[api-digester] Don’t synthesize type nodes with no public members

Upon seeing an extension for a type outside the current module, the digester
creates a dummy type node and puts all the extensions’ members and conformances
in that type. This allows us to track new API endpoints that are retroactively
added.

Unfortunately, if there are no public members/conformances (only internal or
private ones), the type itself ends up in the SDK dump without any public
children. This causes an issue when you dump the SDK from a parseable
interface, where the internal extension was not printed.

Author: Harlan Haskins

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -1,6 +1,7 @@
 Func _SmallString.computeIsASCII() has been removed
 Func _SmallString.withMutableCapacity(_:) has been removed
 Struct _StringObject.Discriminator has been removed
+Struct __swift_stdlib_UErrorCode has been removed
 Var _SmallString.discriminator has been removed
 Var _StringObject.CountAndFlags._storage has declared type change from UInt to UInt64
 Var _StringObject.CountAndFlags.countMask has declared type change from UInt to UInt64

test/api-digester/internal-extension.swift

@@ -0,0 +1,37 @@
+// RUN: %empty-directory(%t.mod)
+// RUN: %empty-directory(%t.sdk)
+// RUN: %empty-directory(%t.module-cache)
+// RUN: %swift -emit-module -o %t.mod/cake.swiftmodule %S/Inputs/cake.swift -parse-as-library -I %S/Inputs/ClangCake %clang-importer-sdk-nosource
+// RUN: %swift -emit-module -o %t.mod/main.swiftmodule %s -parse-as-library -I %t.mod -I %S/Inputs/ClangCake %clang-importer-sdk-nosource
+// RUN: %api-digester -dump-sdk -module main -o %t.dump.json -module-cache-path %t.module-cache -swift-only -I %t.mod -I %S/Inputs/ClangCake %clang-importer-sdk-nosource
+// RUN: %FileCheck %s < %t.dump.json
+
+import cake
+
+// CHECK: publicSymbol
+public func publicSymbol() {}
+
+internal protocol InternalProto {}
+public protocol PublicProto {}
+
+// This internal extension doesn't declare any new members on S1, so it
+// shouldn't show up at all in the output.
+// CHECK-NOT: S1
+internal extension S1 {
+  var x: Int { return 4 }
+}
+
+// There should only be one conformance declared: the conformance of C0 to
+// PublicProto. InternalProto should not show up in the list.
+// CHECK:      "name": "C0",
+// CHECK:      "conformances": [
+// CHECK-NEXT:   {
+// CHECK-NEXT:     "kind": "Conformance",
+// CHECK-NEXT:     "name": "PublicProto",
+// CHECK-NEXT:     "printedName": "PublicProto"
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
+extension C0: InternalProto {
+}
+extension C0: PublicProto {
+}

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -1368,11 +1368,31 @@ SwiftDeclCollector::constructExternalExtensionNode(NominalTypeDecl *NTD,
                                             ArrayRef<ExtensionDecl*> AllExts) {
   auto *TypeNode = SDKNodeInitInfo(Ctx, NTD).createSDKNode(SDKNodeKind::DeclType);
   addConformancesToTypeDecl(cast<SDKNodeDeclType>(TypeNode), NTD);
+
+  bool anyConformancesAdded = false;
   // The members of the extensions are the only members of this synthesized type.
   for (auto *Ext: AllExts) {
     HandledExtensions.insert(Ext);
     addMembersToRoot(TypeNode, Ext);
+
+    // Keep track if we've declared any conformances in this extension.
+    // FIXME: This is too conservative. We only _really_ care if this extension
+    //        declares a conformance to any public protocols outside the module
+    //        where the extended type originated. Eventually this should be
+    //        updated to filter extensions that declare conformances to internal
+    //        protocols that either don't inherit from any protocols or only
+    //        inherit from other internal protocols. It should also consider
+    //        conditional conformances with internal requirements that are still
+    //        part of the ABI.
+    if (!Ext->getInherited().empty())
+      anyConformancesAdded = true;
   }
+
+  // If none of the extensions added any public members or conformances, don't
+  // synthesize the type node.
+  if (TypeNode->getChildrenCount() == 0 && !anyConformancesAdded)
+    return nullptr;
+
   return TypeNode;
 }
 
@@ -1532,7 +1552,8 @@ void SwiftDeclCollector::lookupVisibleDecls(ArrayRef<ModuleDecl *> Modules) {
     }
   }
   for (auto Pair: ExtensionMap) {
-    RootNode->addChild(constructExternalExtensionNode(Pair.first, Pair.second));
+    if (auto child = constructExternalExtensionNode(Pair.first, Pair.second))
+      RootNode->addChild(child);
   }
 }
 

tools/swift-api-digester/ModuleAnalyzerNodes.h

@@ -648,6 +648,8 @@ class SwiftDeclCollector: public VisibleDeclConsumer {
 
   void printTopLevelNames();
 
+  /// Adds all conformances from the provided NominalTypeDecl to the provided
+  /// SDK node for that type decl.
   void addConformancesToTypeDecl(SDKNodeDeclType *Root, NominalTypeDecl* NTD);
   void addMembersToRoot(SDKNode *Root, IterableDeclContext *Context);