Rollup merge of #127189 - GrigorenkoPV:linkedlist-cursor-list, r=Nilstrieb

LinkedList's Cursor: method to get a ref to the cursor's list

We're already providing `.back()` & `.front()`, for which we hold onto a reference to the parent list, so why not share it? Useful for when you got `LinkedList` -> `CursorMut` -> `Cursor` and cannot take another ref to the list, even though you should be able to. This seems to be completely safe & sound.

The name is, of course, bikesheddable.

library/alloc/src/collections/linked_list.rs

@@ -1495,6 +1495,14 @@ impl<'a, T, A: Allocator> Cursor<'a, T, A> {
     pub fn back(&self) -> Option<&'a T> {
         self.list.back()
     }
+
+    /// Provides a reference to the cursor's parent list.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn as_list(&self) -> &'a LinkedList<T, A> {
+        self.list
+    }
 }
 
 impl<'a, T, A: Allocator> CursorMut<'a, T, A> {
@@ -1605,6 +1613,18 @@ impl<'a, T, A: Allocator> CursorMut<'a, T, A> {
     pub fn as_cursor(&self) -> Cursor<'_, T, A> {
         Cursor { list: self.list, current: self.current, index: self.index }
     }
+
+    /// Provides a read-only reference to the cursor's parent list.
+    ///
+    /// The lifetime of the returned reference is bound to that of the
+    /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
+    /// `CursorMut` is frozen for the lifetime of the reference.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "linked_list_cursors", issue = "58533")]
+    pub fn as_list(&self) -> &LinkedList<T, A> {
+        self.list
+    }
 }
 
 // Now the list editing operations