Fix LinkedList::CursorMut::pop_front to correctly update index

When `pop_front` was called while the cursor pointed to the front element, `move_next` incremented the index but it was never decremented afterwards, causing the index to incorrectly report 1 instead of 0.

Always decrement the index after popping from front using `saturating_sub` to handle edge cases safely.

Fixes #147616

Author: heathdutton

library/alloc/src/collections/linked_list.rs

@@ -1855,9 +1855,11 @@ impl<'a, T, A: Allocator> CursorMut<'a, T, A> {
             // node at index 0, which is expected.
             if self.list.head == self.current {
                 self.move_next();
-            } else {
-                self.index -= 1;
             }
+            // An element was removed before (or at) our current position, so
+            // the index must be decremented. `saturating_sub` handles the
+            // ghost node case where index could be 0.
+            self.index = self.index.saturating_sub(1);
             self.list.pop_front()
         }
     }

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

@@ -748,6 +748,60 @@ fn test_cursor_pop_front_back() {
     assert_eq!(c.index, 2);
 }
 
+#[test]
+fn test_cursor_pop_front_index() {
+    // Regression test for issue #147616: `pop_front` was not correctly
+    // updating the cursor index when the cursor was pointing to the front.
+
+    // Test case 1: pop_front when cursor is not at front, then at front
+    let mut ll: LinkedList<u32> = LinkedList::new();
+    ll.extend(&[0, 1, 2]);
+    let mut c = ll.cursor_front_mut();
+
+    c.move_next();
+    assert_eq!(c.index(), Some(1));
+    assert_eq!(c.current(), Some(&mut 1));
+
+    // Pop front when cursor is not at front - index should decrement
+    c.pop_front();
+    assert_eq!(c.index(), Some(0));
+    assert_eq!(c.current(), Some(&mut 1));
+
+    // Now cursor is at front, pop_front again - index should remain 0
+    c.pop_front();
+    assert_eq!(c.index(), Some(0));
+    assert_eq!(c.current(), Some(&mut 2));
+    check_links(&ll);
+
+    // Test case 2: minimal reproduction - cursor at front, pop_front
+    let mut ll: LinkedList<u32> = LinkedList::new();
+    ll.extend(&[0, 1]);
+    let mut c = ll.cursor_front_mut();
+
+    assert_eq!(c.index(), Some(0));
+    assert_eq!(c.current(), Some(&mut 0));
+
+    // Pop front when cursor is at front - should move to next and index stays 0
+    c.pop_front();
+    assert_eq!(c.index(), Some(0));
+    assert_eq!(c.current(), Some(&mut 1));
+    check_links(&ll);
+
+    // Test case 3: single element list
+    let mut ll: LinkedList<u32> = LinkedList::new();
+    ll.push_back(42);
+    let mut c = ll.cursor_front_mut();
+
+    assert_eq!(c.index(), Some(0));
+    assert_eq!(c.current(), Some(&mut 42));
+
+    // Pop the only element - cursor should be at ghost node with index 0
+    c.pop_front();
+    assert_eq!(c.index(), None);
+    assert_eq!(c.current(), None);
+    check_links(&ll);
+}
+
 #[test]
 fn test_extend_ref() {
     let mut a = LinkedList::new();