Add non-mutable methods to Cursor

Author: iwahbe

library/alloc/src/collections/linked_list.rs

@@ -1243,6 +1243,20 @@ impl<'a, T> Cursor<'a, T> {
             prev.map(|prev| &(*prev.as_ptr()).element)
         }
     }
+
+    /// Provides a reference to the front element of the cursor's parent list,
+    /// or None if the list is empty.
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn front(&self) -> Option<&T> {
+        self.list.front()
+    }
+
+    /// Provides a reference to the back element of the cursor's parent list,
+    /// or None if the list is empty.
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn back(&self) -> Option<&T> {
+        self.list.back()
+    }
 }
 
 impl<'a, T> CursorMut<'a, T> {

library/alloc/src/collections/linked_list/tests.rs

@@ -456,6 +456,9 @@ fn test_cursor_pop_front_back() {
     c.move_prev();
     c.move_prev();
     assert_eq!(c.pop_back(), Some(6));
+    let c = c.as_cursor();
+    assert_eq!(c.front(), Some(&2));
+    assert_eq!(c.back(), Some(&5));
     drop(c);
     assert_eq!(ll, (2..6).collect());
     check_links(&ll);