Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

10 files changed, 1272 insertions, 0 deletions

diff --git a/library/panic_unwind/src/dummy.rs b/library/panic_unwind/src/dummy.rs
new file mode 100644
index 000000000..a4bcd216c
--- /dev/null
+++ b/library/panic_unwind/src/dummy.rs
@@ -0,0 +1,15 @@
+//! Unwinding for unsupported target.
+//!
+//! Stubs that simply abort for targets that don't support unwinding otherwise.
+
+use alloc::boxed::Box;
+use core::any::Any;
+use core::intrinsics;
+
+pub unsafe fn cleanup(_ptr: *mut u8) -> Box<dyn Any + Send> {
+    intrinsics::abort()
+}
+
+pub unsafe fn panic(_data: Box<dyn Any + Send>) -> u32 {
+    intrinsics::abort()
+}
diff --git a/library/panic_unwind/src/dwarf/eh.rs b/library/panic_unwind/src/dwarf/eh.rs
new file mode 100644
index 000000000..7394feab8
--- /dev/null
+++ b/library/panic_unwind/src/dwarf/eh.rs
@@ -0,0 +1,192 @@
+//! Parsing of GCC-style Language-Specific Data Area (LSDA)
+//! For details see:
+//!  * <https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html>
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/exceptions.pdf>
+//!  * <https://www.airs.com/blog/archives/460>
+//!  * <https://www.airs.com/blog/archives/464>
+//!
+//! A reference implementation may be found in the GCC source tree
+//! (`<root>/libgcc/unwind-c.c` as of this writing).
+
+#![allow(non_upper_case_globals)]
+#![allow(unused)]
+
+use crate::dwarf::DwarfReader;
+use core::mem;
+
+pub const DW_EH_PE_omit: u8 = 0xFF;
+pub const DW_EH_PE_absptr: u8 = 0x00;
+
+pub const DW_EH_PE_uleb128: u8 = 0x01;
+pub const DW_EH_PE_udata2: u8 = 0x02;
+pub const DW_EH_PE_udata4: u8 = 0x03;
+pub const DW_EH_PE_udata8: u8 = 0x04;
+pub const DW_EH_PE_sleb128: u8 = 0x09;
+pub const DW_EH_PE_sdata2: u8 = 0x0A;
+pub const DW_EH_PE_sdata4: u8 = 0x0B;
+pub const DW_EH_PE_sdata8: u8 = 0x0C;
+
+pub const DW_EH_PE_pcrel: u8 = 0x10;
+pub const DW_EH_PE_textrel: u8 = 0x20;
+pub const DW_EH_PE_datarel: u8 = 0x30;
+pub const DW_EH_PE_funcrel: u8 = 0x40;
+pub const DW_EH_PE_aligned: u8 = 0x50;
+
+pub const DW_EH_PE_indirect: u8 = 0x80;
+
+#[derive(Copy, Clone)]
+pub struct EHContext<'a> {
+    pub ip: usize,                             // Current instruction pointer
+    pub func_start: usize,                     // Address of the current function
+    pub get_text_start: &'a dyn Fn() -> usize, // Get address of the code section
+    pub get_data_start: &'a dyn Fn() -> usize, // Get address of the data section
+}
+
+pub enum EHAction {
+    None,
+    Cleanup(usize),
+    Catch(usize),
+    Terminate,
+}
+
+pub const USING_SJLJ_EXCEPTIONS: bool = cfg!(all(target_os = "ios", target_arch = "arm"));
+
+pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>) -> Result<EHAction, ()> {
+    if lsda.is_null() {
+        return Ok(EHAction::None);
+    }
+
+    let func_start = context.func_start;
+    let mut reader = DwarfReader::new(lsda);
+
+    let start_encoding = reader.read::<u8>();
+    // base address for landing pad offsets
+    let lpad_base = if start_encoding != DW_EH_PE_omit {
+        read_encoded_pointer(&mut reader, context, start_encoding)?
+    } else {
+        func_start
+    };
+
+    let ttype_encoding = reader.read::<u8>();
+    if ttype_encoding != DW_EH_PE_omit {
+        // Rust doesn't analyze exception types, so we don't care about the type table
+        reader.read_uleb128();
+    }
+
+    let call_site_encoding = reader.read::<u8>();
+    let call_site_table_length = reader.read_uleb128();
+    let action_table = reader.ptr.offset(call_site_table_length as isize);
+    let ip = context.ip;
+
+    if !USING_SJLJ_EXCEPTIONS {
+        while reader.ptr < action_table {
+            let cs_start = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_len = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_lpad = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_action = reader.read_uleb128();
+            // Callsite table is sorted by cs_start, so if we've passed the ip, we
+            // may stop searching.
+            if ip < func_start + cs_start {
+                break;
+            }
+            if ip < func_start + cs_start + cs_len {
+                if cs_lpad == 0 {
+                    return Ok(EHAction::None);
+                } else {
+                    let lpad = lpad_base + cs_lpad;
+                    return Ok(interpret_cs_action(cs_action, lpad));
+                }
+            }
+        }
+        // Ip is not present in the table.  This should not happen... but it does: issue #35011.
+        // So rather than returning EHAction::Terminate, we do this.
+        Ok(EHAction::None)
+    } else {
+        // SjLj version:
+        // The "IP" is an index into the call-site table, with two exceptions:
+        // -1 means 'no-action', and 0 means 'terminate'.
+        match ip as isize {
+            -1 => return Ok(EHAction::None),
+            0 => return Ok(EHAction::Terminate),
+            _ => (),
+        }
+        let mut idx = ip;
+        loop {
+            let cs_lpad = reader.read_uleb128();
+            let cs_action = reader.read_uleb128();
+            idx -= 1;
+            if idx == 0 {
+                // Can never have null landing pad for sjlj -- that would have
+                // been indicated by a -1 call site index.
+                let lpad = (cs_lpad + 1) as usize;
+                return Ok(interpret_cs_action(cs_action, lpad));
+            }
+        }
+    }
+}
+
+fn interpret_cs_action(cs_action: u64, lpad: usize) -> EHAction {
+    if cs_action == 0 {
+        // If cs_action is 0 then this is a cleanup (Drop::drop). We run these
+        // for both Rust panics and foreign exceptions.
+        EHAction::Cleanup(lpad)
+    } else {
+        // Stop unwinding Rust panics at catch_unwind.
+        EHAction::Catch(lpad)
+    }
+}
+
+#[inline]
+fn round_up(unrounded: usize, align: usize) -> Result<usize, ()> {
+    if align.is_power_of_two() { Ok((unrounded + align - 1) & !(align - 1)) } else { Err(()) }
+}
+
+unsafe fn read_encoded_pointer(
+    reader: &mut DwarfReader,
+    context: &EHContext<'_>,
+    encoding: u8,
+) -> Result<usize, ()> {
+    if encoding == DW_EH_PE_omit {
+        return Err(());
+    }
+
+    // DW_EH_PE_aligned implies it's an absolute pointer value
+    if encoding == DW_EH_PE_aligned {
+        reader.ptr = round_up(reader.ptr as usize, mem::size_of::<usize>())? as *const u8;
+        return Ok(reader.read::<usize>());
+    }
+
+    let mut result = match encoding & 0x0F {
+        DW_EH_PE_absptr => reader.read::<usize>(),
+        DW_EH_PE_uleb128 => reader.read_uleb128() as usize,
+        DW_EH_PE_udata2 => reader.read::<u16>() as usize,
+        DW_EH_PE_udata4 => reader.read::<u32>() as usize,
+        DW_EH_PE_udata8 => reader.read::<u64>() as usize,
+        DW_EH_PE_sleb128 => reader.read_sleb128() as usize,
+        DW_EH_PE_sdata2 => reader.read::<i16>() as usize,
+        DW_EH_PE_sdata4 => reader.read::<i32>() as usize,
+        DW_EH_PE_sdata8 => reader.read::<i64>() as usize,
+        _ => return Err(()),
+    };
+
+    result += match encoding & 0x70 {
+        DW_EH_PE_absptr => 0,
+        // relative to address of the encoded value, despite the name
+        DW_EH_PE_pcrel => reader.ptr as usize,
+        DW_EH_PE_funcrel => {
+            if context.func_start == 0 {
+                return Err(());
+            }
+            context.func_start
+        }
+        DW_EH_PE_textrel => (*context.get_text_start)(),
+        DW_EH_PE_datarel => (*context.get_data_start)(),
+        _ => return Err(()),
+    };
+
+    if encoding & DW_EH_PE_indirect != 0 {
+        result = *(result as *const usize);
+    }
+
+    Ok(result)
+}
diff --git a/library/panic_unwind/src/dwarf/mod.rs b/library/panic_unwind/src/dwarf/mod.rs
new file mode 100644
index 000000000..652fbe95a
--- /dev/null
+++ b/library/panic_unwind/src/dwarf/mod.rs
@@ -0,0 +1,73 @@
+//! Utilities for parsing DWARF-encoded data streams.
+//! See <http://www.dwarfstd.org>,
+//! DWARF-4 standard, Section 7 - "Data Representation"
+
+// This module is used only by x86_64-pc-windows-gnu for now, but we
+// are compiling it everywhere to avoid regressions.
+#![allow(unused)]
+
+#[cfg(test)]
+mod tests;
+
+pub mod eh;
+
+use core::mem;
+
+pub struct DwarfReader {
+    pub ptr: *const u8,
+}
+
+#[repr(C, packed)]
+struct Unaligned<T>(T);
+
+impl DwarfReader {
+    pub fn new(ptr: *const u8) -> DwarfReader {
+        DwarfReader { ptr }
+    }
+
+    // DWARF streams are packed, so e.g., a u32 would not necessarily be aligned
+    // on a 4-byte boundary. This may cause problems on platforms with strict
+    // alignment requirements. By wrapping data in a "packed" struct, we are
+    // telling the backend to generate "misalignment-safe" code.
+    pub unsafe fn read<T: Copy>(&mut self) -> T {
+        let Unaligned(result) = *(self.ptr as *const Unaligned<T>);
+        self.ptr = self.ptr.add(mem::size_of::<T>());
+        result
+    }
+
+    // ULEB128 and SLEB128 encodings are defined in Section 7.6 - "Variable
+    // Length Data".
+    pub unsafe fn read_uleb128(&mut self) -> u64 {
+        let mut shift: usize = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        result
+    }
+
+    pub unsafe fn read_sleb128(&mut self) -> i64 {
+        let mut shift: u32 = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        // sign-extend
+        if shift < u64::BITS && (byte & 0x40) != 0 {
+            result |= (!0 as u64) << shift;
+        }
+        result as i64
+    }
+}
diff --git a/library/panic_unwind/src/dwarf/tests.rs b/library/panic_unwind/src/dwarf/tests.rs
new file mode 100644
index 000000000..1644f3708
--- /dev/null
+++ b/library/panic_unwind/src/dwarf/tests.rs
@@ -0,0 +1,19 @@
+use super::*;
+
+#[test]
+fn dwarf_reader() {
+    let encoded: &[u8] = &[1, 2, 3, 4, 5, 6, 7, 0xE5, 0x8E, 0x26, 0x9B, 0xF1, 0x59, 0xFF, 0xFF];
+
+    let mut reader = DwarfReader::new(encoded.as_ptr());
+
+    unsafe {
+        assert!(reader.read::<u8>() == u8::to_be(1u8));
+        assert!(reader.read::<u16>() == u16::to_be(0x0203));
+        assert!(reader.read::<u32>() == u32::to_be(0x04050607));
+
+        assert!(reader.read_uleb128() == 624485);
+        assert!(reader.read_sleb128() == -624485);
+
+        assert!(reader.read::<i8>() == i8::to_be(-1));
+    }
+}
diff --git a/library/panic_unwind/src/emcc.rs b/library/panic_unwind/src/emcc.rs
new file mode 100644
index 000000000..1ee69ff9c
--- /dev/null
+++ b/library/panic_unwind/src/emcc.rs
@@ -0,0 +1,132 @@
+//! Unwinding for *emscripten* target.
+//!
+//! Whereas Rust's usual unwinding implementation for Unix platforms
+//! calls into the libunwind APIs directly, on Emscripten we instead
+//! call into the C++ unwinding APIs. This is just an expedience since
+//! Emscripten's runtime always implements those APIs and does not
+//! implement libunwind.
+
+use alloc::boxed::Box;
+use core::any::Any;
+use core::intrinsics;
+use core::mem;
+use core::ptr;
+use core::sync::atomic::{AtomicBool, Ordering};
+use libc::{self, c_int};
+use unwind as uw;
+
+// This matches the layout of std::type_info in C++
+#[repr(C)]
+struct TypeInfo {
+    vtable: *const usize,
+    name: *const u8,
+}
+unsafe impl Sync for TypeInfo {}
+
+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.
+    //
+    // Note that the real size is larger than 3 usize, but we only need our
+    // vtable to point to the third element.
+    #[link_name = "\x01_ZTVN10__cxxabiv117__class_type_infoE"]
+    static CLASS_TYPE_INFO_VTABLE: [usize; 3];
+}
+
+// std::type_info for a rust_panic class
+#[lang = "eh_catch_typeinfo"]
+static EXCEPTION_TYPE_INFO: TypeInfo = TypeInfo {
+    // Normally we would use .as_ptr().add(2) but this doesn't work in a const context.
+    vtable: unsafe { &CLASS_TYPE_INFO_VTABLE[2] },
+    // This intentionally doesn't use the normal name mangling scheme because
+    // we don't want C++ to be able to produce or catch Rust panics.
+    name: b"rust_panic\0".as_ptr(),
+};
+
+struct Exception {
+    // This is necessary because C++ code can capture our exception with
+    // std::exception_ptr and rethrow it multiple times, possibly even in
+    // another thread.
+    caught: AtomicBool,
+
+    // This needs to be an Option because the object's lifetime follows C++
+    // semantics: when catch_unwind moves the Box out of the exception it must
+    // still leave the exception object in a valid state because its destructor
+    // is still going to be called by __cxa_end_catch.
+    data: Option<Box<dyn Any + Send>>,
+}
+
+pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
+    // intrinsics::try actually gives us a pointer to this structure.
+    #[repr(C)]
+    struct CatchData {
+        ptr: *mut u8,
+        is_rust_panic: bool,
+    }
+    let catch_data = &*(ptr as *mut CatchData);
+
+    let adjusted_ptr = __cxa_begin_catch(catch_data.ptr as *mut libc::c_void) as *mut Exception;
+    let out = if catch_data.is_rust_panic {
+        let was_caught = (*adjusted_ptr).caught.swap(true, Ordering::SeqCst);
+        if was_caught {
+            // Since cleanup() isn't allowed to panic, we just abort instead.
+            intrinsics::abort();
+        }
+        (*adjusted_ptr).data.take().unwrap()
+    } else {
+        super::__rust_foreign_exception();
+    };
+    __cxa_end_catch();
+    out
+}
+
+pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
+    let sz = mem::size_of_val(&data);
+    let exception = __cxa_allocate_exception(sz) as *mut Exception;
+    if exception.is_null() {
+        return uw::_URC_FATAL_PHASE1_ERROR as u32;
+    }
+    ptr::write(exception, Exception { caught: AtomicBool::new(false), data: Some(data) });
+    __cxa_throw(exception as *mut _, &EXCEPTION_TYPE_INFO, exception_cleanup);
+}
+
+extern "C" fn exception_cleanup(ptr: *mut libc::c_void) -> *mut libc::c_void {
+    unsafe {
+        if let Some(b) = (ptr as *mut Exception).read().data {
+            drop(b);
+            super::__rust_drop_panic();
+        }
+        ptr
+    }
+}
+
+// This is required by the compiler to exist (e.g., it's a lang item), but it's
+// never actually called by the compiler.  Emscripten EH doesn't use a
+// personality function at all, it instead uses __cxa_find_matching_catch.
+// Wasm error handling would use __gxx_personality_wasm0.
+#[lang = "eh_personality"]
+unsafe extern "C" fn rust_eh_personality(
+    _version: c_int,
+    _actions: uw::_Unwind_Action,
+    _exception_class: uw::_Unwind_Exception_Class,
+    _exception_object: *mut uw::_Unwind_Exception,
+    _context: *mut uw::_Unwind_Context,
+) -> uw::_Unwind_Reason_Code {
+    core::intrinsics::abort()
+}
+
+extern "C" {
+    fn __cxa_allocate_exception(thrown_size: libc::size_t) -> *mut libc::c_void;
+    fn __cxa_begin_catch(thrown_exception: *mut libc::c_void) -> *mut libc::c_void;
+    fn __cxa_end_catch();
+    fn __cxa_throw(
+        thrown_exception: *mut libc::c_void,
+        tinfo: *const TypeInfo,
+        dest: extern "C" fn(*mut libc::c_void) -> *mut libc::c_void,
+    ) -> !;
+}
diff --git a/library/panic_unwind/src/gcc.rs b/library/panic_unwind/src/gcc.rs
new file mode 100644
index 000000000..a59659231
--- /dev/null
+++ b/library/panic_unwind/src/gcc.rs
@@ -0,0 +1,351 @@
+//! Implementation of panics backed by libgcc/libunwind (in some form).
+//!
+//! For background on exception handling and stack unwinding please see
+//! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
+//! documents linked from it.
+//! These are also good reads:
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html>
+//!  * <https://monoinfinito.wordpress.com/series/exception-handling-in-c/>
+//!  * <https://www.airs.com/blog/index.php?s=exception+frames>
+//!
+//! ## A brief summary
+//!
+//! Exception handling happens in two phases: a search phase and a cleanup
+//! phase.
+//!
+//! In both phases the unwinder walks stack frames from top to bottom using
+//! information from the stack frame unwind sections of the current process's
+//! modules ("module" here refers to an OS module, i.e., an executable or a
+//! dynamic library).
+//!
+//! For each stack frame, it invokes the associated "personality routine", whose
+//! address is also stored in the unwind info section.
+//!
+//! In the search phase, the job of a personality routine is to examine
+//! exception object being thrown, and to decide whether it should be caught at
+//! that stack frame. Once the handler frame has been identified, cleanup phase
+//! begins.
+//!
+//! In the cleanup phase, the unwinder invokes each personality routine again.
+//! This time it decides which (if any) cleanup code needs to be run for
+//! the current stack frame. If so, the control is transferred to a special
+//! branch in the function body, the "landing pad", which invokes destructors,
+//! frees memory, etc. At the end of the landing pad, control is transferred
+//! back to the unwinder and unwinding resumes.
+//!
+//! Once stack has been unwound down to the handler frame level, unwinding stops
+//! and the last personality routine transfers control to the catch block.
+
+use alloc::boxed::Box;
+use core::any::Any;
+
+use crate::dwarf::eh::{self, EHAction, EHContext};
+use libc::{c_int, uintptr_t};
+use unwind as uw;
+
+#[repr(C)]
+struct Exception {
+    _uwe: uw::_Unwind_Exception,
+    cause: Box<dyn Any + Send>,
+}
+
+pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
+    let exception = Box::new(Exception {
+        _uwe: uw::_Unwind_Exception {
+            exception_class: rust_exception_class(),
+            exception_cleanup,
+            private: [0; uw::unwinder_private_data_size],
+        },
+        cause: data,
+    });
+    let exception_param = Box::into_raw(exception) as *mut uw::_Unwind_Exception;
+    return uw::_Unwind_RaiseException(exception_param) as u32;
+
+    extern "C" fn exception_cleanup(
+        _unwind_code: uw::_Unwind_Reason_Code,
+        exception: *mut uw::_Unwind_Exception,
+    ) {
+        unsafe {
+            let _: Box<Exception> = Box::from_raw(exception as *mut Exception);
+            super::__rust_drop_panic();
+        }
+    }
+}
+
+pub unsafe fn cleanup(ptr: *mut u8) -> Box<dyn Any + Send> {
+    let exception = ptr as *mut uw::_Unwind_Exception;
+    if (*exception).exception_class != rust_exception_class() {
+        uw::_Unwind_DeleteException(exception);
+        super::__rust_foreign_exception();
+    } else {
+        let exception = Box::from_raw(exception as *mut Exception);
+        exception.cause
+    }
+}
+
+// Rust's exception class identifier.  This is used by personality routines to
+// determine whether the exception was thrown by their own runtime.
+fn rust_exception_class() -> uw::_Unwind_Exception_Class {
+    // M O Z \0  R U S T -- vendor, language
+    0x4d4f5a_00_52555354
+}
+
+// Register ids were lifted from LLVM's TargetLowering::getExceptionPointerRegister()
+// and TargetLowering::getExceptionSelectorRegister() for each architecture,
+// then mapped to DWARF register numbers via register definition tables
+// (typically <arch>RegisterInfo.td, search for "DwarfRegNum").
+// See also https://llvm.org/docs/WritingAnLLVMBackend.html#defining-a-register.
+
+#[cfg(target_arch = "x86")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 2); // EAX, EDX
+
+#[cfg(target_arch = "x86_64")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // RAX, RDX
+
+#[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1 / X0, X1
+
+#[cfg(target_arch = "m68k")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // D0, D1
+
+#[cfg(any(target_arch = "mips", target_arch = "mips64"))]
+const UNWIND_DATA_REG: (i32, i32) = (4, 5); // A0, A1
+
+#[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))]
+const UNWIND_DATA_REG: (i32, i32) = (3, 4); // R3, R4 / X3, X4
+
+#[cfg(target_arch = "s390x")]
+const UNWIND_DATA_REG: (i32, i32) = (6, 7); // R6, R7
+
+#[cfg(any(target_arch = "sparc", target_arch = "sparc64"))]
+const UNWIND_DATA_REG: (i32, i32) = (24, 25); // I0, I1
+
+#[cfg(target_arch = "hexagon")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1
+
+#[cfg(any(target_arch = "riscv64", target_arch = "riscv32"))]
+const UNWIND_DATA_REG: (i32, i32) = (10, 11); // x10, x11
+
+// The following code is based on GCC's C and C++ personality routines.  For reference, see:
+// https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/libsupc++/eh_personality.cc
+// https://github.com/gcc-mirror/gcc/blob/trunk/libgcc/unwind-c.c
+
+cfg_if::cfg_if! {
+    if #[cfg(all(target_arch = "arm", not(target_os = "ios"), not(target_os = "watchos"), not(target_os = "netbsd")))] {
+        // ARM EHABI personality routine.
+        // https://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf
+        //
+        // iOS uses the default routine instead since it uses SjLj unwinding.
+        #[lang = "eh_personality"]
+        unsafe extern "C" fn rust_eh_personality(state: uw::_Unwind_State,
+                                                 exception_object: *mut uw::_Unwind_Exception,
+                                                 context: *mut uw::_Unwind_Context)
+                                                 -> uw::_Unwind_Reason_Code {
+            let state = state as c_int;
+            let action = state & uw::_US_ACTION_MASK as c_int;
+            let search_phase = if action == uw::_US_VIRTUAL_UNWIND_FRAME as c_int {
+                // Backtraces on ARM will call the personality routine with
+                // state == _US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWIND. In those cases
+                // we want to continue unwinding the stack, otherwise all our backtraces
+                // would end at __rust_try
+                if state & uw::_US_FORCE_UNWIND as c_int != 0 {
+                    return continue_unwind(exception_object, context);
+                }
+                true
+            } else if action == uw::_US_UNWIND_FRAME_STARTING as c_int {
+                false
+            } else if action == uw::_US_UNWIND_FRAME_RESUME as c_int {
+                return continue_unwind(exception_object, context);
+            } else {
+                return uw::_URC_FAILURE;
+            };
+
+            // The DWARF unwinder assumes that _Unwind_Context holds things like the function
+            // and LSDA pointers, however ARM EHABI places them into the exception object.
+            // To preserve signatures of functions like _Unwind_GetLanguageSpecificData(), which
+            // take only the context pointer, GCC personality routines stash a pointer to
+            // exception_object in the context, using location reserved for ARM's
+            // "scratch register" (r12).
+            uw::_Unwind_SetGR(context,
+                              uw::UNWIND_POINTER_REG,
+                              exception_object as uw::_Unwind_Ptr);
+            // ...A more principled approach would be to provide the full definition of ARM's
+            // _Unwind_Context in our libunwind bindings and fetch the required data from there
+            // directly, bypassing DWARF compatibility functions.
+
+            let eh_action = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FAILURE,
+            };
+            if search_phase {
+                match eh_action {
+                    EHAction::None |
+                    EHAction::Cleanup(_) => return continue_unwind(exception_object, context),
+                    EHAction::Catch(_) => {
+                        // EHABI requires the personality routine to update the
+                        // SP value in the barrier cache of the exception object.
+                        (*exception_object).private[5] =
+                            uw::_Unwind_GetGR(context, uw::UNWIND_SP_REG);
+                        return uw::_URC_HANDLER_FOUND;
+                    }
+                    EHAction::Terminate => return uw::_URC_FAILURE,
+                }
+            } else {
+                match eh_action {
+                    EHAction::None => return continue_unwind(exception_object, context),
+                    EHAction::Cleanup(lpad) |
+                    EHAction::Catch(lpad) => {
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.0,
+                                          exception_object as uintptr_t);
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
+                        uw::_Unwind_SetIP(context, lpad);
+                        return uw::_URC_INSTALL_CONTEXT;
+                    }
+                    EHAction::Terminate => return uw::_URC_FAILURE,
+                }
+            }
+
+            // On ARM EHABI the personality routine is responsible for actually
+            // unwinding a single stack frame before returning (ARM EHABI Sec. 6.1).
+            unsafe fn continue_unwind(exception_object: *mut uw::_Unwind_Exception,
+                                      context: *mut uw::_Unwind_Context)
+                                      -> uw::_Unwind_Reason_Code {
+                if __gnu_unwind_frame(exception_object, context) == uw::_URC_NO_REASON {
+                    uw::_URC_CONTINUE_UNWIND
+                } else {
+                    uw::_URC_FAILURE
+                }
+            }
+            // defined in libgcc
+            extern "C" {
+                fn __gnu_unwind_frame(exception_object: *mut uw::_Unwind_Exception,
+                                      context: *mut uw::_Unwind_Context)
+                                      -> uw::_Unwind_Reason_Code;
+            }
+        }
+    } else {
+        // Default personality routine, which is used directly on most targets
+        // and indirectly on Windows x86_64 via SEH.
+        unsafe extern "C" fn rust_eh_personality_impl(version: c_int,
+                                                      actions: uw::_Unwind_Action,
+                                                      _exception_class: uw::_Unwind_Exception_Class,
+                                                      exception_object: *mut uw::_Unwind_Exception,
+                                                      context: *mut uw::_Unwind_Context)
+                                                      -> uw::_Unwind_Reason_Code {
+            if version != 1 {
+                return uw::_URC_FATAL_PHASE1_ERROR;
+            }
+            let eh_action = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FATAL_PHASE1_ERROR,
+            };
+            if actions as i32 & uw::_UA_SEARCH_PHASE as i32 != 0 {
+                match eh_action {
+                    EHAction::None |
+                    EHAction::Cleanup(_) => uw::_URC_CONTINUE_UNWIND,
+                    EHAction::Catch(_) => uw::_URC_HANDLER_FOUND,
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE1_ERROR,
+                }
+            } else {
+                match eh_action {
+                    EHAction::None => uw::_URC_CONTINUE_UNWIND,
+                    EHAction::Cleanup(lpad) |
+                    EHAction::Catch(lpad) => {
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.0,
+                            exception_object as uintptr_t);
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
+                        uw::_Unwind_SetIP(context, lpad);
+                        uw::_URC_INSTALL_CONTEXT
+                    }
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE2_ERROR,
+                }
+            }
+        }
+
+        cfg_if::cfg_if! {
+            if #[cfg(all(windows, target_arch = "x86_64", target_env = "gnu"))] {
+                // On x86_64 MinGW targets, the unwinding mechanism is SEH however the unwind
+                // handler data (aka LSDA) uses GCC-compatible encoding.
+                #[lang = "eh_personality"]
+                #[allow(nonstandard_style)]
+                unsafe extern "C" fn rust_eh_personality(exceptionRecord: *mut uw::EXCEPTION_RECORD,
+                        establisherFrame: uw::LPVOID,
+                        contextRecord: *mut uw::CONTEXT,
+                        dispatcherContext: *mut uw::DISPATCHER_CONTEXT)
+                        -> uw::EXCEPTION_DISPOSITION {
+                    uw::_GCC_specific_handler(exceptionRecord,
+                                             establisherFrame,
+                                             contextRecord,
+                                             dispatcherContext,
+                                             rust_eh_personality_impl)
+                }
+            } else {
+                // The personality routine for most of our targets.
+                #[lang = "eh_personality"]
+                unsafe extern "C" fn rust_eh_personality(version: c_int,
+                        actions: uw::_Unwind_Action,
+                        exception_class: uw::_Unwind_Exception_Class,
+                        exception_object: *mut uw::_Unwind_Exception,
+                        context: *mut uw::_Unwind_Context)
+                        -> uw::_Unwind_Reason_Code {
+                    rust_eh_personality_impl(version,
+                                             actions,
+                                             exception_class,
+                                             exception_object,
+                                             context)
+                }
+            }
+        }
+    }
+}
+
+unsafe fn find_eh_action(context: *mut uw::_Unwind_Context) -> Result<EHAction, ()> {
+    let lsda = uw::_Unwind_GetLanguageSpecificData(context) as *const u8;
+    let mut ip_before_instr: c_int = 0;
+    let ip = uw::_Unwind_GetIPInfo(context, &mut ip_before_instr);
+    let eh_context = EHContext {
+        // The return address points 1 byte past the call instruction,
+        // which could be in the next IP range in LSDA range table.
+        //
+        // `ip = -1` has special meaning, so use wrapping sub to allow for that
+        ip: if ip_before_instr != 0 { ip } else { ip.wrapping_sub(1) },
+        func_start: uw::_Unwind_GetRegionStart(context),
+        get_text_start: &|| uw::_Unwind_GetTextRelBase(context),
+        get_data_start: &|| uw::_Unwind_GetDataRelBase(context),
+    };
+    eh::find_eh_action(lsda, &eh_context)
+}
+
+// Frame unwind info registration
+//
+// Each module's image contains a frame unwind info section (usually
+// ".eh_frame").  When a module is loaded/unloaded into the process, the
+// unwinder must be informed about the location of this section in memory. The
+// methods of achieving that vary by the platform.  On some (e.g., Linux), the
+// unwinder can discover unwind info sections on its own (by dynamically
+// enumerating currently loaded modules via the dl_iterate_phdr() API and
+// finding their ".eh_frame" sections); Others, like Windows, require modules
+// to actively register their unwind info sections via unwinder API.
+//
+// This module defines two symbols which are referenced and called from
+// rsbegin.rs to register our information with the GCC runtime. The
+// implementation of stack unwinding is (for now) deferred to libgcc_eh, however
+// Rust crates use these Rust-specific entry points to avoid potential clashes
+// with any GCC runtime.
+#[cfg(all(target_os = "windows", target_arch = "x86", target_env = "gnu"))]
+pub mod eh_frame_registry {
+    extern "C" {
+        fn __register_frame_info(eh_frame_begin: *const u8, object: *mut u8);
+        fn __deregister_frame_info(eh_frame_begin: *const u8, object: *mut u8);
+    }
+
+    #[rustc_std_internal_symbol]
+    pub unsafe extern "C" fn rust_eh_register_frames(eh_frame_begin: *const u8, object: *mut u8) {
+        __register_frame_info(eh_frame_begin, object);
+    }
+
+    #[rustc_std_internal_symbol]
+    pub unsafe extern "C" fn rust_eh_unregister_frames(eh_frame_begin: *const u8, object: *mut u8) {
+        __deregister_frame_info(eh_frame_begin, object);
+    }
+}
diff --git a/library/panic_unwind/src/hermit.rs b/library/panic_unwind/src/hermit.rs
new file mode 100644
index 000000000..69b9edb77
--- /dev/null
+++ b/library/panic_unwind/src/hermit.rs
@@ -0,0 +1,20 @@
+//! Unwinding for *hermit* target.
+//!
+//! Right now we don't support this, so this is just stubs.
+
+use alloc::boxed::Box;
+use core::any::Any;
+
+pub unsafe fn cleanup(_ptr: *mut u8) -> Box<dyn Any + Send> {
+    extern "C" {
+        pub fn __rust_abort() -> !;
+    }
+    __rust_abort();
+}
+
+pub unsafe fn panic(_data: Box<dyn Any + Send>) -> u32 {
+    extern "C" {
+        pub fn __rust_abort() -> !;
+    }
+    __rust_abort();
+}
diff --git a/library/panic_unwind/src/lib.rs b/library/panic_unwind/src/lib.rs
new file mode 100644
index 000000000..f9acb42c4
--- /dev/null
+++ b/library/panic_unwind/src/lib.rs
@@ -0,0 +1,110 @@
+//! Implementation of panics via stack unwinding
+//!
+//! This crate is an implementation of panics in Rust using "most native" stack
+//! unwinding mechanism of the platform this is being compiled for. This
+//! essentially gets categorized into three buckets currently:
+//!
+//! 1. MSVC targets use SEH in the `seh.rs` file.
+//! 2. Emscripten uses C++ exceptions in the `emcc.rs` file.
+//! 3. All other targets use libunwind/libgcc in the `gcc.rs` file.
+//!
+//! More documentation about each implementation can be found in the respective
+//! module.
+
+#![no_std]
+#![unstable(feature = "panic_unwind", issue = "32837")]
+#![doc(issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
+#![feature(core_intrinsics)]
+#![feature(lang_items)]
+#![feature(panic_unwind)]
+#![feature(staged_api)]
+#![feature(std_internals)]
+#![feature(abi_thiscall)]
+#![feature(rustc_attrs)]
+#![panic_runtime]
+#![feature(panic_runtime)]
+#![feature(c_unwind)]
+// `real_imp` is unused with Miri, so silence warnings.
+#![cfg_attr(miri, allow(dead_code))]
+
+use alloc::boxed::Box;
+use core::any::Any;
+use core::panic::BoxMeUp;
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "emscripten")] {
+        #[path = "emcc.rs"]
+        mod real_imp;
+    } else if #[cfg(target_os = "hermit")] {
+        #[path = "hermit.rs"]
+        mod real_imp;
+    } else if #[cfg(target_os = "l4re")] {
+        // L4Re is unix family but does not yet support unwinding.
+        #[path = "dummy.rs"]
+        mod real_imp;
+    } else if #[cfg(target_env = "msvc")] {
+        #[path = "seh.rs"]
+        mod real_imp;
+    } else if #[cfg(any(
+        all(target_family = "windows", target_env = "gnu"),
+        target_os = "psp",
+        target_os = "solid_asp3",
+        all(target_family = "unix", not(target_os = "espidf")),
+        all(target_vendor = "fortanix", target_env = "sgx"),
+    ))] {
+        // Rust runtime's startup objects depend on these symbols, so make them public.
+        #[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
+        pub use real_imp::eh_frame_registry::*;
+        #[path = "gcc.rs"]
+        mod real_imp;
+    } else {
+        // Targets that don't support unwinding.
+        // - family=wasm
+        // - os=none ("bare metal" targets)
+        // - os=uefi
+        // - os=espidf
+        // - nvptx64-nvidia-cuda
+        // - arch=avr
+        #[path = "dummy.rs"]
+        mod real_imp;
+    }
+}
+
+cfg_if::cfg_if! {
+    if #[cfg(miri)] {
+        // Use the Miri runtime.
+        // We still need to also load the normal runtime above, as rustc expects certain lang
+        // items from there to be defined.
+        #[path = "miri.rs"]
+        mod imp;
+    } else {
+        // Use the real runtime.
+        use real_imp as imp;
+    }
+}
+
+extern "C" {
+    /// Handler in libstd called when a panic object is dropped outside of
+    /// `catch_unwind`.
+    fn __rust_drop_panic() -> !;
+
+    /// Handler in libstd called when a foreign exception is caught.
+    fn __rust_foreign_exception() -> !;
+}
+
+mod dwarf;
+
+#[rustc_std_internal_symbol]
+#[allow(improper_ctypes_definitions)]
+pub unsafe extern "C" fn __rust_panic_cleanup(payload: *mut u8) -> *mut (dyn Any + Send + 'static) {
+    Box::into_raw(imp::cleanup(payload))
+}
+
+// Entry point for raising an exception, just delegates to the platform-specific
+// implementation.
+#[rustc_std_internal_symbol]
+pub unsafe fn __rust_start_panic(payload: *mut &mut dyn BoxMeUp) -> u32 {
+    let payload = Box::from_raw((*payload).take_box());
+
+    imp::panic(payload)
+}
diff --git a/library/panic_unwind/src/miri.rs b/library/panic_unwind/src/miri.rs
new file mode 100644
index 000000000..d941b73b5
--- /dev/null
+++ b/library/panic_unwind/src/miri.rs
@@ -0,0 +1,25 @@
+//! Unwinding panics for Miri.
+use alloc::boxed::Box;
+use core::any::Any;
+
+// The type of the payload that the Miri engine propagates through unwinding for us.
+// Must be pointer-sized.
+type Payload = Box<Box<dyn Any + Send>>;
+
+extern "Rust" {
+    /// Miri-provided extern function to begin unwinding.
+    fn miri_start_panic(payload: *mut u8) -> !;
+}
+
+pub unsafe fn panic(payload: Box<dyn Any + Send>) -> u32 {
+    // The payload we pass to `miri_start_panic` will be exactly the argument we get
+    // in `cleanup` below. So we just box it up once, to get something pointer-sized.
+    let payload_box: Payload = Box::new(payload);
+    miri_start_panic(Box::into_raw(payload_box) as *mut u8)
+}
+
+pub unsafe fn cleanup(payload_box: *mut u8) -> Box<dyn Any + Send> {
+    // Recover the underlying `Box`.
+    let payload_box: Payload = Box::from_raw(payload_box as *mut _);
+    *payload_box
+}
diff --git a/library/panic_unwind/src/seh.rs b/library/panic_unwind/src/seh.rs
new file mode 100644
index 000000000..9f1eb411f
--- /dev/null
+++ b/library/panic_unwind/src/seh.rs
@@ -0,0 +1,335 @@
+//! 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::mem::{self, ManuallyDrop};
+use libc::{c_int, c_uint, c_void};
+
+struct Exception {
+    // 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")]
+#[macro_use]
+mod imp {
+    pub type ptr_t = *mut u8;
+
+    macro_rules! ptr {
+        (0) => {
+            core::ptr::null_mut()
+        };
+        ($e:expr) => {
+            $e as *mut u8
+        };
+    }
+}
+
+#[cfg(not(target_arch = "x86"))]
+#[macro_use]
+mod imp {
+    pub type ptr_t = u32;
+
+    extern "C" {
+        pub static __ImageBase: u8;
+    }
+
+    macro_rules! ptr {
+        (0) => (0);
+        ($e:expr) => {
+            (($e as usize) - (&imp::__ImageBase as *const _ as usize)) as u32
+        }
+    }
+}
+
+#[repr(C)]
+pub struct _ThrowInfo {
+    pub attributes: c_uint,
+    pub pmfnUnwind: imp::ptr_t,
+    pub pForwardCompat: imp::ptr_t,
+    pub pCatchableTypeArray: imp::ptr_t,
+}
+
+#[repr(C)]
+pub struct _CatchableTypeArray {
+    pub nCatchableTypes: c_int,
+    pub arrayOfCatchableTypes: [imp::ptr_t; 1],
+}
+
+#[repr(C)]
+pub struct _CatchableType {
+    pub properties: c_uint,
+    pub pType: imp::ptr_t,
+    pub thisDisplacement: _PMD,
+    pub sizeOrOffset: c_int,
+    pub copyFunction: imp::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!(0),
+    pForwardCompat: ptr!(0),
+    pCatchableTypeArray: ptr!(0),
+};
+
+static mut CATCHABLE_TYPE_ARRAY: _CatchableTypeArray =
+    _CatchableTypeArray { nCatchableTypes: 1, arrayOfCatchableTypes: [ptr!(0)] };
+
+static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
+    properties: 0,
+    pType: ptr!(0),
+    thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
+    sizeOrOffset: mem::size_of::<Exception>() as c_int,
+    copyFunction: ptr!(0),
+};
+
+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 {
+    pVFTable: unsafe { &TYPE_INFO_VTABLE } as *const _ 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;
+
+    // _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 { data: Some(data) });
+    let throw_ptr = &mut exception as *mut _ 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(&mut THROW_INFO.pmfnUnwind as *mut _ as *mut u32, ptr!(exception_cleanup) as u32);
+    atomic_store(
+        &mut THROW_INFO.pCatchableTypeArray as *mut _ as *mut u32,
+        ptr!(&CATCHABLE_TYPE_ARRAY as *const _) as u32,
+    );
+    atomic_store(
+        &mut CATCHABLE_TYPE_ARRAY.arrayOfCatchableTypes[0] as *mut _ as *mut u32,
+        ptr!(&CATCHABLE_TYPE as *const _) as u32,
+    );
+    atomic_store(
+        &mut CATCHABLE_TYPE.pType as *mut _ as *mut u32,
+        ptr!(&TYPE_DESCRIPTOR as *const _) as u32,
+    );
+    atomic_store(
+        &mut CATCHABLE_TYPE.copyFunction as *mut _ as *mut u32,
+        ptr!(exception_copy) as u32,
+    );
+
+    extern "system-unwind" {
+        fn _CxxThrowException(pExceptionObject: *mut c_void, pThrowInfo: *mut u8) -> !;
+    }
+
+    _CxxThrowException(throw_ptr, &mut THROW_INFO as *mut _ 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();
+    } else {
+        let exception = &mut *(payload as *mut Exception);
+        exception.data.take().unwrap()
+    }
+}
+
+// This is required by the compiler to exist (e.g., it's a lang item), but
+// it's never actually called by the compiler because __C_specific_handler
+// or _except_handler3 is the personality function that is always used.
+// Hence this is just an aborting stub.
+#[lang = "eh_personality"]
+#[cfg(not(test))]
+fn rust_eh_personality() {
+    core::intrinsics::abort()
+}