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    //! Implementation of various bits and pieces of the `panic!` macro and
    //! associated runtime pieces.
    //!
    //! Specifically, this module contains the implementation of:
    //!
    //! * Panic hooks
    //! * Executing a panic up to doing the actual implementation
    //! * Shims around "try"

    use core::panic::BoxMeUp;

    use io::prelude::*;

    use any::Any;
    use cell::RefCell;
    use core::panic::{PanicInfo, Location};
    use fmt;
    use intrinsics;
    use mem;
    use ptr;
    use raw;
    use sys::stdio::panic_output;
    use sys_common::rwlock::RWLock;
    use sys_common::thread_info;
    use sys_common::util;
    use thread;

    thread_local! {
        pub static LOCAL_STDERR: RefCell<Option<Box<dyn Write + Send>>> = {
            RefCell::new(None)
        }
    }

    // Binary interface to the panic runtime that the standard library depends on.
    //
    // The standard library is tagged with `#![needs_panic_runtime]` (introduced in
    // RFC 1513) to indicate that it requires some other crate tagged with
    // `#![panic_runtime]` to exist somewhere. Each panic runtime is intended to
    // implement these symbols (with the same signatures) so we can get matched up
    // to them.
    //
    // One day this may look a little less ad-hoc with the compiler helping out to
    // hook up these functions, but it is not this day!
    #[allow(improper_ctypes)]
    extern {
        fn __rust_maybe_catch_panic(f: fn(*mut u8),
                                    data: *mut u8,
                                    data_ptr: *mut usize,
                                    vtable_ptr: *mut usize) -> u32;
        #[unwind(allowed)]
        fn __rust_start_panic(payload: usize) -> u32;
    }

    #[derive(Copy, Clone)]
    enum Hook {
        Default,
        Custom(*mut (dyn Fn(&PanicInfo) + 'static + Sync + Send)),
    }

    static HOOK_LOCK: RWLock = RWLock::new();
    static mut HOOK: Hook = Hook::Default;

    /// Registers a custom panic hook, replacing any that was previously registered.
    ///
    /// The panic hook is invoked when a thread panics, but before the panic runtime
    /// is invoked. As such, the hook will run with both the aborting and unwinding
    /// runtimes. The default hook prints a message to standard error and generates
    /// a backtrace if requested, but this behavior can be customized with the
    /// `set_hook` and [`take_hook`] functions.
    ///
    /// [`take_hook`]: ./fn.take_hook.html
    ///
    /// The hook is provided with a `PanicInfo` struct which contains information
    /// about the origin of the panic, including the payload passed to `panic!` and
    /// the source code location from which the panic originated.
    ///
    /// The panic hook is a global resource.
    ///
    /// # Panics
    ///
    /// Panics if called from a panicking thread.
    ///
    /// # Examples
    ///
    /// The following will print "Custom panic hook":
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|_| {
    ///     println!("Custom panic hook");
    /// }));
    ///
    /// panic!("Normal panic");
    /// ```
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub fn set_hook(hook: Box<dyn Fn(&PanicInfo) + 'static + Sync + Send>) {
        if thread::panicking() {
            panic!("cannot modify the panic hook from a panicking thread");
        }

        unsafe {
            HOOK_LOCK.write();
            let old_hook = HOOK;
            HOOK = Hook::Custom(Box::into_raw(hook));
            HOOK_LOCK.write_unlock();

            if let Hook::Custom(ptr) = old_hook {
                Box::from_raw(ptr);
            }
        }
    }

    /// Unregisters the current panic hook, returning it.
    ///
    /// *See also the function [`set_hook`].*
    ///
    /// [`set_hook`]: ./fn.set_hook.html
    ///
    /// If no custom hook is registered, the default hook will be returned.
    ///
    /// # Panics
    ///
    /// Panics if called from a panicking thread.
    ///
    /// # Examples
    ///
    /// The following will print "Normal panic":
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|_| {
    ///     println!("Custom panic hook");
    /// }));
    ///
    /// let _ = panic::take_hook();
    ///
    /// panic!("Normal panic");
    /// ```
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub fn take_hook() -> Box<dyn Fn(&PanicInfo) + 'static + Sync + Send> {
        if thread::panicking() {
            panic!("cannot modify the panic hook from a panicking thread");
        }

        unsafe {
            HOOK_LOCK.write();
            let hook = HOOK;
            HOOK = Hook::Default;
            HOOK_LOCK.write_unlock();

            match hook {
                Hook::Default => Box::new(default_hook),
                Hook::Custom(ptr) => Box::from_raw(ptr),
            }
        }
    }

    fn default_hook(info: &PanicInfo) {
        #[cfg(feature = "backtrace")]
        use sys_common::backtrace;

        // If this is a double panic, make sure that we print a backtrace
        // for this panic. Otherwise only print it if logging is enabled.
        #[cfg(feature = "backtrace")]
        let log_backtrace = {
            let panics = update_panic_count(0);

            if panics >= 2 {
                Some(backtrace::PrintFormat::Full)
            } else {
                backtrace::log_enabled()
            }
        };

        let location = info.location().unwrap();  // The current implementation always returns Some

        let msg = match info.payload().downcast_ref::<&'static str>() {
            Some(s) => *s,
            None => match info.payload().downcast_ref::<String>() {
                Some(s) => &s[..],
                None => "Box<Any>",
            }
        };
        let thread = thread_info::current_thread();
        let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");

        let write = |err: &mut dyn (::io::Write)| {
            let _ = writeln!(err, "thread '{}' panicked at '{}', {}",
                             name, msg, location);

            #[cfg(feature = "backtrace")]
            {
                use sync::atomic::{AtomicBool, Ordering};

                static FIRST_PANIC: AtomicBool = AtomicBool::new(true);

                if let Some(format) = log_backtrace {
                    let _ = backtrace::print(err, format);
                } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
                    let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` \
                                           environment variable to display a backtrace.");
                }
            }
        };

        if let Some(mut local) = LOCAL_STDERR.with(|s| s.borrow_mut().take()) {
           write(&mut *local);
           let mut s = Some(local);
           LOCAL_STDERR.with(|slot| {
               *slot.borrow_mut() = s.take();
           });
        } else if let Some(mut out) = panic_output() {
            write(&mut out);
        }
    }

    #[cfg(not(test))]
    #[doc(hidden)]
    #[unstable(feature = "update_panic_count", issue = "0")]
    pub fn update_panic_count(amt: isize) -> usize {
        use cell::Cell;
        thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }

        PANIC_COUNT.with(|c| {
            let next = (c.get() as isize + amt) as usize;
            c.set(next);
            return next
        })
    }

    #[cfg(test)]
    pub use realstd::rt::update_panic_count;

    /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
    pub unsafe fn try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>> {
        #[allow(unions_with_drop_fields)]
        union Data<F, R> {
            f: F,
            r: R,
        }

        // We do some sketchy operations with ownership here for the sake of
        // performance. We can only  pass pointers down to
        // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
        // the ownership tracking here manually using a union.
        //
        // We go through a transition where:
        //
        // * First, we set the data to be the closure that we're going to call.
        // * When we make the function call, the `do_call` function below, we take
        //   ownership of the function pointer. At this point the `Data` union is
        //   entirely uninitialized.
        // * If the closure successfully returns, we write the return value into the
        //   data's return slot. Note that `ptr::write` is used as it's overwriting
        //   uninitialized data.
        // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
        //   in one of two states:
        //
        //      1. The closure didn't panic, in which case the return value was
        //         filled in. We move it out of `data` and return it.
        //      2. The closure panicked, in which case the return value wasn't
        //         filled in. In this case the entire `data` union is invalid, so
        //         there is no need to drop anything.
        //
        // Once we stack all that together we should have the "most efficient'
        // method of calling a catch panic whilst juggling ownership.
        let mut any_data = 0;
        let mut any_vtable = 0;
        let mut data = Data {
            f,
        };

        let r = __rust_maybe_catch_panic(do_call::<F, R>,
                                         &mut data as *mut _ as *mut u8,
                                         &mut any_data,
                                         &mut any_vtable);

        return if r == 0 {
            debug_assert!(update_panic_count(0) == 0);
            Ok(data.r)
        } else {
            update_panic_count(-1);
            debug_assert!(update_panic_count(0) == 0);
            Err(mem::transmute(raw::TraitObject {
                data: any_data as *mut _,
                vtable: any_vtable as *mut _,
            }))
        };

        fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
            unsafe {
                let data = data as *mut Data<F, R>;
                let f = ptr::read(&mut (*data).f);
                ptr::write(&mut (*data).r, f());
            }
        }
    }

    /// Determines whether the current thread is unwinding because of panic.
    pub fn panicking() -> bool {
        update_panic_count(0) != 0
    }

    /// Entry point of panic from the libcore crate.
    #[cfg(not(test))]
    #[panic_handler]
    #[unwind(allowed)]
    pub fn rust_begin_panic(info: &PanicInfo) -> ! {
        continue_panic_fmt(&info)
    }

    /// The entry point for panicking with a formatted message.
    ///
    /// This is designed to reduce the amount of code required at the call
    /// site as much as possible (so that `panic!()` has as low an impact
    /// on (e.g.) the inlining of other functions as possible), by moving
    /// the actual formatting into this shared place.
    #[unstable(feature = "libstd_sys_internals",
               reason = "used by the panic! macro",
               issue = "0")]
    #[cold]
    // If panic_immediate_abort, inline the abort call,
    // otherwise avoid inlining because of it is cold path.
    #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
    #[cfg_attr(    feature="panic_immediate_abort" ,inline)]
    pub fn begin_panic_fmt(msg: &fmt::Arguments,
                           file_line_col: &(&'static str, u32, u32)) -> ! {
        if cfg!(feature = "panic_immediate_abort") {
            unsafe { intrinsics::abort() }
        }

        let (file, line, col) = *file_line_col;
        let info = PanicInfo::internal_constructor(
            Some(msg),
            Location::internal_constructor(file, line, col),
        );
        continue_panic_fmt(&info)
    }

    fn continue_panic_fmt(info: &PanicInfo) -> ! {
        struct PanicPayload<'a> {
            inner: &'a fmt::Arguments<'a>,
            string: Option<String>,
        }

        impl<'a> PanicPayload<'a> {
            fn new(inner: &'a fmt::Arguments<'a>) -> PanicPayload<'a> {
                PanicPayload { inner, string: None }
            }

            fn fill(&mut self) -> &mut String {
                use fmt::Write;

                let inner = self.inner;
                self.string.get_or_insert_with(|| {
                    let mut s = String::new();
                    drop(s.write_fmt(*inner));
                    s
                })
            }
        }

        unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
            fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
                let contents = mem::replace(self.fill(), String::new());
                Box::into_raw(Box::new(contents))
            }

            fn get(&mut self) -> &(dyn Any + Send) {
                self.fill()
            }
        }

        // We do two allocations here, unfortunately. But (a) they're
        // required with the current scheme, and (b) we don't handle
        // panic + OOM properly anyway (see comment in begin_panic
        // below).

        let loc = info.location().unwrap(); // The current implementation always returns Some
        let msg = info.message().unwrap(); // The current implementation always returns Some
        let file_line_col = (loc.file(), loc.line(), loc.column());
        rust_panic_with_hook(
            &mut PanicPayload::new(msg),
            info.message(),
            &file_line_col);
    }

    /// This is the entry point of panicking for panic!() and assert!().
    #[unstable(feature = "libstd_sys_internals",
               reason = "used by the panic! macro",
               issue = "0")]
    #[cfg_attr(not(test), lang = "begin_panic")]
    // never inline unless panic_immediate_abort to avoid code
    // bloat at the call sites as much as possible
    #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
    #[cold]
    pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
        if cfg!(feature = "panic_immediate_abort") {
            unsafe { intrinsics::abort() }
        }

        // Note that this should be the only allocation performed in this code path.
        // Currently this means that panic!() on OOM will invoke this code path,
        // but then again we're not really ready for panic on OOM anyway. If
        // we do start doing this, then we should propagate this allocation to
        // be performed in the parent of this thread instead of the thread that's
        // panicking.

        rust_panic_with_hook(&mut PanicPayload::new(msg), None, file_line_col);

        struct PanicPayload<A> {
            inner: Option<A>,
        }

        impl<A: Send + 'static> PanicPayload<A> {
            fn new(inner: A) -> PanicPayload<A> {
                PanicPayload { inner: Some(inner) }
            }
        }

        unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
            fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
                let data = match self.inner.take() {
                    Some(a) => Box::new(a) as Box<dyn Any + Send>,
                    None => Box::new(()),
                };
                Box::into_raw(data)
            }

            fn get(&mut self) -> &(dyn Any + Send) {
                match self.inner {
                    Some(ref a) => a,
                    None => &(),
                }
            }
        }
    }

    /// Central point for dispatching panics.
    ///
    /// Executes the primary logic for a panic, including checking for recursive
    /// panics, panic hooks, and finally dispatching to the panic runtime to either
    /// abort or unwind.
    fn rust_panic_with_hook(payload: &mut dyn BoxMeUp,
                            message: Option<&fmt::Arguments>,
                            file_line_col: &(&str, u32, u32)) -> ! {
        let (file, line, col) = *file_line_col;

        let panics = update_panic_count(1);

        // If this is the third nested call (e.g., panics == 2, this is 0-indexed),
        // the panic hook probably triggered the last panic, otherwise the
        // double-panic check would have aborted the process. In this case abort the
        // process real quickly as we don't want to try calling it again as it'll
        // probably just panic again.
        if panics > 2 {
            util::dumb_print(format_args!("thread panicked while processing \
                                           panic. aborting.\n"));
            unsafe { intrinsics::abort() }
        }

        unsafe {
            let mut info = PanicInfo::internal_constructor(
                message,
                Location::internal_constructor(file, line, col),
            );
            HOOK_LOCK.read();
            match HOOK {
                // Some platforms know that printing to stderr won't ever actually
                // print anything, and if that's the case we can skip the default
                // hook.
                Hook::Default if panic_output().is_none() => {}
                Hook::Default => {
                    info.set_payload(payload.get());
                    default_hook(&info);
                }
                Hook::Custom(ptr) => {
                    info.set_payload(payload.get());
                    (*ptr)(&info);
                }
            };
            HOOK_LOCK.read_unlock();
        }

        if panics > 1 {
            // If a thread panics while it's already unwinding then we
            // have limited options. Currently our preference is to
            // just abort. In the future we may consider resuming
            // unwinding or otherwise exiting the thread cleanly.
            util::dumb_print(format_args!("thread panicked while panicking. \
                                           aborting.\n"));
            unsafe { intrinsics::abort() }
        }

        rust_panic(payload)
    }

    /// Shim around rust_panic. Called by resume_unwind.
    pub fn update_count_then_panic(msg: Box<dyn Any + Send>) -> ! {
        update_panic_count(1);

        struct RewrapBox(Box<dyn Any + Send>);

        unsafe impl BoxMeUp for RewrapBox {
            fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
                Box::into_raw(mem::replace(&mut self.0, Box::new(())))
            }

            fn get(&mut self) -> &(dyn Any + Send) {
                &*self.0
            }
        }

        rust_panic(&mut RewrapBox(msg))
    }

    /// An unmangled function (through `rustc_std_internal_symbol`) on which to slap
    /// yer breakpoints.
    #[inline(never)]
    #[cfg_attr(not(test), rustc_std_internal_symbol)]
    fn rust_panic(mut msg: &mut dyn BoxMeUp) -> ! {
        let code = unsafe {
            let obj = &mut msg as *mut &mut dyn BoxMeUp;
            __rust_start_panic(obj as usize)
        };
        rtabort!("failed to initiate panic, error {}", code)
    }

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