Add LocalKey<Cell>::update

Author: camsteffen

library/std/src/thread/local.rs

@@ -469,6 +469,29 @@ impl<T: 'static> LocalKey<Cell<T>> {
     pub fn replace(&'static self, value: T) -> T {
         self.with(|cell| cell.replace(value))
     }
+
+    /// Updates the contained value using a function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(local_key_cell_update)]
+    /// use std::cell::Cell;
+    ///
+    /// thread_local! {
+    ///     static X: Cell<i32> = const { Cell::new(5) };
+    /// }
+    ///
+    /// X.update(|x| x + 1);
+    /// assert_eq!(X.get(), 6);
+    /// ```
+    #[unstable(feature = "local_key_cell_update", issue = "143989")]
+    pub fn update(&'static self, f: impl FnOnce(T) -> T)
+    where
+        T: Copy,
+    {
+        self.with(|cell| cell.update(f))
+    }
 }
 
 impl<T: 'static> LocalKey<RefCell<T>> {