Small refactoring to make this code more clear

src/liballoc/arc.rs

@@ -554,9 +554,9 @@ impl<T: ?Sized> Drop for Arc<T> {
        // This structure has #[unsafe_no_drop_flag], so this drop glue may run
        // more than once (but it is guaranteed to be zeroed after the first if
        // it's run more than once)
        let ptr = *self._ptr;
        // if ptr.is_null() { return }
        if ptr as *mut u8 as usize == 0 || ptr as *mut u8 as usize == mem::POST_DROP_USIZE {
        let thin = *self._ptr as *const ();

        if thin as usize == mem::POST_DROP_USIZE {
            return;
        }

@@ -709,9 +709,10 @@ impl<T: ?Sized> Drop for Weak<T> {
    /// ```
    fn drop(&mut self) {
        let ptr = *self._ptr;
        let thin = ptr as *const ();

        // see comments above for why this check is here
        if ptr as *mut u8 as usize == 0 || ptr as *mut u8 as usize == mem::POST_DROP_USIZE {
        if thin as usize == mem::POST_DROP_USIZE {
            return;
        }

src/liballoc/rc.rs

@@ -449,8 +449,9 @@ impl<T: ?Sized> Drop for Rc<T> {
    fn drop(&mut self) {
        unsafe {
            let ptr = *self._ptr;
            if !(*(&ptr as *const _ as *const *const ())).is_null() &&
               ptr as *const () as usize != mem::POST_DROP_USIZE {
            let thin = ptr as *const ();

            if thin as usize != mem::POST_DROP_USIZE {
                self.dec_strong();
                if self.strong() == 0 {
                    // destroy the contained object
@@ -782,8 +783,9 @@ impl<T: ?Sized> Drop for Weak<T> {
    fn drop(&mut self) {
        unsafe {
            let ptr = *self._ptr;
            if !(*(&ptr as *const _ as *const *const ())).is_null() &&
               ptr as *const () as usize != mem::POST_DROP_USIZE {
            let thin = ptr as *const ();

            if thin as usize != mem::POST_DROP_USIZE {
                self.dec_weak();
                // the weak count starts at 1, and will only go to zero if all
                // the strong pointers have disappeared.