src/libstd/panicking.rs - third_party/rust - Git at Google

 //! 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, PanicInfo, Location};

 use crate::any::Any;
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem;
 use crate::ptr;
 use crate::raw;
 use crate::sys::stdio::panic_output;
 use crate::sys_common::rwlock::RWLock;
 use crate::sys_common::{thread_info, util, backtrace};
 use crate::thread;

 #[cfg(not(test))]
 use crate::io::set_panic;
 // make sure to use the stderr output configured
 // by libtest in the real copy of std
 #[cfg(test)]
 use realstd::io::set_panic;

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

         if panics >= 2 {
             Some(backtrace_rs::PrintFmt::Full)
         } else {
             backtrace::log_enabled()
         }
     } else {
         None
     };

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

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

         if cfg!(feature = "backtrace") {
             use crate::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) = set_panic(None) {
         // NB. In `cfg(test)` this uses the forwarding impl
         // for `Box<dyn (::realstd::io::Write) + Send>`.
         write(&mut local);
         set_panic(Some(local));
     } 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 crate::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 r#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 crate::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::take(self.fill());
             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)
 }
	//! 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, PanicInfo, Location};

	use crate::any::Any;
	use crate::fmt;
	use crate::intrinsics;
	use crate::mem;
	use crate::ptr;
	use crate::raw;
	use crate::sys::stdio::panic_output;
	use crate::sys_common::rwlock::RWLock;
	use crate::sys_common::{thread_info, util, backtrace};
	use crate::thread;

	#[cfg(not(test))]
	use crate::io::set_panic;
	// make sure to use the stderr output configured
	// by libtest in the real copy of std
	#[cfg(test)]
	use realstd::io::set_panic;

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

	if panics >= 2 {
	Some(backtrace_rs::PrintFmt::Full)
	} else {
	backtrace::log_enabled()
	}
	} else {
	None
	};

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

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

	if cfg!(feature = "backtrace") {
	use crate::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) = set_panic(None) {
	// NB. In `cfg(test)` this uses the forwarding impl
	// for `Box<dyn (::realstd::io::Write) + Send>`.
	write(&mut local);
	set_panic(Some(local));
	} 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 crate::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 r#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 crate::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::take(self.fill());
	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)
	}