implement Box::take

Author: edwloef

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]> {