4 files changed, 50 insertions, 1 deletions
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 5a63d90b95f..49ff768bed1 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -619,6 +619,37 @@ impl<T, A: Allocator> Box<T, A> {
     pub fn into_inner(boxed: Self) -> T {
         *boxed
     }
+
+    /// Consumes the `Box` without consuming its allocation, returning the wrapped value and a `Box`
+    /// to the uninitialized memory where the wrapped value used to live.
+    ///
+    /// This can be used together with [`write`](Box::write) to reuse the allocation for multiple
+    /// boxed values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(box_take)]
+    ///
+    /// let c = Box::new(5);
+    ///
+    /// // take the value out of the box
+    /// let (value, uninit) = Box::take(c);
+    /// assert_eq!(value, 5);
+    ///
+    /// // reuse the box for a second value
+    /// let c = Box::write(uninit, 6);
+    /// assert_eq!(*c, 6);
+    /// ```
+    #[unstable(feature = "box_take", issue = "147212")]
+    pub fn take(boxed: Self) -> (T, Box<mem::MaybeUninit<T>, A>) {
+        unsafe {
+            let (raw, alloc) = Box::into_raw_with_allocator(boxed);
+            let value = raw.read();
+            let uninit = Box::from_raw_in(raw.cast::<mem::MaybeUninit<T>>(), alloc);
+            (value, uninit)
+        }
+    }
 }
 
 impl<T> Box<[T]> {
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index fc3266b7479..87ad5b0ce30 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -96,6 +96,7 @@
 #![feature(bstr)]
 #![feature(bstr_internals)]
 #![feature(cast_maybe_uninit)]
+#![feature(cell_get_cloned)]
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 023238a00db..2b62b92d438 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -242,7 +242,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use core::any::Any;
-use core::cell::Cell;
+use core::cell::{Cell, CloneFromCell};
 #[cfg(not(no_global_oom_handling))]
 use core::clone::CloneToUninit;
 use core::clone::UseCloned;
@@ -340,6 +340,10 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Rc<U, A>> for
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
 
+// SAFETY: `Rc::clone` doesn't access any `Cell`s which could contain the `Rc` being cloned.
+#[unstable(feature = "cell_get_cloned", issue = "145329")]
+unsafe impl<T: ?Sized> CloneFromCell for Rc<T> {}
+
 impl<T: ?Sized> Rc<T> {
     #[inline]
     unsafe fn from_inner(ptr: NonNull<RcInner<T>>) -> Self {
@@ -3013,6 +3017,10 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> f
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
+// SAFETY: `Weak::clone` doesn't access any `Cell`s which could contain the `Weak` being cloned.
+#[unstable(feature = "cell_get_cloned", issue = "145329")]
+unsafe impl<T: ?Sized> CloneFromCell for Weak<T> {}
+
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.
     /// Calling [`upgrade`] on the return value always gives [`None`].
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 6432bdfbbed..5927d036469 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -9,6 +9,7 @@
 //! `#[cfg(target_has_atomic = "ptr")]`.
 
 use core::any::Any;
+use core::cell::CloneFromCell;
 #[cfg(not(no_global_oom_handling))]
 use core::clone::CloneToUninit;
 use core::clone::UseCloned;
@@ -281,6 +282,10 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> fo
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
 
+// SAFETY: `Arc::clone` doesn't access any `Cell`s which could contain the `Arc` being cloned.
+#[unstable(feature = "cell_get_cloned", issue = "145329")]
+unsafe impl<T: ?Sized> CloneFromCell for Arc<T> {}
+
 impl<T: ?Sized> Arc<T> {
     unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
         unsafe { Self::from_inner_in(ptr, Global) }
@@ -356,6 +361,10 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> f
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
+// SAFETY: `Weak::clone` doesn't access any `Cell`s which could contain the `Weak` being cloned.
+#[unstable(feature = "cell_get_cloned", issue = "145329")]
+unsafe impl<T: ?Sized> CloneFromCell for Weak<T> {}
+
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {