Make diagnostics for NSCoding classes with unstable names more discouraging.

The diagnostic regarding NSCoding classes with unstable names can be
suppressed by adding @objc (the preferred solution for new code) or
@NSKeyedArchiveLegacy (for existing archives). Provide those as
Fix-Its, in that order.

(Thanks, Jordan!)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -1220,11 +1220,14 @@ WARNING(nscoding_unstable_mangled_name_warn,none,
         "%select{private|fileprivate|nested|local|generic}0 class %1 has an "
         "unstable name when archiving via 'NSCoding'",
         (unsigned, Type))
-NOTE(add_nskeyedarchivelegacy_attr,none,
-     "add the '@NSKeyedArchiveLegacy' attribute to specify the class name "
-     "used for archiving", ())
+NOTE(unstable_mangled_name_add_objc,none,
+     "for new classes, add '@objc' to specify a unique, prefixed Objective-C "
+     "runtime name", ())
+NOTE(unstable_mangled_name_add_nskeyedarchivelegacy,none,
+     "for compatibility with existing archives, use '@NSKeyedArchiveLegacy' "
+     "to record the Swift 3 mangled name", ())
 ERROR(attr_nskeyedarchivelegacy_generic,none,
-      "@NSKeyedArchiveLegacy attribute cannot be applied to generic class %0",
+      "'@NSKeyedArchiveLegacy' cannot be applied to generic class %0",
       (Type))
 
 // Generic types

lib/Sema/TypeCheckProtocol.cpp

@@ -5944,10 +5944,15 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
                                : diag::nscoding_unstable_mangled_name,
                    *kind, classDecl->TypeDecl::getDeclaredInterfaceType());
           if (isFixable) {
-            diagnose(classDecl, diag::add_nskeyedarchivelegacy_attr)
-              .fixItInsert(
-                classDecl->getAttributeInsertionLoc(/*forModifier=*/false),
-                "@NSKeyedArchiveLegacy(\"<#class archival name#>\")");
+            auto insertionLoc =
+              classDecl->getAttributeInsertionLoc(/*forModifier=*/false);
+            diagnose(classDecl, diag::unstable_mangled_name_add_objc)
+              .fixItInsert(insertionLoc,
+                           "@objc(<#Objective-C class name#>)");
+            diagnose(classDecl,
+                     diag::unstable_mangled_name_add_nskeyedarchivelegacy)
+              .fixItInsert(insertionLoc,
+                           "@NSKeyedArchiveLegacy(\"<#class archival name#>\")");
           }
         }
 

test/decl/protocol/conforms/nscoding.swift

@@ -17,13 +17,15 @@ class CodingA : NSObject, NSCoding {
 // Nested classes
 extension CodingA {
   class NestedA : NSObject, NSCoding { // expected-error{{nested class 'CodingA.NestedA' has an unstable name when archiving via 'NSCoding'}}
-    // expected-note@-1{{add the '@NSKeyedArchiveLegacy' attribute to specify the class name used for archiving}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
+    // expected-note@-1{{for new classes, add '@objc' to specify a unique, prefixed Objective-C runtime name}}{{3-3=@objc(<#Objective-C class name#>)}}
+    // expected-note@-2{{for compatibility with existing archives, use '@NSKeyedArchiveLegacy' to record the Swift 3 mangled name}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
     required init(coder: NSCoder) { }
     func encode(coder: NSCoder) { }
   }
 
   class NestedB : NSObject {
-    // expected-note@-1{{add the '@NSKeyedArchiveLegacy' attribute to specify the class name used for archiving}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
+    // expected-note@-1{{for new classes, add '@objc' to specify a unique, prefixed Objective-C runtime name}}{{3-3=@objc(<#Objective-C class name#>)}}
+    // expected-note@-2{{for compatibility with existing archives, use '@NSKeyedArchiveLegacy' to record the Swift 3 mangled name}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
     required init(coder: NSCoder) { }
     func encode(coder: NSCoder) { }
   }
@@ -62,22 +64,25 @@ extension CodingB {
 
 // Fileprivate classes.
 fileprivate class CodingC : NSObject, NSCoding {    // expected-error{{fileprivate class 'CodingC' has an unstable name when archiving via 'NSCoding'}}
-  // expected-note@-1{{add the '@NSKeyedArchiveLegacy' attribute to specify the class name used for archiving}}{{1-1=@NSKeyedArchiveLegacy("<#class archival name#>")}}
+  // expected-note@-1{{for new classes, add '@objc' to specify a unique, prefixed Objective-C runtime name}}{{1-1=@objc(<#Objective-C class name#>)}}
+  // expected-note@-2{{for compatibility with existing archives, use '@NSKeyedArchiveLegacy' to record the Swift 3 mangled name}}{{1-1=@NSKeyedArchiveLegacy("<#class archival name#>")}}
   required init(coder: NSCoder) { }
   func encode(coder: NSCoder) { }
 }
 
 // Private classes
 private class CodingD : NSObject, NSCoding {       // expected-error{{private class 'CodingD' has an unstable name when archiving via 'NSCoding'}}
-  // expected-note@-1{{add the '@NSKeyedArchiveLegacy' attribute to specify the class name used for archiving}}{{1-1=@NSKeyedArchiveLegacy("<#class archival name#>")}}
+  // expected-note@-1{{for new classes, add '@objc' to specify a unique, prefixed Objective-C runtime name}}{{1-1=@objc(<#Objective-C class name#>)}}
+  // expected-note@-2{{for compatibility with existing archives, use '@NSKeyedArchiveLegacy' to record the Swift 3 mangled name}}{{1-1=@NSKeyedArchiveLegacy("<#class archival name#>")}}
   required init(coder: NSCoder) { }
   func encode(coder: NSCoder) { }
 }
 
 // Local classes.
 func someFunction() {
   class LocalCoding : NSObject, NSCoding {       // expected-error{{local class 'LocalCoding' has an unstable name when archiving via 'NSCoding'}}
-  // expected-note@-1{{add the '@NSKeyedArchiveLegacy' attribute to specify the class name used for archiving}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
+  // expected-note@-1{{for new classes, add '@objc' to specify a unique, prefixed Objective-C runtime name}}{{3-3=@objc(<#Objective-C class name#>)}}
+  // expected-note@-2{{for compatibility with existing archives, use '@NSKeyedArchiveLegacy' to record the Swift 3 mangled name}}{{3-3=@NSKeyedArchiveLegacy("<#class archival name#>")}}
   required init(coder: NSCoder) { }
   func encode(coder: NSCoder) { }
 }
@@ -114,11 +119,11 @@ private class CodingG : NSObject, NSCoding {
 @NSKeyedArchiveLegacy("TheCodingG") // expected-error{{@NSKeyedArchiveLegacy may only be used on 'class' declarations}}
 struct Foo { }
 
-@NSKeyedArchiveLegacy("TheCodingG") // expected-error{{@NSKeyedArchiveLegacy attribute cannot be applied to generic class 'Bar<T>'}}
+@NSKeyedArchiveLegacy("TheCodingG") // expected-error{{'@NSKeyedArchiveLegacy' cannot be applied to generic class 'Bar<T>'}}
 class Bar<T> : NSObject { }
 
 extension CodingB {
-  @NSKeyedArchiveLegacy("GenericViaParent") // expected-error{{@NSKeyedArchiveLegacy attribute cannot be applied to generic class 'CodingB<T>.GenericViaParent'}}
+  @NSKeyedArchiveLegacy("GenericViaParent") // expected-error{{'@NSKeyedArchiveLegacy' cannot be applied to generic class 'CodingB<T>.GenericViaParent'}}
   class GenericViaParent : NSObject { }
 }