From c7c91173267632dbd4f082d1f27fcb1453be28a9 Mon Sep 17 00:00:00 2001
From: David Gauch <davidgauch@gmail.com>
Date: Mon, 6 Apr 2026 23:53:29 -0700
Subject: [PATCH] Add `const Default` impls for `LazyCell` and `LazyLock`

---
 library/core/src/cell/lazy.rs       | 3 ++-
 library/std/src/sync/lazy_lock.rs   | 3 ++-
 library/std/tests/sync/lazy_lock.rs | 9 +++++++++
 library/std/tests/sync/lib.rs       | 2 ++
 4 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/library/core/src/cell/lazy.rs b/library/core/src/cell/lazy.rs
index ae559e599f48..9d350166c681 100644
--- a/library/core/src/cell/lazy.rs
+++ b/library/core/src/cell/lazy.rs
@@ -346,7 +346,8 @@ fn deref_mut(&mut self) -> &mut T {
 }
 
 #[stable(feature = "lazy_cell", since = "1.80.0")]
-impl<T: Default> Default for LazyCell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl<T: Default> const Default for LazyCell<T> {
     /// Creates a new lazy value using `Default` as the initializing function.
     #[inline]
     fn default() -> LazyCell<T> {
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 9bdde0ae4a53..de1b9c391e8f 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -375,7 +375,8 @@ fn deref_mut(&mut self) -> &mut T {
 }
 
 #[stable(feature = "lazy_cell", since = "1.80.0")]
-impl<T: Default> Default for LazyLock<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl<T: Default> const Default for LazyLock<T> {
     /// Creates a new lazy value using `Default` as the initializing function.
     #[inline]
     fn default() -> LazyLock<T> {
diff --git a/library/std/tests/sync/lazy_lock.rs b/library/std/tests/sync/lazy_lock.rs
index 68aeea834b4f..c549094dd138 100644
--- a/library/std/tests/sync/lazy_lock.rs
+++ b/library/std/tests/sync/lazy_lock.rs
@@ -33,6 +33,15 @@ fn default() -> Self {
     assert_eq!(CALLED.load(SeqCst), 1);
 }
 
+#[test]
+fn const_lazy_default() {
+    // using Box as it cannot be initialized in const contexts
+    const X: LazyLock<Box<u8>> = <_>::default();
+    const Y: LazyCell<Box<u8>> = <_>::default();
+    assert_eq!(**X, 0);
+    assert_eq!(**Y, 0);
+}
+
 #[test]
 #[cfg_attr(any(target_os = "emscripten", target_os = "wasi"), ignore)] // no threads
 fn sync_lazy_new() {
diff --git a/library/std/tests/sync/lib.rs b/library/std/tests/sync/lib.rs
index 32a7efde2a25..92c4c2f511be 100644
--- a/library/std/tests/sync/lib.rs
+++ b/library/std/tests/sync/lib.rs
@@ -1,3 +1,5 @@
+#![feature(const_default)]
+#![feature(const_trait_impl)]
 #![feature(mapped_lock_guards)]
 #![feature(mpmc_channel)]
 #![feature(oneshot_channel)]