Improve Debug impls for LinkedList reference iterators to show items

Author: steffahn

library/alloc/src/collections/linked_list.rs

@@ -64,7 +64,15 @@ pub struct Iter<'a, T: 'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_tuple("Iter").field(&self.len).finish()
+        f.debug_tuple("Iter")
+            .field(&*mem::ManuallyDrop::new(LinkedList {
+                head: self.head,
+                tail: self.tail,
+                len: self.len,
+                marker: PhantomData,
+            }))
+            .field(&self.len)
+            .finish()
     }
 }
 
@@ -91,7 +99,15 @@ pub struct IterMut<'a, T: 'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_tuple("IterMut").field(&self.len).finish()
+        f.debug_tuple("IterMut")
+            .field(&*mem::ManuallyDrop::new(LinkedList {
+                head: self.head,
+                tail: self.tail,
+                len: self.len,
+                marker: PhantomData,
+            }))
+            .field(&self.len)
+            .finish()
     }
 }