Auto merge of #30593 - steveklabnik:small_rc_refactoring, r=Gankro

This hairy conditional doesn't need to be so. It _does_ need to be a
thin pointer, otherwise, it will fail to compile, so let's pull that out
into a temporary for future readers of the source.

/cc @nrc @SimonSapin @Gankro @durka , who brought this up on IRC

src/liballoc/arc.rs

@@ -555,9 +555,9 @@ fn drop(&mut self) {
         // 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;
         }
 
@@ -710,9 +710,10 @@ 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.