From c34b98f1bad5b994d8f4fc89eb88d7334a386379 Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Wed, 9 Oct 2024 14:24:05 -0400
Subject: [PATCH] allocate before calling T::default in <Arc<T>>::default()

Same rationale as in the previous commit.
---
 library/alloc/src/lib.rs       | 1 +
 library/alloc/src/sync.rs      | 8 +++++++-
 tests/codegen/placement-new.rs | 4 ++--
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index c60c0743c7e12..dcfe96be7551d 100644
--- a/library/alloc/src/lib.rs
+++ b/library/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/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 5d099a49854af..0038750d25dd1 100644
--- a/library/alloc/src/sync.rs
+++ b/library/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)) }
     }
 }
 
diff --git a/tests/codegen/placement-new.rs b/tests/codegen/placement-new.rs
index 1b49d780b5273..a102dd51db6a7 100644
--- a/tests/codegen/placement-new.rs
+++ b/tests/codegen/placement-new.rs
@@ -19,9 +19,9 @@ pub fn box_default_inplace() -> Box<(String, String)> {
 // CHECK-LABEL: @arc_default_inplace
 #[no_mangle]
 pub fn arc_default_inplace() -> Arc<(String, String)> {
-    // CHECK: [[ALLOCA:%.*]] = alloca
+    // CHECK-NOT: alloca
     // CHECK: [[ARC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
-    // CHECK: call void @llvm.memcpy.p0.p0.i64(
+    // CHECK-NOT: call void @llvm.memcpy.p0.p0.i64(
     // CHECK: ret ptr [[ARC]]
     Arc::default()
 }