Rollup merge of #88722 - WaffleLapkin:unsafe_cell_const_get_mut, r=dtolnay

Make `UnsafeCell::get_mut` const

library/core/src/cell.rs

@@ -1916,7 +1916,8 @@ pub const fn get(&self) -> *mut T {
     /// ```
     #[inline(always)]
     #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
-    pub fn get_mut(&mut self) -> &mut T {
+    #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
+    pub const fn get_mut(&mut self) -> &mut T {
         &mut self.value
     }