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    use std::cell::UnsafeCell;
    use std::env;
    use std::ffi::OsString;
    use std::fmt;
    use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
    use std::sync::Mutex;

    pub use super::backtrace::Backtrace;

    const GENERAL_BACKTRACE: &str = "RUST_BACKTRACE";
    const FAILURE_BACKTRACE: &str = "RUST_FAILURE_BACKTRACE";

    pub(super) struct InternalBacktrace {
        backtrace: Option<MaybeResolved>,
    }

    struct MaybeResolved {
        resolved: Mutex<bool>,
        backtrace: UnsafeCell<Backtrace>,
    }

    unsafe impl Send for MaybeResolved {}
    unsafe impl Sync for MaybeResolved {}

    impl InternalBacktrace {
        pub(super) fn new() -> InternalBacktrace {
            static ENABLED: AtomicUsize = ATOMIC_USIZE_INIT;

            match ENABLED.load(Ordering::SeqCst) {
                0 => {
                    let enabled = is_backtrace_enabled(|var| env::var_os(var));
                    ENABLED.store(enabled as usize + 1, Ordering::SeqCst);
                    if !enabled {
                        return InternalBacktrace { backtrace: None }
                    }
                }
                1 => return InternalBacktrace { backtrace: None },
                _ => {}
            }

            InternalBacktrace {
                backtrace: Some(MaybeResolved {
                    resolved: Mutex::new(false),
                    backtrace: UnsafeCell::new(Backtrace::new_unresolved()),
                }),
            }
        }

        pub(super) fn none() -> InternalBacktrace {
            InternalBacktrace { backtrace: None }
        }

        pub(super) fn as_backtrace(&self) -> Option<&Backtrace> {
            let bt = match self.backtrace {
                Some(ref bt) => bt,
                None => return None,
            };
            let mut resolved = bt.resolved.lock().unwrap();
            unsafe {
                if !*resolved {
                    (*bt.backtrace.get()).resolve();
                    *resolved = true;
                }
                Some(&*bt.backtrace.get())
            }
        }

        pub(super) fn is_none(&self) -> bool {
            self.backtrace.is_none()
        }
    }

    impl fmt::Debug for InternalBacktrace {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("InternalBacktrace")
                .field("backtrace", &self.as_backtrace())
                .finish()
        }
    }

    fn is_backtrace_enabled<F: Fn(&str) -> Option<OsString>>(get_var: F) -> bool {
        match get_var(FAILURE_BACKTRACE) {
            Some(ref val) if val != "0" => true,
            Some(ref val) if val == "0" => false,
            _ => match get_var(GENERAL_BACKTRACE) {
                Some(ref val) if val != "0" => true,
                _                           => false,
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        const YEA: Option<&str> = Some("1");
        const NAY: Option<&str> = Some("0");
        const NOT_SET: Option<&str> = None;

        macro_rules! test_enabled {
            (failure: $failure:ident, general: $general:ident => $result:expr) => {{
                assert_eq!(is_backtrace_enabled(|var| match var {
                    FAILURE_BACKTRACE   => $failure.map(OsString::from),
                    GENERAL_BACKTRACE   => $general.map(OsString::from),
                    _                   => panic!()
                }), $result);
            }}
        }

        #[test]
        fn always_enabled_if_failure_is_set_to_yes() {
            test_enabled!(failure: YEA, general: YEA => true);
            test_enabled!(failure: YEA, general: NOT_SET => true);
            test_enabled!(failure: YEA, general: NAY => true);
        }

        #[test]
        fn never_enabled_if_failure_is_set_to_no() {
            test_enabled!(failure: NAY, general: YEA => false);
            test_enabled!(failure: NAY, general: NOT_SET => false);
            test_enabled!(failure: NAY, general: NAY => false);
        }

        #[test]
        fn follows_general_if_failure_is_not_set() {
            test_enabled!(failure: NOT_SET, general: YEA => true);
            test_enabled!(failure: NOT_SET, general: NOT_SET => false);
            test_enabled!(failure: NOT_SET, general: NAY => false);
        }
    }

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