Make derive_const usable within libcore again

Also make it *only* usable on nightly

Author: oli-obk

core/src/macros/mod.rs

@@ -1615,7 +1615,7 @@ pub(crate) mod builtin {
     /// See [the reference] for more info.
     ///
     /// [the reference]: ../../../reference/attributes/derive.html
-    #[unstable(feature = "derive_const", issue = "none")]
+    #[unstable(feature = "derive_const", issue = "118304")]
     #[rustc_builtin_macro]
     pub macro derive_const($item:item) {
         /* compiler built-in */

core/src/prelude/v1.rs

@@ -80,7 +80,7 @@ pub use crate::macros::builtin::{
     alloc_error_handler, bench, derive, global_allocator, test, test_case,
 };
 
-#[unstable(feature = "derive_const", issue = "none")]
+#[unstable(feature = "derive_const", issue = "118304")]
 pub use crate::macros::builtin::derive_const;
 
 #[unstable(

std/src/prelude/v1.rs

@@ -67,7 +67,7 @@ pub use core::prelude::v1::{
     alloc_error_handler, bench, derive, global_allocator, test, test_case,
 };
 
-#[unstable(feature = "derive_const", issue = "none")]
+#[unstable(feature = "derive_const", issue = "118304")]
 pub use core::prelude::v1::derive_const;
 
 // Do not `doc(no_inline)` either.