From 072aa9e66f7c5e398d3030c8d41dc89079a4aed2 Mon Sep 17 00:00:00 2001
From: Trevor Gross <tmgross@umich.edu>
Date: Wed, 2 Apr 2025 18:15:50 +0000
Subject: [PATCH] Apply requested API changes to `cell_update`

Do the following:

* Switch to `impl FnOnce` rather than a generic `F`.
* Change `update` to return nothing.

This was discussed at a libs-api meeting [1].

Tracking issue: https://github.com/rust-lang/rust/issues/50186

[1]: https://github.com/rust-lang/rust/pull/134446#issuecomment-2770842949
---
 library/core/src/cell.rs        | 15 ++++-----------
 library/coretests/tests/cell.rs |  4 ++--
 2 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs
index e789601a409af..09117e4968db6 100644
--- a/library/core/src/cell.rs
+++ b/library/core/src/cell.rs
@@ -544,7 +544,7 @@ impl<T: Copy> Cell<T> {
         unsafe { *self.value.get() }
     }
 
-    /// Updates the contained value using a function and returns the new value.
+    /// Updates the contained value using a function.
     ///
     /// # Examples
     ///
@@ -554,21 +554,14 @@ impl<T: Copy> Cell<T> {
     /// use std::cell::Cell;
     ///
     /// let c = Cell::new(5);
-    /// let new = c.update(|x| x + 1);
-    ///
-    /// assert_eq!(new, 6);
+    /// c.update(|x| x + 1);
     /// assert_eq!(c.get(), 6);
     /// ```
     #[inline]
     #[unstable(feature = "cell_update", issue = "50186")]
-    pub fn update<F>(&self, f: F) -> T
-    where
-        F: FnOnce(T) -> T,
-    {
+    pub fn update(&self, f: impl FnOnce(T) -> T) {
         let old = self.get();
-        let new = f(old);
-        self.set(new);
-        new
+        self.set(f(old));
     }
 }
 
diff --git a/library/coretests/tests/cell.rs b/library/coretests/tests/cell.rs
index d6a401c2b4d98..781a46c3744f5 100644
--- a/library/coretests/tests/cell.rs
+++ b/library/coretests/tests/cell.rs
@@ -50,10 +50,10 @@ fn smoketest_cell() {
 fn cell_update() {
     let x = Cell::new(10);
 
-    assert_eq!(x.update(|x| x + 5), 15);
+    x.update(|x| x + 5);
     assert_eq!(x.get(), 15);
 
-    assert_eq!(x.update(|x| x / 3), 5);
+    x.update(|x| x / 3);
     assert_eq!(x.get(), 5);
 }