allocate before calling T::default in <Arc<T>>::default()

Same rationale as in the previous commit.

alloc/src/lib.rs

@@ -104,6 +104,7 @@
 #![feature(async_closure)]
 #![feature(async_fn_traits)]
 #![feature(async_iterator)]
+#![feature(box_uninit_write)]
 #![feature(clone_to_uninit)]
 #![feature(coerce_unsized)]
 #![feature(const_align_of_val)]

alloc/src/sync.rs

@@ -3447,7 +3447,13 @@ impl<T: Default> Default for Arc<T> {
     /// assert_eq!(*x, 0);
     /// ```
     fn default() -> Arc<T> {
-        Arc::new(Default::default())
+        let x = Box::into_raw(Box::write(Box::new_uninit(), ArcInner {
+            strong: atomic::AtomicUsize::new(1),
+            weak: atomic::AtomicUsize::new(1),
+            data: T::default(),
+        }));
+        // SAFETY: `Box::into_raw` consumes the `Box` and never returns null
+        unsafe { Self::from_inner(NonNull::new_unchecked(x)) }
     }
 }