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

    without_std! {
        /// A `Backtrace`.
        ///
        /// This is an opaque wrapper around the backtrace provided by
        /// libbacktrace. A variety of optimizations have been performed to avoid
        /// unnecessary or ill-advised work:
        ///
        /// - If this crate is compiled in `no_std` compatible mode, `Backtrace`
        ///   is an empty struct, and will be completely compiled away.
        /// - If this crate is run without the `RUST_BACKTRACE` environmental
        ///   variable enabled, the backtrace will not be generated at runtime.
        /// - Even if a backtrace is generated, the most expensive part of
        ///   generating a backtrace is symbol resolution. This backtrace does not
        ///   perform symbol resolution until it is actually read (e.g. by
        ///   printing it). If the Backtrace is never used for anything, symbols
        ///   never get resolved.
        ///
        /// Even with these optimizations, including a backtrace in your failure
        /// may not be appropriate to your use case. You are not required to put a
        /// backtrace in a custom `Fail` type.
        ///
        /// > (We have detected that this crate was documented with no_std
        /// > compatibility turned on. The version of this crate that has been
        /// > documented here will never generate a backtrace.)
        pub struct Backtrace {
            _secret: (),
        }

        impl Backtrace {
            /// Constructs a new backtrace. This will only create a real backtrace
            /// if the crate is compiled in std mode and the `RUST_BACKTRACE`
            /// environmental variable is activated.
            ///
            /// > (We have detected that this crate was documented with no_std
            /// > compatibility turned on. The version of this crate that has been
            /// > documented here will never generate a backtrace.)
            pub fn new() -> Backtrace {
                Backtrace { _secret: () }
            }
        }

        impl Default for Backtrace {
            fn default() -> Backtrace {
                Backtrace::new()
            }
        }

        impl Debug for Backtrace {
            fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
                Ok(())
            }
        }

        impl Display for Backtrace {
            fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
                Ok(())
            }
        }
    }

    with_std! {
        extern crate backtrace;

        mod internal;

        use self::internal::InternalBacktrace;

        /// A `Backtrace`.
        ///
        /// This is an opaque wrapper around the backtrace provided by
        /// libbacktrace. A variety of optimizations have been performed to avoid
        /// unnecessary or ill-advised work:
        ///
        /// - If this crate is compiled in `no_std` compatible mode, `Backtrace`
        ///   is an empty struct, and will be completely compiled away.
        /// - If this crate is run without the `RUST_BACKTRACE` environmental
        ///   variable enabled, the backtrace will not be generated at runtime.
        /// - Even if a backtrace is generated, the most expensive part of
        ///   generating a backtrace is symbol resolution. This backtrace does not
        ///   perform symbol resolution until it is actually read (e.g. by
        ///   printing it). If the Backtrace is never used for anything, symbols
        ///   never get resolved.
        ///
        /// Even with these optimizations, including a backtrace in your failure
        /// may not be appropriate to your use case. You are not required to put a
        /// backtrace in a custom `Fail` type.
        pub struct Backtrace {
            internal: InternalBacktrace
        }

        impl Backtrace {
            /// Constructs a new backtrace. This will only create a real backtrace
            /// if the crate is compiled in std mode and the `RUST_BACKTRACE`
            /// environmental variable is activated.
            pub fn new() -> Backtrace {
                Backtrace { internal: InternalBacktrace::new() }
            }

            pub(crate) fn none() -> Backtrace {
                Backtrace { internal: InternalBacktrace::none() }
            }

            pub(crate) fn is_none(&self) -> bool {
                self.internal.is_none()
            }
        }

        impl Default for Backtrace {
            fn default() -> Backtrace {
                Backtrace::new()
            }
        }

        impl Debug for Backtrace {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                if let Some(bt) = self.internal.as_backtrace() {
                    bt.fmt(f)
                } else { Ok(()) }
            }
        }

        impl Display for Backtrace {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                if let Some(bt) = self.internal.as_backtrace() {
                    bt.fmt(f)
                } else { Ok(()) }
            }
        }
    }

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