Rollup merge of rust-lang#131261 - clarfonthey:unsafe-cell-from-mut, r=m-ou-se

Stabilize `UnsafeCell::from_mut`

Closes rust-lang#111645.
FCP: rust-lang#111645 (comment)

Note that because `const_mut_refs` and `const_refs_to_cell` was stabilized, it's okay to const-stabilize this method as well.

Author: matthiaskrgr

library/core/src/cell.rs

@@ -2122,7 +2122,6 @@ impl<T: ?Sized> UnsafeCell<T> {
     /// # Examples
     ///
     /// ```
-    /// # #![feature(unsafe_cell_from_mut)]
     /// use std::cell::UnsafeCell;
     ///
     /// let mut val = 42;
@@ -2132,7 +2131,9 @@ 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")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
     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>) }