[interop][swiftToCxx] emit empty skeleton C++ classes for Swift struc…

lib/PrintAsClang/CMakeLists.txt

@@ -5,7 +5,8 @@ add_swift_host_library(swiftPrintAsClang STATIC
   ModuleContentsWriter.cpp
   PrimitiveTypeMapping.cpp
   PrintAsClang.cpp
-  PrintClangFunction.cpp)
+  PrintClangFunction.cpp
+  PrintClangValueType.cpp)
 target_link_libraries(swiftPrintAsClang PRIVATE
   swiftAST
   swiftClangImporter

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -14,6 +14,7 @@
 #include "ClangSyntaxPrinter.h"
 #include "PrimitiveTypeMapping.h"
 #include "PrintClangFunction.h"
+#include "PrintClangValueType.h"
 
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTMangler.h"
@@ -327,6 +328,14 @@ class DeclAndTypePrinter::Implementation
     os << "@end\n";
   }
 
+  void visitStructDecl(StructDecl *SD) {
+    if (outputLang != OutputLanguageMode::Cxx)
+      return;
+    // FIXME: Print struct's availability.
+    ClangValueTypePrinter printer(os);
+    printer.printStructDecl(SD);
+  }
+
   void visitExtensionDecl(ExtensionDecl *ED) {
     if (isEmptyExtensionDecl(ED))
       return;

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -409,6 +409,13 @@ class ModuleWriter {
     return true;
   }
 
+  bool writeStruct(const StructDecl *SD) {
+    if (addImport(SD))
+      return true;
+    printer.print(SD);
+    return true;
+  }
+
   bool writeProtocol(const ProtocolDecl *PD) {
     if (addImport(PD))
       return true;
@@ -584,6 +591,8 @@ class ModuleWriter {
       if (outputLangMode == OutputLanguageMode::Cxx) {
         if (auto FD = dyn_cast<FuncDecl>(D))
           success = writeFunc(FD);
+        if (auto SD = dyn_cast<StructDecl>(D))
+          success = writeStruct(SD);
         // FIXME: Warn on unsupported exported decl.
       } else if (isa<ValueDecl>(D)) {
         if (auto CD = dyn_cast<ClassDecl>(D))

lib/PrintAsClang/PrintClangValueType.cpp

@@ -0,0 +1,33 @@
+//===--- PrintClangValueType.cpp - Printer for C/C++ value types *- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "PrintClangValueType.h"
+#include "ClangSyntaxPrinter.h"
+#include "DeclAndTypePrinter.h"
+#include "OutputLanguageMode.h"
+#include "PrimitiveTypeMapping.h"
+#include "swift/AST/Decl.h"
+#include "swift/AST/ParameterList.h"
+#include "swift/AST/Type.h"
+#include "swift/AST/TypeVisitor.h"
+#include "swift/ClangImporter/ClangImporter.h"
+#include "llvm/ADT/STLExtras.h"
+
+using namespace swift;
+
+void ClangValueTypePrinter::printStructDecl(const StructDecl *SD) {
+  os << "class ";
+  ClangSyntaxPrinter(os).printIdentifier(SD->getName().str());
+  os << " final {\n";
+  // FIXME: Print the members of the struct.
+  os << "};\n";
+}

lib/PrintAsClang/PrintClangValueType.h

@@ -0,0 +1,39 @@
+//===--- PrintClangValueType.h - Printer for C/C++ value types --*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_PRINTASCLANG_PRINTCLANGVALUETYPE_H
+#define SWIFT_PRINTASCLANG_PRINTCLANGVALUETYPE_H
+
+#include "swift/Basic/LLVM.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace swift {
+
+class StructDecl;
+
+/// Responsible for printing a Swift struct or enum decl or in C or C++ mode, to
+/// be included in a Swift module's generated clang header.
+class ClangValueTypePrinter {
+public:
+  ClangValueTypePrinter(raw_ostream &os) : os(os) {}
+
+  /// Print the C struct thunk or the C++ class definition that
+  /// corresponds to the given structure declaration.
+  void printStructDecl(const StructDecl *SD);
+
+private:
+  raw_ostream &os;
+};
+
+} // end namespace swift
+
+#endif

test/Interop/SwiftToCxx/structs/swift-struct-in-cxx.swift

@@ -0,0 +1,20 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -typecheck -module-name Structs -clang-header-expose-public-decls -emit-clang-header-path %t/structs.h
+// RUN: %FileCheck %s < %t/structs.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/structs.h)
+
+// CHECK: namespace Structs {
+
+// CHECK:      class StructWithIntField final {
+// CHECK-NEXT: };
+struct StructWithIntField {
+  let field: Int
+}
+
+// Special name gets renamed in C++.
+// CHECK: class register_ final {
+struct register {
+}
+
+// CHECK: } // namespace Structs