementations. Two of these
    /// may be appropriate to override depending on the definition of your
    /// particular failure: the `cause` and `backtrace` methods.
    ///
    /// The `failure_derive` crate provides a way to derive the `Fail` trait for
    /// your type. Additionally, all types that already implement
    /// `std::error::Error`, and are also `Send`, `Sync`, and `'static`, implement
    /// `Fail` by a blanket impl.
    pub trait Fail: Display + Debug + Send + Sync + 'static {
        /// Returns a reference to the underlying cause of this failure, if it
        /// is an error that wraps other errors.
        ///
        /// Returns `None` if this failure does not have another error as its
        /// underlying cause. By default, this returns `None`.
        ///
        /// This should **never** return a reference to `self`, but only return
        /// `Some` when it can return a **different** failure. Users may loop
        /// over the cause chain, and returning `self` would result in an infinite
        /// loop.
        fn cause(&self) -> Option<&Fail> {
            None
        }

        /// Returns a reference to the `Backtrace` carried by this failure, if it
        /// carries one.
        ///
        /// Returns `None` if this failure does not carry a backtrace. By
        /// default, this returns `None`.
        fn backtrace(&self) -> Option<&Backtrace> {
            None
        }

        /// Provides context for this failure.
        ///
        /// 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 another failure, or some other custom context-carrying
        /// type.
        fn context<D>(self, context: D) -> Context<D> where
            D: Display + Send + Sync + 'static,
            Self: Sized,
        {
            Context::with_err(context, self)
        }

        /// Wraps this failure in a compatibility wrapper that implements
        /// `std::error::Error`.
        ///
        /// This allows failures  to be compatible with older crates that
        /// expect types that implement the `Error` trait from `std::error`.
        fn compat(self) -> Compat<Self> where
            Self: Sized,
        {
            Compat { error: self }
        }

        /// Returns a iterator over the causes of this `Fail` with itself
        /// as the first item and the `root_cause` as the final item.
        fn causes(&self) -> Causes where Self: Sized {
            Causes { fail: Some(self) }
        }

        /// Returns the "root cause" of this `Fail` - the last value in the
        /// cause chain which does not return an underlying `cause`.
        ///
        /// If this type does not have a cause, `self` is returned, because
        /// it is its own root cause.
        fn root_cause(&self) -> &Fail where
            Self: Sized,
        {
            find_root_cause(self)
        }

        #[doc(hidden)]
        fn __private_get_type_id__(&self) -> TypeId {
            TypeId::of::<Self>()
        }
    }

    impl Fail {
        /// Attempts to downcast this failure to a concrete 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> {
            if self.__private_get_type_id__() == TypeId::of::<T>() {
                unsafe { Some(&*(self as *const Fail as *const T)) }
            } else {
                None
            }
        }

        /// Attempts to downcast this failure to a concrete 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> {
            if self.__private_get_type_id__() == TypeId::of::<T>() {
                unsafe { Some(&mut *(self as *mut Fail as *mut T)) }
            } else {
                None
            }
        }

        /// Returns the "root cause" of this `Fail` - the last value in the
        /// cause chain which does not return an underlying `cause`.
        ///
        /// If this type does not have a cause, `self` is returned, because
        /// it is its own root cause.
        pub fn root_cause(&self) -> &Fail {
            find_root_cause(self)
        }

        /// Returns a iterator over the causes of this `Fail` with itself
        /// as the first item and the `root_cause` as the final item.
        pub fn causes(&self) -> Causes {
            Causes { fail: Some(self) }
        }
    }

    #[cfg(feature = "std")]
    impl<E: StdError + Send + Sync + 'static> Fail for E {}

    /// A iterator over the causes of a `Fail`
    pub struct Causes<'f> {
        fail: Option<&'f Fail>,
    }

    impl<'f> Iterator for Causes<'f> {
        type Item = &'f Fail;
        fn next(&mut self) -> Option<&'f Fail> {
            self.fail.map(|fail| {
                self.fail = fail.cause();
                fail
            })
        }
    }

    fn find_root_cause(mut fail: &Fail) -> &Fail {
        while let Some(cause) = fail.cause() {
            fail = cause;
        }

        fail
    }

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