Disable diagnostics about problem NSCoding-conforming classes for now. (#9808)

Adoption so far shows that the criteria we set up here are too broad.
This is particularly problematic for subclasses of NS/UIView and the
like that might never be encoded at all.

rdar://problem/32306355

Author: jrose-apple

include/swift/Basic/LangOptions.h

@@ -227,6 +227,9 @@ namespace swift {
     /// of Swift do not.
     Swift3ObjCInferenceWarnings WarnSwift3ObjCInference =
       Swift3ObjCInferenceWarnings::None;
+
+    /// Diagnose uses of NSCoding with classes that have unstable mangled names.
+    bool EnableNSKeyedArchiverDiagnostics = false;
 
     /// Enable keypath components that aren't fully implemented.
     bool EnableExperimentalKeyPathComponents = false;

include/swift/Option/FrontendOptions.td

@@ -304,6 +304,13 @@ def disable_swift3_objc_inference :
   Flags<[FrontendOption, HelpHidden]>,
   HelpText<"Disable Swift 3's @objc inference rules for NSObject-derived classes and 'dynamic' members (emulates Swift 4 behavior)">;
 
+def enable_nskeyedarchiver_diagnostics :
+  Flag<["-"], "enable-nskeyedarchiver-diagnostics">,
+  HelpText<"Diagnose classes with unstable mangled names adopting NSCoding">;
+def disable_nskeyedarchiver_diagnostics :
+  Flag<["-"], "disable-nskeyedarchiver-diagnostics">,
+  HelpText<"Allow classes with unstable mangled names to adopt NSCoding">;
+
 def warn_long_function_bodies : Separate<["-"], "warn-long-function-bodies">,
   MetaVarName<"<n>">,
   HelpText<"Warns when type-checking a function takes longer than <n> ms">;

lib/Frontend/CompilerInvocation.cpp

@@ -1038,6 +1038,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.EnableNSKeyedArchiverDiagnostics =
+      Args.hasFlag(OPT_enable_nskeyedarchiver_diagnostics,
+                   OPT_disable_nskeyedarchiver_diagnostics,
+                   Opts.EnableNSKeyedArchiverDiagnostics);
+
   llvm::Triple Target = Opts.Target;
   StringRef TargetArg;
   if (const Arg *A = Args.getLastArg(OPT_target)) {

lib/Sema/TypeCheckProtocol.cpp

@@ -6058,7 +6058,8 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
           }
         }
 
-        if (kind && !hasExplicitObjCName(classDecl) &&
+        if (kind && getLangOpts().EnableNSKeyedArchiverDiagnostics &&
+            !hasExplicitObjCName(classDecl) &&
             !classDecl->getAttrs().hasAttribute<NSKeyedArchiverClassNameAttr>() &&
             !classDecl->getAttrs()
               .hasAttribute<NSKeyedArchiverEncodeNonGenericSubclassesOnlyAttr>()) {

test/decl/protocol/conforms/nscoding.swift

@@ -1,10 +1,14 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -swift-version 4 %s -verify
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -swift-version 4 %s -enable-nskeyedarchiver-diagnostics -verify
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -swift-version 4 %s 2>&1 | %FileCheck -check-prefix CHECK-NO-DIAGS %s
 
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -swift-version 4 %s -dump-ast 2> %t.ast
 // RUN: %FileCheck %s < %t.ast
 
 // REQUIRES: objc_interop
 
+// CHECK-NO-DIAGS-NOT: NSCoding
+// CHECK-NO-DIAGS-NOT: unstable
+
 import Foundation
 
 // Top-level classes