alloc: Add new_zeroed() versions like new_uninit().

MaybeUninit has both uninit() and zeroed(), it seems reasonable to have the same
surface on Box/Rc/Arc.

Needs tests.

1 files changed, 29 insertions, 0 deletions

diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 80d6c6e0d43..8153e63e8f5 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -341,6 +341,35 @@ impl<T> Arc<T> {
         }
     }
 
+    /// Constructs a new `Arc` with uninitialized contents, with the memory
+    /// being filled with `0` bytes.
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let zero = Arc::<u32>::new_zeroed();
+    /// let zero = unsafe { zero.assume_init() };
+    ///
+    /// assert_eq!(*zero, 0)
+    /// ```
+    ///
+    /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
+        unsafe {
+            let mut uninit = Self::new_uninit();
+            ptr::write_bytes::<T>(Arc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
+            uninit
+        }
+    }
+
     /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
     /// `data` will be pinned in memory and unable to be moved.
     #[stable(feature = "pin", since = "1.33.0")]