Fix is_dangling import when Arc is #[cfg]’ed out

Author: SimonSapin

src/liballoc/rc.rs

@@ -261,7 +261,6 @@ use core::convert::From;
 
 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
 use string::String;
-use sync::is_dangling;
 use vec::Vec;
 
 struct RcBox<T: ?Sized> {
@@ -1192,6 +1191,12 @@ impl<T> Weak<T> {
     }
 }
 
+pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
+    let address = ptr.as_ptr() as *mut () as usize;
+    let align = align_of_val(unsafe { ptr.as_ref() });
+    address == align
+}
+
 impl<T: ?Sized> Weak<T> {
     /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
     /// the lifetime of the value if successful.

src/liballoc/sync.rs

@@ -34,6 +34,7 @@ use core::convert::From;
 
 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
 use boxed::Box;
+use rc::is_dangling;
 use string::String;
 use vec::Vec;
 
@@ -1038,12 +1039,6 @@ impl<T> Weak<T> {
     }
 }
 
-pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
-    let address = ptr.as_ptr() as *mut () as usize;
-    let align = align_of_val(unsafe { ptr.as_ref() });
-    address == align
-}
-
 impl<T: ?Sized> Weak<T> {
     /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
     /// the lifetime of the value if successful.