Restrict the set of things that const stability can be applied

Remove the custom special case rejecting const stability on macros as it is now handled by the general check

Author: oli-obk

compiler/rustc_attr_parsing/src/attributes/stability.rs

@@ -243,7 +243,21 @@ impl<S: Stage> AttributeParser<S> for ConstStabilityParser {
             this.promotable = true;
         }),
     ];
-    const ALLOWED_TARGETS: AllowedTargets = ALLOWED_TARGETS;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Fn),
+        Allow(Target::Method(MethodKind::Inherent)),
+        Allow(Target::Method(MethodKind::TraitImpl)),
+        Allow(Target::Method(MethodKind::Trait { body: true })),
+        Allow(Target::Impl { of_trait: false }),
+        Allow(Target::Impl { of_trait: true }),
+        Allow(Target::Use), // FIXME I don't think this does anything?
+        Allow(Target::Const),
+        Allow(Target::AssocConst),
+        Allow(Target::Trait),
+        Allow(Target::Static),
+        Allow(Target::Crate),
+        Allow(Target::MacroDef), // FIXME(oli-obk): remove this and eliminate the manual check for it
+    ]);
 
     fn finalize(mut self, cx: &FinalizeContext<'_, '_, S>) -> Option<AttributeKind> {
         if self.promotable {