Ban the use of @_staticInitializeObjCMetadata; it's an implementation detail.

Author: DougGregor

include/swift/AST/Attr.def

@@ -278,7 +278,7 @@ DECL_ATTR(NSKeyedArchiveLegacy, NSKeyedArchiveLegacy,
           /*Not serialized */ 68)
 
 SIMPLE_DECL_ATTR(_staticInitializeObjCMetadata, StaticInitializeObjCMetadata,
-                 OnClass | NotSerialized | LongAttribute | UserInaccessible,
+                 OnClass | NotSerialized | LongAttribute | RejectByParser,
                  /*Not serialized */ 69)
 
 SIMPLE_DECL_ATTR(NSKeyedArchiveSubclassesOnly,

test/decl/protocol/conforms/nscoding.swift

@@ -139,6 +139,10 @@ extension CodingB {
 // Inference of @_staticInitializeObjCMetadata.
 class SubclassOfCodingE : CodingE<Int> { }
 
+// But don't allow one to write @_staticInitializeObjCMetadata!
+@_staticInitializeObjCMetadata // expected-error{{unknown attribute '_staticInitializeObjCMetadata'}}
+class DontAllowStaticInits { }
+
 // CHECK-NOT: class_decl "CodingA"{{.*}}@_staticInitializeObjCMetadata
 // CHECK: class_decl "NestedA"{{.*}}@_staticInitializeObjCMetadata
 // CHECK: class_decl "NestedC"{{.*}}@_staticInitializeObjCMetadata