Add @NSKeyedArchiveSubclassesOnly to suppress NSCoding unstable name diags.

Introduce the @NSKeyedArchiveSubclassesOnly attribute, which can be
placed on a class that conforms to NSCoding to suppress the
unstable-name diagnostics by promising to only archive
subclasses---not this class directly.

Author: DougGregor

include/swift/AST/Attr.def

@@ -281,6 +281,11 @@ SIMPLE_DECL_ATTR(_staticInitializeObjCMetadata, StaticInitializeObjCMetadata,
                  OnClass | NotSerialized | LongAttribute | UserInaccessible,
                  /*Not serialized */ 69)
 
+SIMPLE_DECL_ATTR(NSKeyedArchiveSubclassesOnly,
+                 NSKeyedArchiveSubclassesOnly,
+                 OnClass | NotSerialized | LongAttribute,
+                 /*Not serialized */ 70)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef SIMPLE_DECL_ATTR

include/swift/AST/DiagnosticsSema.def

@@ -1226,6 +1226,11 @@ NOTE(unstable_mangled_name_add_objc,none,
 NOTE(unstable_mangled_name_add_nskeyedarchivelegacy,none,
      "for compatibility with existing archives, use '@NSKeyedArchiveLegacy' "
      "to record the Swift 3 mangled name", ())
+NOTE(add_nskeyedarchivesubclassesonly_attr,none,
+     "generic classes should not be archived directly; "
+     "add @NSKeyedArchiveSubclassesOnly "
+     "and only archive specific subclasses of this class", (Type))
+
 ERROR(attr_nskeyedarchivelegacy_generic,none,
       "'@NSKeyedArchiveLegacy' cannot be applied to generic class %0",
       (Type))

lib/Sema/TypeCheckAttr.cpp

@@ -106,6 +106,7 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   IGNORED_ATTR(Implements)
   IGNORED_ATTR(NSKeyedArchiveLegacy)
   IGNORED_ATTR(StaticInitializeObjCMetadata)
+  IGNORED_ATTR(NSKeyedArchiveSubclassesOnly)
 #undef IGNORED_ATTR
 
   // @noreturn has been replaced with a 'Never' return type.
@@ -782,6 +783,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     IGNORED_ATTR(ShowInInterface)
     IGNORED_ATTR(ObjCMembers)
     IGNORED_ATTR(StaticInitializeObjCMetadata)
+    IGNORED_ATTR(NSKeyedArchiveSubclassesOnly)
 #undef IGNORED_ATTR
 
   void visitAvailableAttr(AvailableAttr *attr);

lib/Sema/TypeCheckDecl.cpp

@@ -6071,6 +6071,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     UNINTERESTING_ATTR(Implements)
     UNINTERESTING_ATTR(NSKeyedArchiveLegacy)
     UNINTERESTING_ATTR(StaticInitializeObjCMetadata)
+    UNINTERESTING_ATTR(NSKeyedArchiveSubclassesOnly)
 #undef UNINTERESTING_ATTR
 
     void visitAvailableAttr(AvailableAttr *attr) {

lib/Sema/TypeCheckProtocol.cpp

@@ -5931,7 +5931,9 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
         }
 
         if (kind && !hasExplicitObjCName(classDecl) &&
-            !classDecl->getAttrs().hasAttribute<NSKeyedArchiveLegacyAttr>()) {
+            !classDecl->getAttrs().hasAttribute<NSKeyedArchiveLegacyAttr>() &&
+            !classDecl->getAttrs()
+              .hasAttribute<NSKeyedArchiveSubclassesOnlyAttr>()) {
           SourceLoc loc;
           if (auto normal = dyn_cast<NormalProtocolConformance>(conformance))
             loc = normal->getLoc();
@@ -5943,16 +5945,20 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
                    emitWarning ? diag::nscoding_unstable_mangled_name_warn
                                : diag::nscoding_unstable_mangled_name,
                    *kind, classDecl->TypeDecl::getDeclaredInterfaceType());
+          auto insertionLoc =
+            classDecl->getAttributeInsertionLoc(/*forModifier=*/false);
           if (isFixable) {
-            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#>\")");
+          } else {
+            diagnose(classDecl, diag::add_nskeyedarchivesubclassesonly_attr,
+                     classDecl->getDeclaredInterfaceType())
+              .fixItInsert(insertionLoc, "@NSKeyedArchiveSubclassesOnly");
           }
         }
 

test/IDE/complete_decl_attribute.swift

@@ -58,7 +58,7 @@ func method(){}
 @#^KEYWORD3^#
 class C {}
 
-// KEYWORD3:                  Begin completions, 9 items
+// KEYWORD3:                  Begin completions, 10 items
 // KEYWORD3-NEXT:             Keyword/None:                       available[#Class Attribute#]; name=available{{$}}
 // KEYWORD3-NEXT:             Keyword/None:                       objc[#Class Attribute#]; name=objc{{$}}
 // KEYWORD3-NEXT:             Keyword/None:                       IBDesignable[#Class Attribute#]; name=IBDesignable{{$}}
@@ -68,6 +68,7 @@ class C {}
 // KEYWORD3-NEXT:             Keyword/None:                       NSApplicationMain[#Class Attribute#]; name=NSApplicationMain{{$}}
 // KEYWORD3-NEXT:             Keyword/None:                       objc_non_lazy_realization[#Class Attribute#]; name=objc_non_lazy_realization{{$}}
 // KEYWORD3-NEXT:             Keyword/None:                       NSKeyedArchiveLegacy[#Class Attribute#]; name=NSKeyedArchiveLegacy{{$}}
+// KEYWORD3-NEXT:             Keyword/None:                       NSKeyedArchiveSubclassesOnly[#Class Attribute#]; name=NSKeyedArchiveSubclassesOnly{{$}}
 // KEYWORD3-NEXT:             End completions
 
 @#^KEYWORD4^#
@@ -87,7 +88,7 @@ struct S{}
 
 @#^KEYWORD_LAST^#
 
-// KEYWORD_LAST:                  Begin completions, 22 items
+// KEYWORD_LAST:                  Begin completions, 23 items
 // KEYWORD_LAST-NEXT:             Keyword/None:                       available[#Declaration Attribute#]; name=available{{$}}
 // KEYWORD_LAST-NEXT:             Keyword/None:                       objc[#Declaration Attribute#]; name=objc{{$}}
 // KEYWORD_LAST-NEXT:             Keyword/None:                       noreturn[#Declaration Attribute#]; name=noreturn{{$}}
@@ -110,4 +111,5 @@ struct S{}
 // KEYWORD_LAST-NEXT:             Keyword/None:                       discardableResult[#Declaration Attribute#]; name=discardableResult
 // KEYWORD_LAST-NEXT:             Keyword/None:                       GKInspectable[#Declaration Attribute#]; name=GKInspectable{{$}}
 // KEYWORD_LAST-NEXT:             Keyword/None:                       NSKeyedArchiveLegacy[#Declaration Attribute#]; name=NSKeyedArchiveLegacy{{$}}
+// KEYWORD_LAST-NEXT:             Keyword/None:                       NSKeyedArchiveSubclassesOnly[#Declaration Attribute#]; name=NSKeyedArchiveSubclassesOnly{{$}}
 // KEYWORD_LAST-NEXT:             End completions

test/Interpreter/SDK/archiving_generic_swift_class.swift

@@ -6,6 +6,7 @@
 
 import Foundation
 
+@NSKeyedArchiveSubclassesOnly
 final class Foo<T: NSCoding>: NSObject, NSCoding {
   var one, two: T
 

test/decl/protocol/conforms/nscoding.swift

@@ -51,12 +51,14 @@ extension CodingA.NestedD: NSCoding { // okay
 
 // Generic classes
 class CodingB<T> : NSObject, NSCoding {   // expected-error{{generic class 'CodingB<T>' has an unstable name when archiving via 'NSCoding'}}
+  // expected-note@-1{{generic classes should not be archived directly}}{{1-1=@NSKeyedArchiveSubclassesOnly}}
   required init(coder: NSCoder) { }
   func encode(coder: NSCoder) { }
 }
 
 extension CodingB {
   class NestedA : NSObject, NSCoding { // expected-error{{generic class 'CodingB<T>.NestedA' has an unstable name when archiving via 'NSCoding'}}
+    // expected-note@-1{{generic classes should not be archived directly}}{{3-3=@NSKeyedArchiveSubclassesOnly}}
     required init(coder: NSCoder) { }
     func encode(coder: NSCoder) { }
   }
@@ -90,6 +92,7 @@ func someFunction() {
 
 // Inherited conformances.
 class CodingE<T> : CodingB<T> {   // expected-error{{generic class 'CodingE<T>' has an unstable name when archiving via 'NSCoding'}}
+    // expected-note@-1{{generic classes should not be archived directly}}{{1-1=@NSKeyedArchiveSubclassesOnly}}
   required init(coder: NSCoder) { super.init(coder: coder) }
   override func encode(coder: NSCoder) { }
 }
@@ -103,6 +106,12 @@ extension CodingA {
   }
 }
 
+@NSKeyedArchiveSubclassesOnly
+class CodingGeneric<T> : NSObject, NSCoding {
+  required init(coder: NSCoder) { }
+  func encode(coder: NSCoder) { }
+}
+
 @NSKeyedArchiveLegacy("TheCodingF")
 fileprivate class CodingF : NSObject, NSCoding {
   required init(coder: NSCoder) { }