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    //! Backtrace support using libunwind/gcc_s/etc APIs.
    //!
    //! This module contains the ability to unwind the stack using libunwind-style
    //! APIs. Note that there's a whole bunch of implementations of the
    //! libunwind-like API, and this is just trying to be compatible with most of
    //! them all at once instead of being picky.
    //!
    //! The libunwind API is powered by `_Unwind_Backtrace` and is in practice very
    //! reliable at generating a backtrace. It's not entirely clear how it does it
    //! (frame pointers? eh_frame info? both?) but it seems to work!
    //!
    //! Most of the complexity of this module is handling the various platform
    //! differences across libunwind implementations. Otherwise this is a pretty
    //! straightforward Rust binding to the libunwind APIs.
    //!
    //! This is the default unwinding API for all non-Windows platforms currently.

    use super::super::Bomb;
    use core::ffi::c_void;

    pub enum Frame {
        Raw(*mut uw::_Unwind_Context),
        Cloned {
            ip: *mut c_void,
            sp: *mut c_void,
            symbol_address: *mut c_void,
        },
    }

    // With a raw libunwind pointer it should only ever be access in a readonly
    // threadsafe fashion, so it's `Sync`. When sending to other threads via `Clone`
    // we always switch to a version which doesn't retain interior pointers, so we
    // should be `Send` as well.
    unsafe impl Send for Frame {}
    unsafe impl Sync for Frame {}

    impl Frame {
        pub fn ip(&self) -> *mut c_void {
            let ctx = match *self {
                Frame::Raw(ctx) => ctx,
                Frame::Cloned { ip, .. } => return ip,
            };
            unsafe { uw::_Unwind_GetIP(ctx) as *mut c_void }
        }

        pub fn sp(&self) -> *mut c_void {
            match *self {
                Frame::Raw(ctx) => unsafe { uw::get_sp(ctx) as *mut c_void },
                Frame::Cloned { sp, .. } => sp,
            }
        }

        pub fn symbol_address(&self) -> *mut c_void {
            if let Frame::Cloned { symbol_address, .. } = *self {
                return symbol_address;
            }

            // It seems that on OSX `_Unwind_FindEnclosingFunction` returns a
            // pointer to... something that's unclear. It's definitely not always
            // the enclosing function for whatever reason. It's not entirely clear
            // to me what's going on here, so pessimize this for now and just always
            // return the ip.
            //
            // Note the `skip_inner_frames.rs` test is skipped on OSX due to this
            // clause, and if this is fixed that test in theory can be run on OSX!
            if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
                self.ip()
            } else {
                unsafe { uw::_Unwind_FindEnclosingFunction(self.ip()) }
            }
        }

        pub fn module_base_address(&self) -> Option<*mut c_void> {
            None
        }
    }

    impl Clone for Frame {
        fn clone(&self) -> Frame {
            Frame::Cloned {
                ip: self.ip(),
                sp: self.sp(),
                symbol_address: self.symbol_address(),
            }
        }
    }

    #[inline(always)]
    pub unsafe fn trace(mut cb: &mut dyn FnMut(&super::Frame) -> bool) {
        uw::_Unwind_Backtrace(trace_fn, &mut cb as *mut _ as *mut _);

        extern "C" fn trace_fn(
            ctx: *mut uw::_Unwind_Context,
            arg: *mut c_void,
        ) -> uw::_Unwind_Reason_Code {
            let cb = unsafe { &mut *(arg as *mut &mut dyn FnMut(&super::Frame) -> bool) };
            let cx = super::Frame {
                inner: Frame::Raw(ctx),
            };

            let mut bomb = Bomb { enabled: true };
            let keep_going = cb(&cx);
            bomb.enabled = false;

            if keep_going {
                uw::_URC_NO_REASON
            } else {
                uw::_URC_FAILURE
            }
        }
    }

    /// Unwind library interface used for backtraces
    ///
    /// Note that dead code is allowed as here are just bindings
    /// iOS doesn't use all of them it but adding more
    /// platform-specific configs pollutes the code too much
    #[allow(non_camel_case_types)]
    #[allow(non_snake_case)]
    #[allow(dead_code)]
    mod uw {
        pub use self::_Unwind_Reason_Code::*;

        use core::ffi::c_void;

        #[repr(C)]
        pub enum _Unwind_Reason_Code {
            _URC_NO_REASON = 0,
            _URC_FOREIGN_EXCEPTION_CAUGHT = 1,
            _URC_FATAL_PHASE2_ERROR = 2,
            _URC_FATAL_PHASE1_ERROR = 3,
            _URC_NORMAL_STOP = 4,
            _URC_END_OF_STACK = 5,
            _URC_HANDLER_FOUND = 6,
            _URC_INSTALL_CONTEXT = 7,
            _URC_CONTINUE_UNWIND = 8,
            _URC_FAILURE = 9, // used only by ARM EABI
        }

        pub enum _Unwind_Context {}

        pub type _Unwind_Trace_Fn =
            extern "C" fn(ctx: *mut _Unwind_Context, arg: *mut c_void) -> _Unwind_Reason_Code;

        extern "C" {
            // No native _Unwind_Backtrace on iOS
            #[cfg(not(all(target_os = "ios", target_arch = "arm")))]
            pub fn _Unwind_Backtrace(
                trace: _Unwind_Trace_Fn,
                trace_argument: *mut c_void,
            ) -> _Unwind_Reason_Code;

            // available since GCC 4.2.0, should be fine for our purpose
            #[cfg(all(
                not(all(target_os = "android", target_arch = "arm")),
                not(all(target_os = "freebsd", target_arch = "arm")),
                not(all(target_os = "linux", target_arch = "arm"))
            ))]
            pub fn _Unwind_GetIP(ctx: *mut _Unwind_Context) -> libc::uintptr_t;

            #[cfg(all(
                not(all(target_os = "android", target_arch = "arm")),
                not(all(target_os = "freebsd", target_arch = "arm")),
                not(all(target_os = "linux", target_arch = "arm"))
            ))]
            pub fn _Unwind_FindEnclosingFunction(pc: *mut c_void) -> *mut c_void;

            #[cfg(all(
                not(all(target_os = "android", target_arch = "arm")),
                not(all(target_os = "freebsd", target_arch = "arm")),
                not(all(target_os = "linux", target_arch = "arm")),
                not(all(target_os = "linux", target_arch = "s390x"))
            ))]
            // This function is a misnomer: rather than getting this frame's
            // Canonical Frame Address (aka the caller frame's SP) it
            // returns this frame's SP.
            //
            // https://github.com/libunwind/libunwind/blob/d32956507cf29d9b1a98a8bce53c78623908f4fe/src/unwind/GetCFA.c#L28-L35
            #[link_name = "_Unwind_GetCFA"]
            pub fn get_sp(ctx: *mut _Unwind_Context) -> libc::uintptr_t;
        }

        // s390x uses a biased CFA value, therefore we need to use
        // _Unwind_GetGR to get the stack pointer register (%r15)
        // instead of relying on _Unwind_GetCFA.
        #[cfg(all(target_os = "linux", target_arch = "s390x"))]
        pub unsafe fn get_sp(ctx: *mut _Unwind_Context) -> libc::uintptr_t {
            extern "C" {
                pub fn _Unwind_GetGR(ctx: *mut _Unwind_Context, index: libc::c_int) -> libc::uintptr_t;
            }
            _Unwind_GetGR(ctx, 15)
        }

        // On android and arm, the function `_Unwind_GetIP` and a bunch of others
        // are macros, so we define functions containing the expansion of the
        // macros.
        //
        // TODO: link to the header file that defines these macros, if you can find
        // it. (I, fitzgen, cannot find the header file that some of these macro
        // expansions were originally borrowed from.)
        #[cfg(any(
            all(target_os = "android", target_arch = "arm"),
            all(target_os = "freebsd", target_arch = "arm"),
            all(target_os = "linux", target_arch = "arm")
        ))]
        pub use self::arm::*;
        #[cfg(any(
            all(target_os = "android", target_arch = "arm"),
            all(target_os = "freebsd", target_arch = "arm"),
            all(target_os = "linux", target_arch = "arm")
        ))]
        mod arm {
            pub use super::*;
            #[repr(C)]
            enum _Unwind_VRS_Result {
                _UVRSR_OK = 0,
                _UVRSR_NOT_IMPLEMENTED = 1,
                _UVRSR_FAILED = 2,
            }
            #[repr(C)]
            enum _Unwind_VRS_RegClass {
                _UVRSC_CORE = 0,
                _UVRSC_VFP = 1,
                _UVRSC_FPA = 2,
                _UVRSC_WMMXD = 3,
                _UVRSC_WMMXC = 4,
            }
            #[repr(C)]
            enum _Unwind_VRS_DataRepresentation {
                _UVRSD_UINT32 = 0,
                _UVRSD_VFPX = 1,
                _UVRSD_FPAX = 2,
                _UVRSD_UINT64 = 3,
                _UVRSD_FLOAT = 4,
                _UVRSD_DOUBLE = 5,
            }

            type _Unwind_Word = libc::c_uint;
            extern "C" {
                fn _Unwind_VRS_Get(
                    ctx: *mut _Unwind_Context,
                    klass: _Unwind_VRS_RegClass,
                    word: _Unwind_Word,
                    repr: _Unwind_VRS_DataRepresentation,
                    data: *mut c_void,
                ) -> _Unwind_VRS_Result;
            }

            pub unsafe fn _Unwind_GetIP(ctx: *mut _Unwind_Context) -> libc::uintptr_t {
                let mut val: _Unwind_Word = 0;
                let ptr = &mut val as *mut _Unwind_Word;
                let _ = _Unwind_VRS_Get(
                    ctx,
                    _Unwind_VRS_RegClass::_UVRSC_CORE,
                    15,
                    _Unwind_VRS_DataRepresentation::_UVRSD_UINT32,
                    ptr as *mut c_void,
                );
                (val & !1) as libc::uintptr_t
            }

            // R13 is the stack pointer on arm.
            const SP: _Unwind_Word = 13;

            pub unsafe fn get_sp(ctx: *mut _Unwind_Context) -> libc::uintptr_t {
                let mut val: _Unwind_Word = 0;
                let ptr = &mut val as *mut _Unwind_Word;
                let _ = _Unwind_VRS_Get(
                    ctx,
                    _Unwind_VRS_RegClass::_UVRSC_CORE,
                    SP,
                    _Unwind_VRS_DataRepresentation::_UVRSD_UINT32,
                    ptr as *mut c_void,
                );
                val as libc::uintptr_t
            }

            // This function also doesn't exist on Android or ARM/Linux, so make it
            // a no-op.
            pub unsafe fn _Unwind_FindEnclosingFunction(pc: *mut c_void) -> *mut c_void {
                pc
            }
        }
    }