rework on the feedback

Author: felixinkinto

include/swift/AST/DiagnosticsSema.def

@@ -1718,7 +1718,10 @@ ERROR(functions_mutating_and_not,none,
       "method must not be declared both %0 and %1",
       (SelfAccessKind, SelfAccessKind))
 ERROR(static_functions_not_mutating,none,
-      "nonmutating or mutating modifiers cannot be used on static functions because they do not operate on instance properties or self", ())
+      "%0 modifiers cannot be used on static functions",
+      (SelfAccessKind))
+NOTE(static_functions_not_mutating_detail,none,
+      "%0 modifiers cannot access instance properties or self", (SelfAccessKind))
 
 ERROR(readwriter_mutatingness_differs_from_reader_or_writer_mutatingness,none,
       "%0 cannot be %1 when "

lib/Sema/TypeCheckAttr.cpp

@@ -603,7 +603,8 @@ void AttributeChecker::visitMutationAttr(DeclAttribute *attr) {
   }
   // Verify that we don't have a static function.
   if (FD->isStatic())
-    diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating);
+    diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating, attrModifier);
+    diagnose(attr->getLocation(), diag::static_functions_not_mutating_detail, attrModifier);
 }
 
 void AttributeChecker::visitDynamicAttr(DynamicAttr *attr) {