From 669e25597c8244ce4991414a8dfb47869854fda3 Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Wed, 9 Oct 2024 14:24:05 -0400
Subject: [PATCH 1/2] allocate before calling T::default in <Box<T>>::default()

The `Box<T: Default>` impl currently calls `T::default()` before allocating
the `Box`.

Most `Default` impls are trivial, which should in theory allow
LLVM to construct `T: Default` directly in the `Box` allocation when calling
`<Box<T>>::default()`.

However, the allocation may fail, which necessitates calling `T's` destructor if it has one.
If the destructor is non-trivial, then LLVM has a hard time proving that it's
sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation.

Create an uninit `Box` first, and then write `T::default` into it, so that LLVM now only needs to prove
that the `T::default` can't panic, which should be trivial for most `Default` impls.
---
 alloc/src/boxed.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/alloc/src/boxed.rs b/alloc/src/boxed.rs
index 5f20729568352..3e791416820ef 100644
--- a/alloc/src/boxed.rs
+++ b/alloc/src/boxed.rs
@@ -1688,7 +1688,7 @@ impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Self {
-        Box::new(T::default())
+        Box::write(Box::new_uninit(), T::default())
     }
 }
 

From 8547f513811ade8044296fa8bda487d066c41afd Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Wed, 9 Oct 2024 14:24:05 -0400
Subject: [PATCH 2/2] allocate before calling T::default in <Arc<T>>::default()

Same rationale as in the previous commit.
---
 alloc/src/lib.rs  | 1 +
 alloc/src/sync.rs | 8 +++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/alloc/src/lib.rs b/alloc/src/lib.rs
index c60c0743c7e12..dcfe96be7551d 100644
--- 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
index 5d099a49854af..0038750d25dd1 100644
--- 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)) }
     }
 }