From 80faf203515bf3be214ad3ffc019b55b026b1220 Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Sun, 26 Jan 2025 03:23:02 -0500
Subject: [PATCH 1/2] add test for Box::default's stack usage in debug mode

---
 tests/codegen/box-default-debug-copies.rs | 25 +++++++++++++++++++++++
 1 file changed, 25 insertions(+)
 create mode 100644 tests/codegen/box-default-debug-copies.rs

diff --git a/tests/codegen/box-default-debug-copies.rs b/tests/codegen/box-default-debug-copies.rs
new file mode 100644
index 0000000000000..ed4e0c416b8dc
--- /dev/null
+++ b/tests/codegen/box-default-debug-copies.rs
@@ -0,0 +1,25 @@
+//@ compile-flags: -Copt-level=0
+
+// Test to make sure that `<Box<T>>::default` does not create too many copies of `T` on the stack.
+// in debug mode. This regressed in dd0620b86721ae8cae86736443acd3f72ba6fc32 to
+// four `T` allocas.
+//
+// See https://github.com/rust-lang/rust/issues/136043 for more context.
+
+#![crate_type = "lib"]
+
+#[allow(dead_code)]
+pub struct Thing([u8; 1000000]);
+
+impl Default for Thing {
+    fn default() -> Self {
+        Thing([0; 1000000])
+    }
+}
+
+// CHECK-COUNT-4: %{{.*}} = alloca {{.*}}1000000
+// CHECK-NOT: %{{.*}} = alloca {{.*}}1000000
+#[no_mangle]
+pub fn box_default_single_copy() -> Box<Thing> {
+    Box::default()
+}

From 97005678c38fd391c9b502d011cc3f3d4434a18a Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Sun, 26 Jan 2025 03:36:50 -0500
Subject: [PATCH 2/2] reduce `Box::default` stack copies in debug mode

The `Box::new(T::default())` implementation of `Box::default` only
had two stack copies in debug mode, compared to the current version,
which has four. By avoiding creating any `MaybeUninit<T>`'s and just writing
`T` directly to the `Box` pointer, the stack usage in debug mode remains
the same as the old version.
---
 library/alloc/src/boxed.rs                | 15 ++++++++++++++-
 tests/codegen/box-default-debug-copies.rs |  5 ++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 1b5e44a913467..8f43ebc388889 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1730,7 +1730,20 @@ impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Self {
-        Box::write(Box::new_uninit(), T::default())
+        let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
+        unsafe {
+            // SAFETY: `x` is valid for writing and has the same layout as `T`.
+            // If `T::default()` panics, dropping `x` will just deallocate the Box as `MaybeUninit<T>`
+            // does not have a destructor.
+            //
+            // We use `ptr::write` as `MaybeUninit::write` creates
+            // extra stack copies of `T` in debug mode.
+            //
+            // See https://github.com/rust-lang/rust/issues/136043 for more context.
+            ptr::write(&raw mut *x as *mut T, T::default());
+            // SAFETY: `x` was just initialized above.
+            x.assume_init()
+        }
     }
 }
 
diff --git a/tests/codegen/box-default-debug-copies.rs b/tests/codegen/box-default-debug-copies.rs
index ed4e0c416b8dc..06cc41b21c06f 100644
--- a/tests/codegen/box-default-debug-copies.rs
+++ b/tests/codegen/box-default-debug-copies.rs
@@ -5,6 +5,9 @@
 // four `T` allocas.
 //
 // See https://github.com/rust-lang/rust/issues/136043 for more context.
+//
+// FIXME: This test only wants to ensure that there are at most two allocas of `T` created, instead
+// of checking for exactly two.
 
 #![crate_type = "lib"]
 
@@ -17,7 +20,7 @@ impl Default for Thing {
     }
 }
 
-// CHECK-COUNT-4: %{{.*}} = alloca {{.*}}1000000
+// CHECK-COUNT-2: %{{.*}} = alloca {{.*}}1000000
 // CHECK-NOT: %{{.*}} = alloca {{.*}}1000000
 #[no_mangle]
 pub fn box_default_single_copy() -> Box<Thing> {