Introduce an internal attribute '@_show_in_interface' to be used in stdlib for underscored protocols that

should be shown in the interface.

Also switch the option and hide all underscored protocols by default, unless they are marked with the new attribute.

include/swift/AST/Attr.def

@@ -242,6 +242,9 @@ SIMPLE_DECL_ATTR(indirect, Indirect,
 SIMPLE_DECL_ATTR(warn_unqualified_access, WarnUnqualifiedAccess,
                  OnFunc /*| OnVar*/ | LongAttribute, 61)
 
+SIMPLE_DECL_ATTR(_show_in_interface, ShowInInterface,
+                 OnProtocol | UserInaccessible, 62)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef SIMPLE_DECL_ATTR

include/swift/AST/PrintOptions.h

@@ -156,6 +156,7 @@ struct PrintOptions {
   bool SkipPrivateStdlibDecls = false;
 
   /// Whether to skip underscored stdlib protocols.
+  /// Protocols marked with @_show_in_interface are still printed.
   bool SkipUnderscoredStdlibProtocols = false;
 
   /// Whether to skip extensions that don't add protocols or no members.
@@ -302,6 +303,7 @@ struct PrintOptions {
     result.SkipUnavailable = true;
     result.SkipImplicit = true;
     result.SkipPrivateStdlibDecls = true;
+    result.SkipUnderscoredStdlibProtocols = true;
     result.SkipDeinit = true;
     return result;
   }

lib/AST/Attr.cpp

@@ -279,6 +279,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options)
   case DAK_RawDocComment:
   case DAK_ObjCBridged:
   case DAK_SynthesizedProtocol:
+  case DAK_ShowInInterface:
     return false;
   default:
     break;

lib/AST/Decl.cpp

@@ -427,6 +427,8 @@ bool Decl::isPrivateStdlibDecl(bool whitelistProtocols) const {
   }
 
   if (auto PD = dyn_cast<ProtocolDecl>(D)) {
+    if (PD->getAttrs().hasAttribute<ShowInInterfaceAttr>())
+      return false;
     StringRef NameStr = PD->getNameStr();
     if (NameStr.startswith("_Builtin"))
       return true;

lib/Sema/TypeCheckAttr.cpp

@@ -77,6 +77,7 @@ public:
   IGNORED_ATTR(UIApplicationMain)
   IGNORED_ATTR(UnsafeNoObjCTaggedPointer)
   IGNORED_ATTR(WarnUnusedResult)
+  IGNORED_ATTR(ShowInInterface)
 #undef IGNORED_ATTR
 
   void visitAlignmentAttr(AlignmentAttr *attr) {
@@ -647,6 +648,7 @@ public:
     IGNORED_ATTR(Swift3Migration)
     IGNORED_ATTR(Testable)
     IGNORED_ATTR(WarnUnqualifiedAccess)
+    IGNORED_ATTR(ShowInInterface)
 #undef IGNORED_ATTR
 
   void visitAvailableAttr(AvailableAttr *attr);

lib/Sema/TypeCheckDecl.cpp

@@ -5086,6 +5086,7 @@ public:
     UNINTERESTING_ATTR(ObjCNonLazyRealization)
     UNINTERESTING_ATTR(UnsafeNoObjCTaggedPointer)
     UNINTERESTING_ATTR(SwiftNativeObjCRuntimeBase)
+    UNINTERESTING_ATTR(ShowInInterface)
 
     // These can't appear on overridable declarations.
     UNINTERESTING_ATTR(AutoClosure)

test/IDE/print_stdlib.swift

@@ -1,5 +1,6 @@
 // Check interface produced for the standard library.
 //
+// RUN: %target-swift-frontend -parse %s
 // RUN: %target-swift-ide-test -print-module -module-to-print=Swift -source-filename %s -print-interface > %t.txt
 // RUN: FileCheck -check-prefix=CHECK-ARGC %s < %t.txt
 // RUN: FileCheck %s < %t.txt
@@ -43,6 +44,9 @@
 
 // CHECK-MUTATING-ATTR: mutating func
 
+func foo(x: _NSFastEnumerationType) {} // Checks that this protocol actually exists.
+// CHECK-NOT: _NSFastEnumerationType
+
 // NO-FIXMES-NOT: FIXME
 // RUN: %target-swift-ide-test -print-module-groups -module-to-print=Swift -source-filename %s -print-interface > %t-group.txt
 // RUN: FileCheck -check-prefix=CHECK-GROUPS1 %s < %t-group.txt

test/attr/attributes.swift

@@ -226,3 +226,6 @@ class B {
 class SILStored {
   @sil_stored var x : Int = 42  // expected-error {{'sil_stored' only allowed in SIL modules}}
 }
+
+@_show_in_interface protocol _underscored {}
+@_show_in_interface class _notapplicable {} // expected-error {{may only be used on 'protocol' declarations}}

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1133,6 +1133,8 @@ std::vector<UIdent> UIDsFromDeclAttributes(const DeclAttributes &Attrs) {
     // We handle accessibility explicitly.
     case DAK_Accessibility:
     case DAK_SetterAccessibility:
+    // Ignore these.
+    case DAK_ShowInInterface:
       continue;
     default:
       break;