Rollup merge of #154929 - davidgauch:const-default-lazy, r=jhpratt

Add `const Default` impls for `LazyCell` and `LazyLock`

Follow up to these commits by @estebank https://github.com/rust-lang/rust/pull/134628 and https://github.com/rust-lang/rust/pull/151190.
Tracking issue https://github.com/rust-lang/rust/issues/143894.

cc @fmease @fee1-dead @oli-obk

This enables `static L: LazyLock<D> = Default::default()`  for any type `D: Default` which is safe as `D::default()` is only evaluated at runtime.

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> {

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() {

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)]