Stabilize UnsafeCell::from_mut

Author: clarfonthey

library/core/src/cell.rs

@@ -2125,7 +2125,8 @@ impl<T: ?Sized> UnsafeCell<T> {
     /// assert_eq!(*uc.get_mut(), 41);
     /// ```
     #[inline(always)]
-    #[unstable(feature = "unsafe_cell_from_mut", issue = "111645")]
+    #[stable(feature = "unsafe_cell_from_mut", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "unsafe_cell_from_mut", since = "CURRENT_RUSTC_VERSION")]
     pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
         // SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
         unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }