From 5b12d906bb66d308fb3e94d6d79e076fa87c7f7c Mon Sep 17 00:00:00 2001
From: Slanterns <slanterns.w@gmail.com>
Date: Tue, 22 Oct 2024 01:08:15 -0700
Subject: [PATCH 1/3] optimize `Rc<T>::default`

---
 library/alloc/src/rc.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index e98ae7c31ad..582d850e14b 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -2312,7 +2312,16 @@ impl<T: Default> Default for Rc<T> {
     /// ```
     #[inline]
     fn default() -> Rc<T> {
-        Rc::new(Default::default())
+        unsafe {
+            Self::from_inner(
+                Box::leak(Box::write(Box::new_uninit(), RcInner {
+                    strong: Cell::new(1),
+                    weak: Cell::new(1),
+                    value: T::default(),
+                }))
+                .into(),
+            )
+        }
     }
 }
 

From 7782401c520ab1e744dbcf58d0f0254672da9026 Mon Sep 17 00:00:00 2001
From: Slanterns <slanterns.w@gmail.com>
Date: Tue, 22 Oct 2024 01:33:22 -0700
Subject: [PATCH 2/3] add codegen test

---
 tests/codegen/placement-new.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tests/codegen/placement-new.rs b/tests/codegen/placement-new.rs
index edb25df5eb4..0ec2b6a6f20 100644
--- a/tests/codegen/placement-new.rs
+++ b/tests/codegen/placement-new.rs
@@ -1,9 +1,11 @@
 //@ compile-flags: -O
+//@ compile-flags: -Zmerge-functions=disabled
 #![crate_type = "lib"]
 
 // Test to check that types with "complex" destructors, but trivial `Default` impls
 // are constructed directly into the allocation in `Box::default` and `Arc::default`.
 
+use std::rc::Rc;
 use std::sync::Arc;
 
 // CHECK-LABEL: @box_default_inplace
@@ -16,6 +18,16 @@ pub fn box_default_inplace() -> Box<(String, String)> {
     Box::default()
 }
 
+// CHECK-LABEL: @rc_default_inplace
+#[no_mangle]
+pub fn rc_default_inplace() -> Rc<(String, String)> {
+    // CHECK-NOT: alloca
+    // CHECK: [[RC:%.*]] = {{.*}}call {{.*}}__rust_alloc(
+    // CHECK-NOT: call void @llvm.memcpy
+    // CHECK: ret ptr [[RC]]
+    Rc::default()
+}
+
 // CHECK-LABEL: @arc_default_inplace
 #[no_mangle]
 pub fn arc_default_inplace() -> Arc<(String, String)> {

From 0a963ab2da139b177ef1e7f09efed7e4e1e8e930 Mon Sep 17 00:00:00 2001
From: Slanterns <slanterns.w@gmail.com>
Date: Tue, 22 Oct 2024 01:37:13 -0700
Subject: [PATCH 3/3] refactor `Arc<T>::default`

---
 library/alloc/src/sync.rs | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index acbc325a514..13677245e98 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -3447,13 +3447,16 @@ impl<T: Default> Default for Arc<T> {
     /// assert_eq!(*x, 0);
     /// ```
     fn default() -> Arc<T> {
-        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)) }
+        unsafe {
+            Self::from_inner(
+                Box::leak(Box::write(Box::new_uninit(), ArcInner {
+                    strong: atomic::AtomicUsize::new(1),
+                    weak: atomic::AtomicUsize::new(1),
+                    data: T::default(),
+                }))
+                .into(),
+            )
+        }
     }
 }