Expose Freeze trait again

Author: oli-obk

core/src/marker.rs

@@ -810,15 +810,21 @@ pub trait DiscriminantKind {
     type Discriminant: Clone + Copy + Debug + Eq + PartialEq + Hash + Send + Sync + Unpin;
 }
 
-/// Compiler-internal trait used to determine whether a type contains
+/// Used to determine whether a type contains
 /// any `UnsafeCell` internally, but not through an indirection.
 /// This affects, for example, whether a `static` of that type is
 /// placed in read-only static memory or writable static memory.
+/// This can be used to declare that a constant with a generic type
+/// will not contain interior mutability, and subsequently allow
+/// placing the constant behind references.
 #[lang = "freeze"]
-pub(crate) unsafe auto trait Freeze {}
+#[unstable(feature = "freeze", issue = "60715")]
+pub unsafe auto trait Freeze {}
 
+#[unstable(feature = "freeze", issue = "60715")]
 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
 marker_impls! {
+    #[unstable(feature = "freeze", issue = "60715")]
     unsafe Freeze for
         {T: ?Sized} PhantomData<T>,
         {T: ?Sized} *const T,