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    #![cfg(target_thread_local)]
    #![unstable(feature = "thread_local_internals", issue = "0")]

    // Since what appears to be glibc 2.18 this symbol has been shipped which
    // GCC and clang both use to invoke destructors in thread_local globals, so
    // let's do the same!
    //
    // Note, however, that we run on lots older linuxes, as well as cross
    // compiling from a newer linux to an older linux, so we also have a
    // fallback implementation to use as well.
    //
    // Due to rust-lang/rust#18804, make sure this is not generic!
    #[cfg(any(target_os = "linux", target_os = "fuchsia", target_os = "hermit", target_os = "redox"))]
    pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
        use crate::mem;
        use crate::sys_common::thread_local::register_dtor_fallback;

        extern {
            #[linkage = "extern_weak"]
            static __dso_handle: *mut u8;
            #[linkage = "extern_weak"]
            static __cxa_thread_atexit_impl: *const libc::c_void;
        }
        if !__cxa_thread_atexit_impl.is_null() {
            type F = unsafe extern fn(dtor: unsafe extern fn(*mut u8),
                                      arg: *mut u8,
                                      dso_handle: *mut u8) -> libc::c_int;
            mem::transmute::<*const libc::c_void, F>(__cxa_thread_atexit_impl)
                (dtor, t, &__dso_handle as *const _ as *mut _);
            return
        }
        register_dtor_fallback(t, dtor);
    }

    // This implementation is very similar to register_dtor_fallback in
    // sys_common/thread_local.rs. The main difference is that we want to hook into
    // macOS's analog of the above linux function, _tlv_atexit. OSX will run the
    // registered dtors before any TLS slots get freed, and when the main thread
    // exits.
    //
    // Unfortunately, calling _tlv_atexit while tls dtors are running is UB. The
    // workaround below is to register, via _tlv_atexit, a custom DTOR list once per
    // thread. thread_local dtors are pushed to the DTOR list without calling
    // _tlv_atexit.
    #[cfg(target_os = "macos")]
    pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
        use crate::cell::Cell;
        use crate::ptr;

        #[thread_local]
        static REGISTERED: Cell<bool> = Cell::new(false);
        if !REGISTERED.get() {
            _tlv_atexit(run_dtors, ptr::null_mut());
            REGISTERED.set(true);
        }

        type List = Vec<(*mut u8, unsafe extern fn(*mut u8))>;

        #[thread_local]
        static DTORS: Cell<*mut List> = Cell::new(ptr::null_mut());
        if DTORS.get().is_null() {
            let v: Box<List> = box Vec::new();
            DTORS.set(Box::into_raw(v));
        }

        extern {
            fn _tlv_atexit(dtor: unsafe extern fn(*mut u8),
                           arg: *mut u8);
        }

        let list: &mut List = &mut *DTORS.get();
        list.push((t, dtor));

        unsafe extern fn run_dtors(_: *mut u8) {
            let mut ptr = DTORS.replace(ptr::null_mut());
            while !ptr.is_null() {
                let list = Box::from_raw(ptr);
                for (ptr, dtor) in list.into_iter() {
                    dtor(ptr);
                }
                ptr = DTORS.replace(ptr::null_mut());
            }
        }
    }

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