Add RefCell::take

src/libcore/cell.rs

@@ -1023,6 +1023,31 @@ impl<T: ?Sized> RefCell<T> {
     }
 }
 
+impl<T: Default> RefCell<T> {
+    /// Takes the wrapped value, leaving `Default::default()` in its place.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is currently borrowed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(refcell_take)]
+    /// use std::cell::RefCell;
+    ///
+    /// let c = RefCell::new(5);
+    /// let five = c.take();
+    ///
+    /// assert_eq!(five, 5);
+    /// assert_eq!(c.into_inner(), 0);
+    /// ```
+    #[unstable(feature = "refcell_take", issue = "71395")]
+    pub fn take(&self) -> T {
+        self.replace(Default::default())
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 

src/libstd/sync/once.rs

@@ -497,7 +497,7 @@ impl Drop for WaiterQueue<'_> {
             let mut queue = (state_and_queue & !STATE_MASK) as *const Waiter;
             while !queue.is_null() {
                 let next = (*queue).next;
-                let thread = (*queue).thread.replace(None).unwrap();
+                let thread = (*queue).thread.take().unwrap();
                 (*queue).signaled.store(true, Ordering::Release);
                 // ^- FIXME (maybe): This is another case of issue #55005
                 // `store()` has a potentially dangling ref to `signaled`.

src/test/run-make/wasm-panic-small/foo.rs

@@ -23,5 +23,5 @@ pub fn foo() {
 pub fn foo() -> usize {
     use std::cell::Cell;
     thread_local!(static A: Cell<Vec<u32>> = Cell::new(Vec::new()));
-    A.try_with(|x| x.replace(Vec::new()).len()).unwrap_or(0)
+    A.try_with(|x| x.take().len()).unwrap_or(0)
 }

src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.rs

@@ -7,7 +7,7 @@ struct MyType<'a>(Cell<Option<&'a mut MyType<'a>>>, PhantomPinned);
 impl<'a> Clone for &'a mut MyType<'a> {
     //~^ ERROR E0751
     fn clone(&self) -> &'a mut MyType<'a> {
-        self.0.replace(None).unwrap()
+        self.0.take().unwrap()
     }
 }
 

src/test/ui/traits/negative-impls/pin-unsound-issue-66544-derefmut.rs

@@ -12,7 +12,7 @@ struct MyType<'a>(Cell<Option<&'a mut MyType<'a>>>, PhantomPinned);
 impl<'a> DerefMut for &'a MyType<'a> {
     //~^ ERROR E0751
     fn deref_mut(&mut self) -> &mut MyType<'a> {
-        self.0.replace(None).unwrap()
+        self.0.take().unwrap()
     }
 }