library/panic_unwind/src/seh.rs - third_party/rust - Git at Google

 //! Windows SEH
 //!
 //! On Windows (currently only on MSVC), the default exception handling
 //! mechanism is Structured Exception Handling (SEH). This is quite different
 //! than Dwarf-based exception handling (e.g., what other unix platforms use) in
 //! terms of compiler internals, so LLVM is required to have a good deal of
 //! extra support for SEH.
 //!
 //! In a nutshell, what happens here is:
 //!
 //! 1. The `panic` function calls the standard Windows function
 //!    `_CxxThrowException` to throw a C++-like exception, triggering the
 //!    unwinding process.
 //! 2. All landing pads generated by the compiler use the personality function
 //!    `__CxxFrameHandler3`, a function in the CRT, and the unwinding code in
 //!    Windows will use this personality function to execute all cleanup code on
 //!    the stack.
 //! 3. All compiler-generated calls to `invoke` have a landing pad set as a
 //!    `cleanuppad` LLVM instruction, which indicates the start of the cleanup
 //!    routine. The personality (in step 2, defined in the CRT) is responsible
 //!    for running the cleanup routines.
 //! 4. Eventually the "catch" code in the `try` intrinsic (generated by the
 //!    compiler) is executed and indicates that control should come back to
 //!    Rust. This is done via a `catchswitch` plus a `catchpad` instruction in
 //!    LLVM IR terms, finally returning normal control to the program with a
 //!    `catchret` instruction.
 //!
 //! Some specific differences from the gcc-based exception handling are:
 //!
 //! * Rust has no custom personality function, it is instead *always*
 //!   `__CxxFrameHandler3`. Additionally, no extra filtering is performed, so we
 //!   end up catching any C++ exceptions that happen to look like the kind we're
 //!   throwing. Note that throwing an exception into Rust is undefined behavior
 //!   anyway, so this should be fine.
 //! * We've got some data to transmit across the unwinding boundary,
 //!   specifically a `Box<dyn Any + Send>`. Like with Dwarf exceptions
 //!   these two pointers are stored as a payload in the exception itself. On
 //!   MSVC, however, there's no need for an extra heap allocation because the
 //!   call stack is preserved while filter functions are being executed. This
 //!   means that the pointers are passed directly to `_CxxThrowException` which
 //!   are then recovered in the filter function to be written to the stack frame
 //!   of the `try` intrinsic.
 //!
 //! [win64]: https://docs.microsoft.com/en-us/cpp/build/exception-handling-x64
 //! [llvm]: https://llvm.org/docs/ExceptionHandling.html#background-on-windows-exceptions

 #![allow(nonstandard_style)]

 use alloc::boxed::Box;
 use core::any::Any;
 use core::ffi::{c_int, c_uint, c_void};
 use core::mem::{self, ManuallyDrop};
 use core::ptr::{addr_of, addr_of_mut};

 // NOTE(nbdd0121): The `canary` field is part of stable ABI.
 #[repr(C)]
 struct Exception {
     // See `gcc.rs` on why this is present. We already have a static here so just use it.
     canary: *const _TypeDescriptor,

     // This needs to be an Option because we catch the exception by reference
     // and its destructor is executed by the C++ runtime. When we take the Box
     // out of the exception, we need to leave the exception in a valid state
     // for its destructor to run without double-dropping the Box.
     data: Option<Box<dyn Any + Send>>,
 }

 // First up, a whole bunch of type definitions. There's a few platform-specific
 // oddities here, and a lot that's just blatantly copied from LLVM. The purpose
 // of all this is to implement the `panic` function below through a call to
 // `_CxxThrowException`.
 //
 // This function takes two arguments. The first is a pointer to the data we're
 // passing in, which in this case is our trait object. Pretty easy to find! The
 // next, however, is more complicated. This is a pointer to a `_ThrowInfo`
 // structure, and it generally is just intended to just describe the exception
 // being thrown.
 //
 // Currently the definition of this type [1] is a little hairy, and the main
 // oddity (and difference from the online article) is that on 32-bit the
 // pointers are pointers but on 64-bit the pointers are expressed as 32-bit
 // offsets from the `__ImageBase` symbol. The `ptr_t` and `ptr!` macro in the
 // modules below are used to express this.
 //
 // The maze of type definitions also closely follows what LLVM emits for this
 // sort of operation. For example, if you compile this C++ code on MSVC and emit
 // the LLVM IR:
 //
 //      #include <stdint.h>
 //
 //      struct rust_panic {
 //          rust_panic(const rust_panic&);
 //          ~rust_panic();
 //
 //          uint64_t x[2];
 //      };
 //
 //      void foo() {
 //          rust_panic a = {0, 1};
 //          throw a;
 //      }
 //
 // That's essentially what we're trying to emulate. Most of the constant values
 // below were just copied from LLVM,
 //
 // In any case, these structures are all constructed in a similar manner, and
 // it's just somewhat verbose for us.
 //
 // [1]: https://www.geoffchappell.com/studies/msvc/language/predefined/

 #[cfg(target_arch = "x86")]
 mod imp {
     #[repr(transparent)]
     #[derive(Copy, Clone)]
     pub struct ptr_t(*mut u8);

     impl ptr_t {
         pub const fn null() -> Self {
             Self(core::ptr::null_mut())
         }

         pub const fn new(ptr: *mut u8) -> Self {
             Self(ptr)
         }

         pub const fn raw(self) -> *mut u8 {
             self.0
         }
     }
 }

 #[cfg(not(target_arch = "x86"))]
 mod imp {
     use core::ptr::addr_of;

     // On 64-bit systems, SEH represents pointers as 32-bit offsets from `__ImageBase`.
     #[repr(transparent)]
     #[derive(Copy, Clone)]
     pub struct ptr_t(u32);

     extern "C" {
         pub static __ImageBase: u8;
     }

     impl ptr_t {
         pub const fn null() -> Self {
             Self(0)
         }

         pub fn new(ptr: *mut u8) -> Self {
             // We need to expose the provenance of the pointer because it is not carried by
             // the `u32`, while the FFI needs to have this provenance to excess our statics.
             //
             // NOTE(niluxv): we could use `MaybeUninit<u32>` instead to leak the provenance
             // into the FFI. In theory then the other side would need to do some processing
             // to get a pointer with correct provenance, but these system functions aren't
             // going to be cross-lang LTOed anyway. However, using expose is shorter and
             // requires less unsafe.
             let addr: usize = ptr.expose_provenance();
             #[cfg(bootstrap)]
             let image_base = unsafe { addr_of!(__ImageBase) }.addr();
             #[cfg(not(bootstrap))]
             let image_base = addr_of!(__ImageBase).addr();
             let offset: usize = addr - image_base;
             Self(offset as u32)
         }

         pub const fn raw(self) -> u32 {
             self.0
         }
     }
 }

 use imp::ptr_t;

 #[repr(C)]
 pub struct _ThrowInfo {
     pub attributes: c_uint,
     pub pmfnUnwind: ptr_t,
     pub pForwardCompat: ptr_t,
     pub pCatchableTypeArray: ptr_t,
 }

 #[repr(C)]
 pub struct _CatchableTypeArray {
     pub nCatchableTypes: c_int,
     pub arrayOfCatchableTypes: [ptr_t; 1],
 }

 #[repr(C)]
 pub struct _CatchableType {
     pub properties: c_uint,
     pub pType: ptr_t,
     pub thisDisplacement: _PMD,
     pub sizeOrOffset: c_int,
     pub copyFunction: ptr_t,
 }

 #[repr(C)]
 pub struct _PMD {
     pub mdisp: c_int,
     pub pdisp: c_int,
     pub vdisp: c_int,
 }

 #[repr(C)]
 pub struct _TypeDescriptor {
     pub pVFTable: *const u8,
     pub spare: *mut u8,
     pub name: [u8; 11],
 }

 // Note that we intentionally ignore name mangling rules here: we don't want C++
 // to be able to catch Rust panics by simply declaring a `struct rust_panic`.
 //
 // When modifying, make sure that the type name string exactly matches
 // the one used in `compiler/rustc_codegen_llvm/src/intrinsic.rs`.
 const TYPE_NAME: [u8; 11] = *b"rust_panic\0";

 static mut THROW_INFO: _ThrowInfo = _ThrowInfo {
     attributes: 0,
     pmfnUnwind: ptr_t::null(),
     pForwardCompat: ptr_t::null(),
     pCatchableTypeArray: ptr_t::null(),
 };

 static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray =
     _CatchableTypeArray { nCatchableTypes: 1, arrayOfCatchableTypes: [ptr_t::null()] };

 static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
     properties: 0,
     pType: ptr_t::null(),
     thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
     sizeOrOffset: mem::size_of::<Exception>() as c_int,
     copyFunction: ptr_t::null(),
 };

 extern "C" {
     // The leading `\x01` byte here is actually a magical signal to LLVM to
     // *not* apply any other mangling like prefixing with a `_` character.
     //
     // This symbol is the vtable used by C++'s `std::type_info`. Objects of type
     // `std::type_info`, type descriptors, have a pointer to this table. Type
     // descriptors are referenced by the C++ EH structures defined above and
     // that we construct below.
     #[link_name = "\x01??_7type_info@@6B@"]
     static TYPE_INFO_VTABLE: *const u8;
 }

 // This type descriptor is only used when throwing an exception. The catch part
 // is handled by the try intrinsic, which generates its own TypeDescriptor.
 //
 // This is fine since the MSVC runtime uses string comparison on the type name
 // to match TypeDescriptors rather than pointer equality.
 static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
     #[cfg(bootstrap)]
     pVFTable: unsafe { addr_of!(TYPE_INFO_VTABLE) } as *const _,
     #[cfg(not(bootstrap))]
     pVFTable: addr_of!(TYPE_INFO_VTABLE) as *const _,
     spare: core::ptr::null_mut(),
     name: TYPE_NAME,
 };

 // Destructor used if the C++ code decides to capture the exception and drop it
 // without propagating it. The catch part of the try intrinsic will set the
 // first word of the exception object to 0 so that it is skipped by the
 // destructor.
 //
 // Note that x86 Windows uses the "thiscall" calling convention for C++ member
 // functions instead of the default "C" calling convention.
 //
 // The exception_copy function is a bit special here: it is invoked by the MSVC
 // runtime under a try/catch block and the panic that we generate here will be
 // used as the result of the exception copy. This is used by the C++ runtime to
 // support capturing exceptions with std::exception_ptr, which we can't support
 // because Box<dyn Any> isn't clonable.
 macro_rules! define_cleanup {
     ($abi:tt $abi2:tt) => {
         unsafe extern $abi fn exception_cleanup(e: *mut Exception) {
             if let Exception { data: Some(b), .. } = e.read() {
                 drop(b);
                 super::__rust_drop_panic();
             }
         }
         unsafe extern $abi2 fn exception_copy(
             _dest: *mut Exception, _src: *mut Exception
         ) -> *mut Exception {
             panic!("Rust panics cannot be copied");
         }
     }
 }
 cfg_if::cfg_if! {
    if #[cfg(target_arch = "x86")] {
        define_cleanup!("thiscall" "thiscall-unwind");
    } else {
        define_cleanup!("C" "C-unwind");
    }
 }

 pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     use core::intrinsics::atomic_store_seqcst;

     // _CxxThrowException executes entirely on this stack frame, so there's no
     // need to otherwise transfer `data` to the heap. We just pass a stack
     // pointer to this function.
     //
     // The ManuallyDrop is needed here since we don't want Exception to be
     // dropped when unwinding. Instead it will be dropped by exception_cleanup
     // which is invoked by the C++ runtime.
     let mut exception =
         ManuallyDrop::new(Exception { canary: addr_of!(TYPE_DESCRIPTOR), data: Some(data) });
     let throw_ptr = addr_of_mut!(exception) as *mut _;

     // This... may seems surprising, and justifiably so. On 32-bit MSVC the
     // pointers between these structure are just that, pointers. On 64-bit MSVC,
     // however, the pointers between structures are rather expressed as 32-bit
     // offsets from `__ImageBase`.
     //
     // Consequently, on 32-bit MSVC we can declare all these pointers in the
     // `static`s above. On 64-bit MSVC, we would have to express subtraction of
     // pointers in statics, which Rust does not currently allow, so we can't
     // actually do that.
     //
     // The next best thing, then is to fill in these structures at runtime
     // (panicking is already the "slow path" anyway). So here we reinterpret all
     // of these pointer fields as 32-bit integers and then store the
     // relevant value into it (atomically, as concurrent panics may be
     // happening). Technically the runtime will probably do a nonatomic read of
     // these fields, but in theory they never read the *wrong* value so it
     // shouldn't be too bad...
     //
     // In any case, we basically need to do something like this until we can
     // express more operations in statics (and we may never be able to).
     atomic_store_seqcst(
         addr_of_mut!(THROW_INFO.pmfnUnwind).cast(),
         ptr_t::new(exception_cleanup as *mut u8).raw(),
     );
     atomic_store_seqcst(
         addr_of_mut!(THROW_INFO.pCatchableTypeArray).cast(),
         ptr_t::new(addr_of_mut!(CATCHABLE_TYPE_ARRAY).cast()).raw(),
     );
     atomic_store_seqcst(
         addr_of_mut!(CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[0]).cast(),
         ptr_t::new(addr_of_mut!(CATCHABLE_TYPE).cast()).raw(),
     );
     atomic_store_seqcst(
         addr_of_mut!(CATCHABLE_TYPE.pType).cast(),
         ptr_t::new(addr_of_mut!(TYPE_DESCRIPTOR).cast()).raw(),
     );
     atomic_store_seqcst(
         addr_of_mut!(CATCHABLE_TYPE.copyFunction).cast(),
         ptr_t::new(exception_copy as *mut u8).raw(),
     );

     extern "system-unwind" {
         fn _CxxThrowException(pExceptionObject: *mut c_void, pThrowInfo: *mut u8) -> !;
     }

     _CxxThrowException(throw_ptr, addr_of_mut!(THROW_INFO) as *mut _);
 }

 pub unsafe fn cleanup(payload: *mut u8) -> Box<dyn Any + Send> {
     // A null payload here means that we got here from the catch (...) of
     // __rust_try. This happens when a non-Rust foreign exception is caught.
     if payload.is_null() {
         super::__rust_foreign_exception();
     }
     let exception = payload as *mut Exception;
     let canary = addr_of!((*exception).canary).read();
     if !core::ptr::eq(canary, addr_of!(TYPE_DESCRIPTOR)) {
         // A foreign Rust exception.
         super::__rust_foreign_exception();
     }
     (*exception).data.take().unwrap()
 }
	//! Windows SEH
	//!
	//! On Windows (currently only on MSVC), the default exception handling
	//! mechanism is Structured Exception Handling (SEH). This is quite different
	//! than Dwarf-based exception handling (e.g., what other unix platforms use) in
	//! terms of compiler internals, so LLVM is required to have a good deal of
	//! extra support for SEH.
	//!
	//! In a nutshell, what happens here is:
	//!
	//! 1. The `panic` function calls the standard Windows function
	//! `_CxxThrowException` to throw a C++-like exception, triggering the
	//! unwinding process.
	//! 2. All landing pads generated by the compiler use the personality function
	//! `__CxxFrameHandler3`, a function in the CRT, and the unwinding code in
	//! Windows will use this personality function to execute all cleanup code on
	//! the stack.
	//! 3. All compiler-generated calls to `invoke` have a landing pad set as a
	//! `cleanuppad` LLVM instruction, which indicates the start of the cleanup
	//! routine. The personality (in step 2, defined in the CRT) is responsible
	//! for running the cleanup routines.
	//! 4. Eventually the "catch" code in the `try` intrinsic (generated by the
	//! compiler) is executed and indicates that control should come back to
	//! Rust. This is done via a `catchswitch` plus a `catchpad` instruction in
	//! LLVM IR terms, finally returning normal control to the program with a
	//! `catchret` instruction.
	//!
	//! Some specific differences from the gcc-based exception handling are:
	//!
	//! * Rust has no custom personality function, it is instead always
	//! `__CxxFrameHandler3`. Additionally, no extra filtering is performed, so we
	//! end up catching any C++ exceptions that happen to look like the kind we're
	//! throwing. Note that throwing an exception into Rust is undefined behavior
	//! anyway, so this should be fine.
	//! * We've got some data to transmit across the unwinding boundary,
	//! specifically a `Box<dyn Any + Send>`. Like with Dwarf exceptions
	//! these two pointers are stored as a payload in the exception itself. On
	//! MSVC, however, there's no need for an extra heap allocation because the
	//! call stack is preserved while filter functions are being executed. This
	//! means that the pointers are passed directly to `_CxxThrowException` which
	//! are then recovered in the filter function to be written to the stack frame
	//! of the `try` intrinsic.
	//!
	//! [win64]: https://docs.microsoft.com/en-us/cpp/build/exception-handling-x64
	//! [llvm]: https://llvm.org/docs/ExceptionHandling.html#background-on-windows-exceptions

	#![allow(nonstandard_style)]

	use alloc::boxed::Box;
	use core::any::Any;
	use core::ffi::{c_int, c_uint, c_void};
	use core::mem::{self, ManuallyDrop};
	use core::ptr::{addr_of, addr_of_mut};

	// NOTE(nbdd0121): The `canary` field is part of stable ABI.
	#[repr(C)]
	struct Exception {
	// See `gcc.rs` on why this is present. We already have a static here so just use it.
	canary: *const _TypeDescriptor,

	// This needs to be an Option because we catch the exception by reference
	// and its destructor is executed by the C++ runtime. When we take the Box
	// out of the exception, we need to leave the exception in a valid state
	// for its destructor to run without double-dropping the Box.
	data: Option<Box<dyn Any + Send>>,
	}

	// First up, a whole bunch of type definitions. There's a few platform-specific
	// oddities here, and a lot that's just blatantly copied from LLVM. The purpose
	// of all this is to implement the `panic` function below through a call to
	// `_CxxThrowException`.
	//
	// This function takes two arguments. The first is a pointer to the data we're
	// passing in, which in this case is our trait object. Pretty easy to find! The
	// next, however, is more complicated. This is a pointer to a `_ThrowInfo`
	// structure, and it generally is just intended to just describe the exception
	// being thrown.
	//
	// Currently the definition of this type [1] is a little hairy, and the main
	// oddity (and difference from the online article) is that on 32-bit the
	// pointers are pointers but on 64-bit the pointers are expressed as 32-bit
	// offsets from the `__ImageBase` symbol. The `ptr_t` and `ptr!` macro in the
	// modules below are used to express this.
	//
	// The maze of type definitions also closely follows what LLVM emits for this
	// sort of operation. For example, if you compile this C++ code on MSVC and emit
	// the LLVM IR:
	//
	// #include <stdint.h>
	//
	// struct rust_panic {
	// rust_panic(const rust_panic&);
	// ~rust_panic();
	//
	// uint64_t x[2];
	// };
	//
	// void foo() {
	// rust_panic a = {0, 1};
	// throw a;
	// }
	//
	// That's essentially what we're trying to emulate. Most of the constant values
	// below were just copied from LLVM,
	//
	// In any case, these structures are all constructed in a similar manner, and
	// it's just somewhat verbose for us.
	//
	// [1]: https://www.geoffchappell.com/studies/msvc/language/predefined/

	#[cfg(target_arch = "x86")]
	mod imp {
	#[repr(transparent)]
	#[derive(Copy, Clone)]
	pub struct ptr_t(*mut u8);

	impl ptr_t {
	pub const fn null() -> Self {
	Self(core::ptr::null_mut())
	}

	pub const fn new(ptr: *mut u8) -> Self {
	Self(ptr)
	}

	pub const fn raw(self) -> *mut u8 {
	self.0
	}
	}
	}

	#[cfg(not(target_arch = "x86"))]
	mod imp {
	use core::ptr::addr_of;

	// On 64-bit systems, SEH represents pointers as 32-bit offsets from `__ImageBase`.
	#[repr(transparent)]
	#[derive(Copy, Clone)]
	pub struct ptr_t(u32);

	extern "C" {
	pub static __ImageBase: u8;
	}

	impl ptr_t {
	pub const fn null() -> Self {
	Self(0)
	}

	pub fn new(ptr: *mut u8) -> Self {
	// We need to expose the provenance of the pointer because it is not carried by
	// the `u32`, while the FFI needs to have this provenance to excess our statics.
	//
	// NOTE(niluxv): we could use `MaybeUninit<u32>` instead to leak the provenance
	// into the FFI. In theory then the other side would need to do some processing
	// to get a pointer with correct provenance, but these system functions aren't
	// going to be cross-lang LTOed anyway. However, using expose is shorter and
	// requires less unsafe.
	let addr: usize = ptr.expose_provenance();
	#[cfg(bootstrap)]
	let image_base = unsafe { addr_of!(__ImageBase) }.addr();
	#[cfg(not(bootstrap))]
	let image_base = addr_of!(__ImageBase).addr();
	let offset: usize = addr - image_base;
	Self(offset as u32)
	}

	pub const fn raw(self) -> u32 {
	self.0
	}
	}
	}

	use imp::ptr_t;

	#[repr(C)]
	pub struct _ThrowInfo {
	pub attributes: c_uint,
	pub pmfnUnwind: ptr_t,
	pub pForwardCompat: ptr_t,
	pub pCatchableTypeArray: ptr_t,
	}

	#[repr(C)]
	pub struct _CatchableTypeArray {
	pub nCatchableTypes: c_int,
	pub arrayOfCatchableTypes: [ptr_t; 1],
	}

	#[repr(C)]
	pub struct _CatchableType {
	pub properties: c_uint,
	pub pType: ptr_t,
	pub thisDisplacement: _PMD,
	pub sizeOrOffset: c_int,
	pub copyFunction: ptr_t,
	}

	#[repr(C)]
	pub struct _PMD {
	pub mdisp: c_int,
	pub pdisp: c_int,
	pub vdisp: c_int,
	}

	#[repr(C)]
	pub struct _TypeDescriptor {
	pub pVFTable: *const u8,
	pub spare: *mut u8,
	pub name: [u8; 11],
	}

	// Note that we intentionally ignore name mangling rules here: we don't want C++
	// to be able to catch Rust panics by simply declaring a `struct rust_panic`.
	//
	// When modifying, make sure that the type name string exactly matches
	// the one used in `compiler/rustc_codegen_llvm/src/intrinsic.rs`.
	const TYPE_NAME: [u8; 11] = *b"rust_panic\0";

	static mut THROW_INFO: _ThrowInfo = _ThrowInfo {
	attributes: 0,
	pmfnUnwind: ptr_t::null(),
	pForwardCompat: ptr_t::null(),
	pCatchableTypeArray: ptr_t::null(),
	};

	static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray =
	_CatchableTypeArray { nCatchableTypes: 1, arrayOfCatchableTypes: [ptr_t::null()] };

	static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
	properties: 0,
	pType: ptr_t::null(),
	thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
	sizeOrOffset: mem::size_of::<Exception>() as c_int,
	copyFunction: ptr_t::null(),
	};

	extern "C" {
	// The leading `\x01` byte here is actually a magical signal to LLVM to
	// not apply any other mangling like prefixing with a `_` character.
	//
	// This symbol is the vtable used by C++'s `std::type_info`. Objects of type
	// `std::type_info`, type descriptors, have a pointer to this table. Type
	// descriptors are referenced by the C++ EH structures defined above and
	// that we construct below.
	#[link_name = "\x01??_7type_info@@6B@"]
	static TYPE_INFO_VTABLE: *const u8;
	}

	// This type descriptor is only used when throwing an exception. The catch part
	// is handled by the try intrinsic, which generates its own TypeDescriptor.
	//
	// This is fine since the MSVC runtime uses string comparison on the type name
	// to match TypeDescriptors rather than pointer equality.
	static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
	#[cfg(bootstrap)]
	pVFTable: unsafe { addr_of!(TYPE_INFO_VTABLE) } as *const _,
	#[cfg(not(bootstrap))]
	pVFTable: addr_of!(TYPE_INFO_VTABLE) as *const _,
	spare: core::ptr::null_mut(),
	name: TYPE_NAME,
	};

	// Destructor used if the C++ code decides to capture the exception and drop it
	// without propagating it. The catch part of the try intrinsic will set the
	// first word of the exception object to 0 so that it is skipped by the
	// destructor.
	//
	// Note that x86 Windows uses the "thiscall" calling convention for C++ member
	// functions instead of the default "C" calling convention.
	//
	// The exception_copy function is a bit special here: it is invoked by the MSVC
	// runtime under a try/catch block and the panic that we generate here will be
	// used as the result of the exception copy. This is used by the C++ runtime to
	// support capturing exceptions with std::exception_ptr, which we can't support
	// because Box<dyn Any> isn't clonable.
	macro_rules! define_cleanup {
	($abi:tt $abi2:tt) => {
	unsafe extern $abi fn exception_cleanup(e: *mut Exception) {
	if let Exception { data: Some(b), .. } = e.read() {
	drop(b);
	super::__rust_drop_panic();
	}
	}
	unsafe extern $abi2 fn exception_copy(
	_dest: mut Exception, _src: mut Exception
	) -> *mut Exception {
	panic!("Rust panics cannot be copied");
	}
	}
	}
	cfg_if::cfg_if! {
	if #[cfg(target_arch = "x86")] {
	define_cleanup!("thiscall" "thiscall-unwind");
	} else {
	define_cleanup!("C" "C-unwind");
	}
	}

	pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
	use core::intrinsics::atomic_store_seqcst;

	// _CxxThrowException executes entirely on this stack frame, so there's no
	// need to otherwise transfer `data` to the heap. We just pass a stack
	// pointer to this function.
	//
	// The ManuallyDrop is needed here since we don't want Exception to be
	// dropped when unwinding. Instead it will be dropped by exception_cleanup
	// which is invoked by the C++ runtime.
	let mut exception =
	ManuallyDrop::new(Exception { canary: addr_of!(TYPE_DESCRIPTOR), data: Some(data) });
	let throw_ptr = addr_of_mut!(exception) as *mut _;

	// This... may seems surprising, and justifiably so. On 32-bit MSVC the
	// pointers between these structure are just that, pointers. On 64-bit MSVC,
	// however, the pointers between structures are rather expressed as 32-bit
	// offsets from `__ImageBase`.
	//
	// Consequently, on 32-bit MSVC we can declare all these pointers in the
	// `static`s above. On 64-bit MSVC, we would have to express subtraction of
	// pointers in statics, which Rust does not currently allow, so we can't
	// actually do that.
	//
	// The next best thing, then is to fill in these structures at runtime
	// (panicking is already the "slow path" anyway). So here we reinterpret all
	// of these pointer fields as 32-bit integers and then store the
	// relevant value into it (atomically, as concurrent panics may be
	// happening). Technically the runtime will probably do a nonatomic read of
	// these fields, but in theory they never read the wrong value so it
	// shouldn't be too bad...
	//
	// In any case, we basically need to do something like this until we can
	// express more operations in statics (and we may never be able to).
	atomic_store_seqcst(
	addr_of_mut!(THROW_INFO.pmfnUnwind).cast(),
	ptr_t::new(exception_cleanup as *mut u8).raw(),
	);
	atomic_store_seqcst(
	addr_of_mut!(THROW_INFO.pCatchableTypeArray).cast(),
	ptr_t::new(addr_of_mut!(CATCHABLE_TYPE_ARRAY).cast()).raw(),
	);
	atomic_store_seqcst(
	addr_of_mut!(CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[0]).cast(),
	ptr_t::new(addr_of_mut!(CATCHABLE_TYPE).cast()).raw(),
	);
	atomic_store_seqcst(
	addr_of_mut!(CATCHABLE_TYPE.pType).cast(),
	ptr_t::new(addr_of_mut!(TYPE_DESCRIPTOR).cast()).raw(),
	);
	atomic_store_seqcst(
	addr_of_mut!(CATCHABLE_TYPE.copyFunction).cast(),
	ptr_t::new(exception_copy as *mut u8).raw(),
	);

	extern "system-unwind" {
	fn _CxxThrowException(pExceptionObject: mut c_void, pThrowInfo: mut u8) -> !;
	}

	_CxxThrowException(throw_ptr, addr_of_mut!(THROW_INFO) as *mut _);
	}

	pub unsafe fn cleanup(payload: *mut u8) -> Box<dyn Any + Send> {
	// A null payload here means that we got here from the catch (...) of
	// __rust_try. This happens when a non-Rust foreign exception is caught.
	if payload.is_null() {
	super::__rust_foreign_exception();
	}
	let exception = payload as *mut Exception;
	let canary = addr_of!((*exception).canary).read();
	if !core::ptr::eq(canary, addr_of!(TYPE_DESCRIPTOR)) {
	// A foreign Rust exception.
	super::__rust_foreign_exception();
	}
	(*exception).data.take().unwrap()
	}