add tracking issue

Signed-off-by: tison <[email protected]>

Author: tisonkun

library/core/src/cell/once.rs

@@ -189,7 +189,7 @@ impl<T> OnceCell<T> {
     /// assert_eq!(value, &94);
     /// ```
     #[inline]
-    #[unstable(feature = "once_cell_get_mut")]
+    #[unstable(feature = "once_cell_get_mut", issue = "121641")]
     pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T
     where
         F: FnOnce() -> T,
@@ -267,7 +267,7 @@ impl<T> OnceCell<T> {
     /// *value.unwrap() += 2;
     /// assert_eq!(cell.get(), Some(&94))
     /// ```
-    #[unstable(feature = "once_cell_get_mut")]
+    #[unstable(feature = "once_cell_get_mut", issue = "121641")]
     pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
     where
         F: FnOnce() -> Result<T, E>,