Add const Default for *Cell

Author: estebank

library/core/src/cell.rs

@@ -332,7 +332,8 @@ impl<T: Copy> Clone for Cell<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Default> Default for Cell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: ~const Default> const Default for Cell<T> {
     /// Creates a `Cell<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Cell<T> {
@@ -1318,7 +1319,8 @@ impl<T: Clone> Clone for RefCell<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Default> Default for RefCell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: ~const Default> const Default for RefCell<T> {
     /// Creates a `RefCell<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> RefCell<T> {
@@ -2316,7 +2318,8 @@ impl<T: ?Sized> UnsafeCell<T> {
 }
 
 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
-impl<T: Default> Default for UnsafeCell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: ~const Default> const Default for UnsafeCell<T> {
     /// Creates an `UnsafeCell`, with the `Default` value for T.
     fn default() -> UnsafeCell<T> {
         UnsafeCell::new(Default::default())
@@ -2423,7 +2426,8 @@ impl<T: ?Sized> SyncUnsafeCell<T> {
 }
 
 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
-impl<T: Default> Default for SyncUnsafeCell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "none")]
+impl<T: ~const Default> const Default for SyncUnsafeCell<T> {
     /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
     fn default() -> SyncUnsafeCell<T> {
         SyncUnsafeCell::new(Default::default())

tests/ui/traits/const-traits/const-traits-core.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-#![feature(const_trait_impl, ptr_alignment_type, ascii_char, f16, f128)]
+#![feature(const_trait_impl, ptr_alignment_type, ascii_char, f16, f128, sync_unsafe_cell)]
 #![allow(dead_code)]
 // core::default
 const UNIT: () = Default::default();
@@ -32,8 +32,13 @@ const ALIGNMENT: std::ptr::Alignment = Default::default();
 // core::slice
 const SLICE: &[()] = Default::default();
 const MUT_SLICE: &mut [()] = Default::default();
-//core::str
+// core::str
 const STR: &str = Default::default();
 const MUT_STR: &mut str = Default::default();
+// core::cell
+const CELL: std::cell::Cell<()> = Default::default();
+const REF_CELL: std::cell::RefCell<()> = Default::default();
+const UNSAFE_CELL: std::cell::UnsafeCell<()> = Default::default();
+const SYNC_UNSAFE_CELL: std::cell::SyncUnsafeCell<()> = Default::default();
 
 fn main() {}