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

jwong101 · jwong101 · commit 8547f513811a · 2024-10-10T09:50:35.000-04:00
Same rationale as in the previous commit.
diff --git a/alloc/src/lib.rs b/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)]
diff --git a/alloc/src/sync.rs b/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)) }
     }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -3447,7 +3447,13 @@ impl<T: Default> Default for Arc<T> {`
`3447`	`3447`	`/// assert_eq!(*x, 0);`
`3448`	`3448`	/// ```
`3449`	`3449`	`fn default() -> Arc<T> {`
`3450`		`- Arc::new(Default::default())`
	`3450`	`+ let x = Box::into_raw(Box::write(Box::new_uninit(), ArcInner {`
	`3451`	`+ strong: atomic::AtomicUsize::new(1),`
	`3452`	`+ weak: atomic::AtomicUsize::new(1),`
	`3453`	`+ data: T::default(),`
	`3454`	`+ }));`
	`3455`	+ // SAFETY: `Box::into_raw` consumes the `Box` and never returns null
	`3456`	`+ unsafe { Self::from_inner(NonNull::new_unchecked(x)) }`
`3451`	`3457`	`}`
`3452`	`3458`	`}`
`3453`	`3459`