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

 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

 //! 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 prelude::v1::*;
 use io::prelude::*;

 use any::Any;
 use cell::Cell;
 use cell::RefCell;
 use fmt;
 use intrinsics;
 use mem;
 use raw;
 use sys_common::rwlock::RWLock;
 use sys::stdio::Stderr;
 use sys_common::thread_info;
 use sys_common::util;
 use thread;

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

 thread_local! { pub static PANIC_COUNT: Cell<usize> = Cell::new(0) }

 // 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]
     fn __rust_start_panic(data: usize, vtable: usize) -> u32;
 }

 #[derive(Copy, Clone)]
 enum Hook {
     Default,
     Custom(*mut (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.
 ///
 /// 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.
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub fn set_hook(hook: Box<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.
 ///
 /// If no custom hook is registered, the default hook will be returned.
 ///
 /// # Panics
 ///
 /// Panics if called from a panicking thread.
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub fn take_hook() -> Box<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)} // FIXME #30530
         }
     }
 }

 /// A struct providing information about a panic.
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub struct PanicInfo<'a> {
     payload: &'a (Any + Send),
     location: Location<'a>,
 }

 impl<'a> PanicInfo<'a> {
     /// Returns the payload associated with the panic.
     ///
     /// This will commonly, but not always, be a `&'static str` or `String`.
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn payload(&self) -> &(Any + Send) {
         self.payload
     }

     /// Returns information about the location from which the panic originated,
     /// if available.
     ///
     /// This method will currently always return `Some`, but this may change
     /// in future versions.
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn location(&self) -> Option<&Location> {
         Some(&self.location)
     }
 }

 /// A struct containing information about the location of a panic.
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub struct Location<'a> {
     file: &'a str,
     line: u32,
 }

 impl<'a> Location<'a> {
     /// Returns the name of the source file from which the panic originated.
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn file(&self) -> &str {
         self.file
     }

     /// Returns the line number from which the panic originated.
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn line(&self) -> u32 {
         self.line
     }
 }

 fn default_hook(info: &PanicInfo) {
     #[cfg(any(not(cargobuild), 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(any(not(cargobuild), feature = "backtrace"))]
     let log_backtrace = {
         let panics = PANIC_COUNT.with(|c| c.get());

         panics >= 2 || backtrace::log_enabled()
     };

     let file = info.location.file;
     let line = info.location.line;

     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 mut err = Stderr::new().ok();
     let thread = thread_info::current_thread();
     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");

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

         #[cfg(any(not(cargobuild), feature = "backtrace"))]
         {
             use sync::atomic::{AtomicBool, Ordering};

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

             if log_backtrace {
                 let _ = backtrace::write(err);
             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
                 let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
             }
         }
     };

     let prev = LOCAL_STDERR.with(|s| s.borrow_mut().take());
     match (prev, err.as_mut()) {
         (Some(mut stderr), _) => {
             write(&mut *stderr);
             let mut s = Some(stderr);
             LOCAL_STDERR.with(|slot| {
                 *slot.borrow_mut() = s.take();
             });
         }
         (None, Some(ref mut err)) => { write(err) }
         _ => {}
     }
 }

 /// 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<Any + Send>> {
     let mut slot = None;
     let mut f = Some(f);
     let ret = PANIC_COUNT.with(|s| {
         let prev = s.get();
         s.set(0);

         let mut to_run = || {
             slot = Some(f.take().unwrap()());
         };
         let fnptr = get_call(&mut to_run);
         let dataptr = &mut to_run as *mut _ as *mut u8;
         let mut any_data = 0;
         let mut any_vtable = 0;
         let fnptr = mem::transmute::<fn(&mut _), fn(*mut u8)>(fnptr);
         let r = __rust_maybe_catch_panic(fnptr,
                                          dataptr,
                                          &mut any_data,
                                          &mut any_vtable);
         s.set(prev);

         if r == 0 {
             Ok(())
         } else {
             Err(mem::transmute(raw::TraitObject {
                 data: any_data as *mut _,
                 vtable: any_vtable as *mut _,
             }))
         }
     });

     return ret.map(|()| {
         slot.take().unwrap()
     });

     fn get_call<F: FnMut()>(_: &mut F) -> fn(&mut F) {
         call
     }

     fn call<F: FnMut()>(f: &mut F) {
         f()
     }
 }

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

 /// Entry point of panic from the libcore crate.
 #[cfg(not(test))]
 #[lang = "panic_fmt"]
 #[unwind]
 pub extern fn rust_begin_panic(msg: fmt::Arguments,
                                file: &'static str,
                                line: u32) -> ! {
     begin_panic_fmt(&msg, &(file, line))
 }

 /// 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")]
 #[inline(never)] #[cold]
 pub fn begin_panic_fmt(msg: &fmt::Arguments,
                        file_line: &(&'static str, u32)) -> ! {
     use fmt::Write;

     // 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 mut s = String::new();
     let _ = s.write_fmt(*msg);
     begin_panic(s, file_line)
 }

 /// This is the entry point of panicking for panic!() and assert!().
 #[unstable(feature = "libstd_sys_internals",
            reason = "used by the panic! macro",
            issue = "0")]
 #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
 pub fn begin_panic<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> ! {
     // 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(Box::new(msg), file_line)
 }

 /// Executes the primary logic for a panic, including checking for recursive
 /// panics and panic hooks.
 ///
 /// This is the entry point or panics from libcore, formatted panics, and
 /// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
 /// run panic hooks, and then delegate to the actual implementation of panics.
 #[inline(never)]
 #[cold]
 fn rust_panic_with_hook(msg: Box<Any + Send>,
                         file_line: &(&'static str, u32)) -> ! {
     let (file, line) = *file_line;

     let panics = PANIC_COUNT.with(|c| {
         let prev = c.get();
         c.set(prev + 1);
         prev
     });

     // 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 > 1 {
         util::dumb_print(format_args!("thread panicked while processing \
                                        panic. aborting.\n"));
         unsafe { intrinsics::abort() }
     }

     unsafe {
         let info = PanicInfo {
             payload: &*msg,
             location: Location {
                 file: file,
                 line: line,
             },
         };
         HOOK_LOCK.read();
         match HOOK {
             Hook::Default => default_hook(&info),
             Hook::Custom(ptr) => (*ptr)(&info),
         }
         HOOK_LOCK.read_unlock();
     }

     if panics > 0 {
         // 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(msg)
 }

 /// A private no-mangle function on which to slap yer breakpoints.
 #[no_mangle]
 #[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
 pub fn rust_panic(msg: Box<Any + Send>) -> ! {
     let code = unsafe {
         let obj = mem::transmute::<_, raw::TraitObject>(msg);
         __rust_start_panic(obj.data as usize, obj.vtable as usize)
     };
     rtabort!("failed to initiate panic, error {}", code)
 }
	// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
	// file at the top-level directory of this distribution and at
	// http://rust-lang.org/COPYRIGHT.
	//
	// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
	// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
	// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
	// option. This file may not be copied, modified, or distributed
	// except according to those terms.

	//! 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 prelude::v1::*;
	use io::prelude::*;

	use any::Any;
	use cell::Cell;
	use cell::RefCell;
	use fmt;
	use intrinsics;
	use mem;
	use raw;
	use sys_common::rwlock::RWLock;
	use sys::stdio::Stderr;
	use sys_common::thread_info;
	use sys_common::util;
	use thread;

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

	thread_local! { pub static PANIC_COUNT: Cell<usize> = Cell::new(0) }

	// 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]
	fn __rust_start_panic(data: usize, vtable: usize) -> u32;
	}

	#[derive(Copy, Clone)]
	enum Hook {
	Default,
	Custom(*mut (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.
	///
	/// 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.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn set_hook(hook: Box<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.
	///
	/// If no custom hook is registered, the default hook will be returned.
	///
	/// # Panics
	///
	/// Panics if called from a panicking thread.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn take_hook() -> Box<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)} // FIXME #30530
	}
	}
	}

	/// A struct providing information about a panic.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub struct PanicInfo<'a> {
	payload: &'a (Any + Send),
	location: Location<'a>,
	}

	impl<'a> PanicInfo<'a> {
	/// Returns the payload associated with the panic.
	///
	/// This will commonly, but not always, be a `&'static str` or `String`.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn payload(&self) -> &(Any + Send) {
	self.payload
	}

	/// Returns information about the location from which the panic originated,
	/// if available.
	///
	/// This method will currently always return `Some`, but this may change
	/// in future versions.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn location(&self) -> Option<&Location> {
	Some(&self.location)
	}
	}

	/// A struct containing information about the location of a panic.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub struct Location<'a> {
	file: &'a str,
	line: u32,
	}

	impl<'a> Location<'a> {
	/// Returns the name of the source file from which the panic originated.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn file(&self) -> &str {
	self.file
	}

	/// Returns the line number from which the panic originated.
	#[stable(feature = "panic_hooks", since = "1.10.0")]
	pub fn line(&self) -> u32 {
	self.line
	}
	}

	fn default_hook(info: &PanicInfo) {
	#[cfg(any(not(cargobuild), 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(any(not(cargobuild), feature = "backtrace"))]
	let log_backtrace = {
	let panics = PANIC_COUNT.with(\|c\| c.get());

	panics >= 2 \|\| backtrace::log_enabled()
	};

	let file = info.location.file;
	let line = info.location.line;

	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 mut err = Stderr::new().ok();
	let thread = thread_info::current_thread();
	let name = thread.as_ref().and_then(\|t\| t.name()).unwrap_or("<unnamed>");

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

	#[cfg(any(not(cargobuild), feature = "backtrace"))]
	{
	use sync::atomic::{AtomicBool, Ordering};

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

	if log_backtrace {
	let _ = backtrace::write(err);
	} else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
	let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
	}
	}
	};

	let prev = LOCAL_STDERR.with(\|s\| s.borrow_mut().take());
	match (prev, err.as_mut()) {
	(Some(mut stderr), _) => {
	write(&mut *stderr);
	let mut s = Some(stderr);
	LOCAL_STDERR.with(\|slot\| {
	*slot.borrow_mut() = s.take();
	});
	}
	(None, Some(ref mut err)) => { write(err) }
	_ => {}
	}
	}

	/// 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<Any + Send>> {
	let mut slot = None;
	let mut f = Some(f);
	let ret = PANIC_COUNT.with(\|s\| {
	let prev = s.get();
	s.set(0);

	let mut to_run = \|\| {
	slot = Some(f.take().unwrap()());
	};
	let fnptr = get_call(&mut to_run);
	let dataptr = &mut to_run as mut _ as mut u8;
	let mut any_data = 0;
	let mut any_vtable = 0;
	let fnptr = mem::transmute::<fn(&mut _), fn(*mut u8)>(fnptr);
	let r = __rust_maybe_catch_panic(fnptr,
	dataptr,
	&mut any_data,
	&mut any_vtable);
	s.set(prev);

	if r == 0 {
	Ok(())
	} else {
	Err(mem::transmute(raw::TraitObject {
	data: any_data as *mut _,
	vtable: any_vtable as *mut _,
	}))
	}
	});

	return ret.map(\|()\| {
	slot.take().unwrap()
	});

	fn get_call<F: FnMut()>(_: &mut F) -> fn(&mut F) {
	call
	}

	fn call<F: FnMut()>(f: &mut F) {
	f()
	}
	}

	/// Determines whether the current thread is unwinding because of panic.
	pub fn panicking() -> bool {
	PANIC_COUNT.with(\|c\| c.get() != 0)
	}

	/// Entry point of panic from the libcore crate.
	#[cfg(not(test))]
	#[lang = "panic_fmt"]
	#[unwind]
	pub extern fn rust_begin_panic(msg: fmt::Arguments,
	file: &'static str,
	line: u32) -> ! {
	begin_panic_fmt(&msg, &(file, line))
	}

	/// 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")]
	#[inline(never)] #[cold]
	pub fn begin_panic_fmt(msg: &fmt::Arguments,
	file_line: &(&'static str, u32)) -> ! {
	use fmt::Write;

	// 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 mut s = String::new();
	let _ = s.write_fmt(*msg);
	begin_panic(s, file_line)
	}

	/// This is the entry point of panicking for panic!() and assert!().
	#[unstable(feature = "libstd_sys_internals",
	reason = "used by the panic! macro",
	issue = "0")]
	#[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
	pub fn begin_panic<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> ! {
	// 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(Box::new(msg), file_line)
	}

	/// Executes the primary logic for a panic, including checking for recursive
	/// panics and panic hooks.
	///
	/// This is the entry point or panics from libcore, formatted panics, and
	/// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
	/// run panic hooks, and then delegate to the actual implementation of panics.
	#[inline(never)]
	#[cold]
	fn rust_panic_with_hook(msg: Box<Any + Send>,
	file_line: &(&'static str, u32)) -> ! {
	let (file, line) = *file_line;

	let panics = PANIC_COUNT.with(\|c\| {
	let prev = c.get();
	c.set(prev + 1);
	prev
	});

	// 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 > 1 {
	util::dumb_print(format_args!("thread panicked while processing \
	panic. aborting.\n"));
	unsafe { intrinsics::abort() }
	}

	unsafe {
	let info = PanicInfo {
	payload: &*msg,
	location: Location {
	file: file,
	line: line,
	},
	};
	HOOK_LOCK.read();
	match HOOK {
	Hook::Default => default_hook(&info),
	Hook::Custom(ptr) => (*ptr)(&info),
	}
	HOOK_LOCK.read_unlock();
	}

	if panics > 0 {
	// 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(msg)
	}

	/// A private no-mangle function on which to slap yer breakpoints.
	#[no_mangle]
	#[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
	pub fn rust_panic(msg: Box<Any + Send>) -> ! {
	let code = unsafe {
	let obj = mem::transmute::<_, raw::TraitObject>(msg);
	__rust_start_panic(obj.data as usize, obj.vtable as usize)
	};
	rtabort!("failed to initiate panic, error {}", code)
	}