libcore: Make it unsafe to create NonZero and impl Deref.

src/liballoc/arc.rs

@@ -164,7 +164,7 @@ impl<T> Arc<T> {
             weak: atomic::AtomicUint::new(1),
             data: data,
         };
-        Arc { _ptr: NonZero(unsafe { mem::transmute(x) }) }
+        Arc { _ptr: unsafe { NonZero::new(mem::transmute(x)) } }
     }
 
     /// Downgrades the `Arc<T>` to a `Weak<T>` reference.
@@ -193,8 +193,7 @@ impl<T> Arc<T> {
         // pointer is valid. Furthermore, we know that the `ArcInner` structure itself is `Sync`
         // because the inner data is `Sync` as well, so we're ok loaning out an immutable pointer
         // to these contents.
-        let NonZero(ptr) = self._ptr;
-        unsafe { &*ptr }
+        unsafe { &**self._ptr }
     }
 }
 
@@ -281,8 +280,7 @@ impl<T: Send + Sync + Clone> Arc<T> {
         // pointer that will ever be returned to T. Our reference count is guaranteed to be 1 at
         // this point, and we required the Arc itself to be `mut`, so we're returning the only
         // possible reference to the inner data.
-        let NonZero(ptr) = self._ptr;
-        let inner = unsafe { &mut *ptr };
+        let inner = unsafe { &mut **self._ptr };
         &mut inner.data
     }
 }
@@ -317,7 +315,7 @@ impl<T: Sync + Send> Drop for Arc<T> {
     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 NonZero(ptr) = self._ptr;
+        let ptr = *self._ptr;
         if ptr.is_null() { return }
 
         // Because `fetch_sub` is already atomic, we do not need to synchronize with other threads
@@ -388,8 +386,7 @@ impl<T: Sync + Send> Weak<T> {
     #[inline]
     fn inner(&self) -> &ArcInner<T> {
         // See comments above for why this is "safe"
-        let NonZero(ptr) = self._ptr;
-        unsafe { &*ptr }
+        unsafe { &**self._ptr }
     }
 }
 
@@ -445,7 +442,7 @@ impl<T: Sync + Send> Drop for Weak<T> {
     /// } // implicit drop
     /// ```
     fn drop(&mut self) {
-        let NonZero(ptr) = self._ptr;
+        let ptr = *self._ptr;
 
         // see comments above for why this check is here
         if ptr.is_null() { return }