make rustc_encodable_decodable feature properly unstable

Author: RalfJung

core/src/macros/mod.rs

@@ -1773,8 +1773,7 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[unstable(
         feature = "rustc_encodable_decodable",
-        issue = "none",
-        soft,
+        issue = "134301",
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
@@ -1787,8 +1786,7 @@ pub(crate) mod builtin {
     #[rustc_builtin_macro]
     #[unstable(
         feature = "rustc_encodable_decodable",
-        issue = "none",
-        soft,
+        issue = "134301",
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]

core/src/prelude/mod.rs

@@ -22,8 +22,7 @@ pub mod v1 {
     // Do not `doc(inline)` these `doc(hidden)` items.
     #[unstable(
         feature = "rustc_encodable_decodable",
-        issue = "none",
-        soft,
+        issue = "134301",
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[allow(deprecated)]

std/src/prelude/mod.rs

@@ -124,8 +124,7 @@ pub mod v1 {
     // Do not `doc(inline)` these `doc(hidden)` items.
     #[unstable(
         feature = "rustc_encodable_decodable",
-        issue = "none",
-        soft,
+        issue = "134301",
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[allow(deprecated)]