ModuleContentsWriter.cpp

//===--- ModuleContentsWriter.cpp - Walk module decls to print ObjC/C++ ---===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2019 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 "ModuleContentsWriter.h"

#include "ClangSyntaxPrinter.h"
#include "DeclAndTypePrinter.h"
#include "OutputLanguageMode.h"
#include "PrimitiveTypeMapping.h"
#include "PrintClangValueType.h"
#include "PrintSwiftToClangCoreScaffold.h"
#include "SwiftToClangInteropContext.h"

#include "swift/AST/DiagnosticsSema.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/Module.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/SwiftNameTranslation.h"
#include "swift/AST/TypeDeclFinder.h"
#include "swift/Basic/SourceManager.h"
#include "swift/ClangImporter/ClangImporter.h"
#include "swift/Strings.h"

#include "clang/AST/Decl.h"
#include "clang/Basic/Module.h"

#include "llvm/Support/raw_ostream.h"

using namespace swift;
using namespace swift::objc_translation;

using DelayedMemberSet = DeclAndTypePrinter::DelayedMemberSet;

/// Returns true if \p decl represents an <os/object.h> type.
static bool isOSObjectType(const clang::Decl *decl) {
  auto *named = dyn_cast_or_null<clang::NamedDecl>(decl);
  if (!named)
    return false;
  return !DeclAndTypePrinter::maybeGetOSObjectBaseName(named).empty();
}

namespace {
class ReferencedTypeFinder : public TypeDeclFinder {
  friend TypeDeclFinder;

  llvm::function_ref<void(ReferencedTypeFinder &, const TypeDecl *)> Callback;
  bool NeedsDefinition = false;

  explicit ReferencedTypeFinder(decltype(Callback) callback)
    : Callback(callback) {}

  Action visitNominalType(NominalType *nominal) override {
    Callback(*this, nominal->getDecl());
    return Action::SkipNode;
  }

  Action visitTypeAliasType(TypeAliasType *aliasTy) override {
    if (aliasTy->getDecl()->hasClangNode() &&
        !aliasTy->getDecl()->isCompatibilityAlias()) {
      Callback(*this, aliasTy->getDecl());
    } else {
      Type(aliasTy->getSinglyDesugaredType()).walk(*this);
    }
    return Action::SkipNode;
  }

  /// Returns true if \p paramTy has any constraints other than being
  /// class-bound ("conforms to" AnyObject).
  static bool isConstrained(GenericSignature sig,
                            GenericTypeParamType *paramTy) {
    auto existentialTy = sig->getExistentialType(paramTy);
    return !(existentialTy->isAny() || existentialTy->isAnyObject());
  }

  Action visitBoundGenericType(BoundGenericType *boundGeneric) override {
    auto *decl = boundGeneric->getDecl();

    NeedsDefinition = true;
    Callback(*this, decl);
    NeedsDefinition = false;

    bool isObjCGeneric = decl->hasClangNode();
    auto sig = decl->getGenericSignature();

    for_each(boundGeneric->getGenericArgs(),
             sig.getInnermostGenericParams(),
             [&](Type argTy, GenericTypeParamType *paramTy) {
      // FIXME: I think there's a bug here with recursive generic types.
      if (isObjCGeneric && isConstrained(sig, paramTy))
        NeedsDefinition = true;
      argTy.walk(*this);
      NeedsDefinition = false;
    });
    return Action::SkipNode;
  }

public:
  bool needsDefinition() const {
    return NeedsDefinition;
  }

  static void walk(Type ty, decltype(Callback) callback) {
    ty.walk(ReferencedTypeFinder(callback));
  }
};

class ModuleWriter {
  enum class EmissionState {
    NotYetDefined = 0,
    DefinitionRequested,
    Defined
  };

  raw_ostream &os;
  SmallPtrSetImpl<ImportModuleTy> &imports;
  ModuleDecl &M;

  llvm::DenseMap<const TypeDecl *, std::pair<EmissionState, bool>> seenTypes;
  llvm::DenseSet<const clang::Type *> seenClangTypes;
  std::vector<const Decl *> declsToWrite;
  DelayedMemberSet objcDelayedMembers;
  CxxDeclEmissionScope topLevelEmissionScope;
  PrimitiveTypeMapping typeMapping;
  std::string outOfLineDefinitions;
  llvm::raw_string_ostream outOfLineDefinitionsOS;
  DeclAndTypePrinter printer;
  OutputLanguageMode outputLangMode;
  bool dependsOnStdlib = false;

public:
  ModuleWriter(raw_ostream &os, raw_ostream &prologueOS,
               llvm::SmallPtrSetImpl<ImportModuleTy> &imports, ModuleDecl &mod,
               SwiftToClangInteropContext &interopContext, AccessLevel access,
               bool requiresExposedAttribute, llvm::StringSet<> &exposedModules,
               OutputLanguageMode outputLang)
      : os(os), imports(imports), M(mod),
        outOfLineDefinitionsOS(outOfLineDefinitions),
        printer(M, os, prologueOS, outOfLineDefinitionsOS, objcDelayedMembers,
                topLevelEmissionScope, typeMapping, interopContext, access,
                requiresExposedAttribute, exposedModules, outputLang),
        outputLangMode(outputLang) {}

  PrimitiveTypeMapping &getTypeMapping() { return typeMapping; }

  /// Returns true if a Stdlib dependency was seen during the emission of this module.
  bool isStdlibRequired() const {
    return dependsOnStdlib;
  }

  /// Returns true if we added the decl's module to the import set, false if
  /// the decl is a local decl.
  ///
  /// The standard library is special-cased: we assume that any types from it
  /// will be handled explicitly rather than needing an explicit @import.
  bool addImport(const Decl *D) {
    ModuleDecl *otherModule = D->getModuleContext();

    if (otherModule == &M)
      return false;
    if (otherModule->isStdlibModule()) {
      dependsOnStdlib = true;
      return true;
    } else if (otherModule->isBuiltinModule())
      return true;
    // Don't need a module for SIMD types in C.
    if (otherModule->getName() == M.getASTContext().Id_simd)
      return true;

    // If there's a Clang node, see if it comes from an explicit submodule.
    // Import that instead, looking through any implicit submodules.
    if (auto clangNode = D->getClangNode()) {
      auto importer =
        static_cast<ClangImporter *>(M.getASTContext().getClangModuleLoader());
      if (const auto *clangModule = importer->getClangOwningModule(clangNode)) {
        while (clangModule && !clangModule->IsExplicit)
          clangModule = clangModule->Parent;
        if (clangModule) {
          imports.insert(clangModule);
          return true;
        }
      }
    }

    if (outputLangMode == OutputLanguageMode::Cxx) {
      // Do not expose compiler private '_ObjC' module.
      if (otherModule->getName().str() == CLANG_HEADER_MODULE_NAME)
        return true;
      // Add C++ module imports in C++ mode explicitly, to ensure that their
      // import is always emitted in the header.
      if (D->hasClangNode()) {
        if (auto *clangMod = otherModule->findUnderlyingClangModule())
          imports.insert(clangMod);
      }
    }

    imports.insert(otherModule);
    return true;
  }

  bool hasBeenRequested(const TypeDecl *D) const {
    return seenTypes.lookup(D).first >= EmissionState::DefinitionRequested;
  }

  bool tryRequire(const TypeDecl *D) {
    if (addImport(D)) {
      seenTypes[D] = { EmissionState::Defined, true };
      return true;
    }
    auto &state = seenTypes[D];
    return state.first == EmissionState::Defined;
  }

  bool require(const TypeDecl *D) {
    if (addImport(D)) {
      seenTypes[D] = { EmissionState::Defined, true };
      return true;
    }

    auto &state = seenTypes[D];
    switch (state.first) {
    case EmissionState::NotYetDefined:
    case EmissionState::DefinitionRequested:
      state.first = EmissionState::DefinitionRequested;
      declsToWrite.push_back(D);
      return false;
    case EmissionState::Defined:
      return true;
    }

    llvm_unreachable("Unhandled EmissionState in switch.");
  }

  void forwardDeclare(const NominalTypeDecl *NTD,
                      llvm::function_ref<void(void)> Printer) {
    if (NTD->getModuleContext()->isStdlibModule()) {
      if (outputLangMode != OutputLanguageMode::Cxx ||
          !printer.shouldInclude(NTD))
        return;
    }
    auto &state = seenTypes[NTD];
    if (state.second)
      return;
    Printer();
    state.second = true;
  }

  bool forwardDeclare(const ClassDecl *CD) {
    if (!CD->isObjC() ||
        CD->getForeignClassKind() == ClassDecl::ForeignKind::CFType ||
        isOSObjectType(CD->getClangDecl())) {
      return false;
    }
    forwardDeclare(CD, [&]{ os << "@class " << getNameForObjC(CD) << ";\n"; });
    return true;
  }

  void forwardDeclare(const ProtocolDecl *PD) {
    assert(PD->isObjC() ||
           *PD->getKnownProtocolKind() == KnownProtocolKind::Error);
    forwardDeclare(PD, [&]{
      os << "@protocol " << getNameForObjC(PD) << ";\n";
    });
  }

  void forwardDeclare(const EnumDecl *ED) {
    assert(ED->isObjC() || ED->hasClangNode());
    
    forwardDeclare(ED, [&]{
      os << "enum " << getNameForObjC(ED) << " : ";
      printer.print(ED->getRawType());
      os << ";\n";
    });
  }

  void emitReferencedClangTypeMetadata(const TypeDecl *typeDecl) {
    if (!isa<clang::TypeDecl>(typeDecl->getClangDecl()))
      return;
    // Get the underlying clang type from a type alias decl or record decl.
    auto clangType =
        clang::QualType(
            cast<clang::TypeDecl>(typeDecl->getClangDecl())->getTypeForDecl(),
            0)
            .getCanonicalType();
    if (!isa<clang::RecordType>(clangType.getTypePtr()))
      return;
    auto it = seenClangTypes.insert(clangType.getTypePtr());
    if (it.second)
      ClangValueTypePrinter::printClangTypeSwiftGenericTraits(os, typeDecl, &M,
                                                              printer);
  }

  void forwardDeclareCxxValueTypeIfNeeded(const NominalTypeDecl *NTD) {
    forwardDeclare(NTD, [&]() {
      ClangValueTypePrinter::forwardDeclType(os, NTD, printer);
    });
  }

  void forwardDeclareType(const TypeDecl *TD) {
    if (outputLangMode == OutputLanguageMode::Cxx) {
      if (isa<StructDecl>(TD) || isa<EnumDecl>(TD)) {
        auto *NTD = cast<NominalTypeDecl>(TD);
        if (!addImport(NTD))
          forwardDeclareCxxValueTypeIfNeeded(NTD);
        else if (isa<StructDecl>(TD) && NTD->hasClangNode())
          emitReferencedClangTypeMetadata(NTD);
      } else if (auto TAD = dyn_cast<TypeAliasDecl>(TD)) {
        if (TAD->hasClangNode())
          emitReferencedClangTypeMetadata(TAD);
      }
      return;
    }
    if (auto CD = dyn_cast<ClassDecl>(TD)) {
      if (!forwardDeclare(CD)) {
        (void)addImport(CD);
      }
    } else if (auto PD = dyn_cast<ProtocolDecl>(TD)) {
      if (!PD->isMarkerProtocol())
        forwardDeclare(PD);
    } else if (auto TAD = dyn_cast<TypeAliasDecl>(TD)) {
      bool imported = false;
      if (TAD->hasClangNode())
        imported = addImport(TD);
      assert((imported || !TAD->isGeneric()) &&
             "referencing non-imported generic typealias?");
    } else if (addImport(TD)) {
      return;
    } else if (auto ED = dyn_cast<EnumDecl>(TD)) {
      forwardDeclare(ED);
    } else if (isa<GenericTypeParamDecl>(TD)) {
      llvm_unreachable("should not see generic parameters here");
    } else if (isa<AssociatedTypeDecl>(TD)) {
      llvm_unreachable("should not see associated types here");
    } else if (isa<StructDecl>(TD) &&
               TD->getModuleContext()->isStdlibModule()) {
      // stdlib has some @_cdecl functions with structs.
      return;
    } else {
      assert(false && "unknown local type decl");
    }
  }

  bool forwardDeclareMemberTypes(DeclRange members, const Decl *container) {
    PrettyStackTraceDecl
        entry("printing forward declarations needed by members of", container);
    switch (container->getKind()) {
    case DeclKind::Class:
    case DeclKind::Protocol:
    case DeclKind::Extension:
      break;
    case DeclKind::Struct:
    case DeclKind::Enum:
      if (outputLangMode == OutputLanguageMode::Cxx)
        break;
      LLVM_FALLTHROUGH;
    default:
      llvm_unreachable("unexpected container kind");
    }

    bool hadAnyDelayedMembers = false;
    SmallVector<ValueDecl *, 4> nestedTypes;
    for (auto member : members) {
      PrettyStackTraceDecl loopEntry("printing for member", member);
      auto VD = dyn_cast<ValueDecl>(member);
      if (!VD || !printer.shouldInclude(VD))
        continue;

      // Catch nested types and emit their definitions /after/ this class.
      if (isa<TypeDecl>(VD)) {
        // Don't emit nested types that are just implicitly @objc.
        // You should have to opt into this, since they are even less
        // namespaced than usual.
        if (std::any_of(VD->getAttrs().begin(), VD->getAttrs().end(),
                        [](const DeclAttribute *attr) {
                          return isa<ObjCAttr>(attr) && !attr->isImplicit();
                        })) {
          nestedTypes.push_back(VD);
        }
        continue;
      }

      bool needsToBeIndividuallyDelayed = false;
      ReferencedTypeFinder::walk(VD->getInterfaceType(),
                                 [&](ReferencedTypeFinder &finder,
                                     const TypeDecl *TD) {
        PrettyStackTraceDecl
            entry("walking its interface type, currently at", TD);
        if (TD == container)
          return;

        // Bridge, if necessary.
        if (outputLangMode != OutputLanguageMode::Cxx)
          TD = printer.getObjCTypeDecl(TD);

        if (finder.needsDefinition() && isa<NominalTypeDecl>(TD)) {
          // We can delay individual members of classes; do so if necessary.
          if (isa<ClassDecl>(container)) {
            if (!tryRequire(TD)) {
              needsToBeIndividuallyDelayed = true;
              hadAnyDelayedMembers = true;
            }
            return;
          }

          // Extensions can always be delayed wholesale.
          if (isa<ExtensionDecl>(container)) {
            if (!require(TD))
              hadAnyDelayedMembers = true;
            return;
          }

          // Protocols should be delayed wholesale unless we might have a cycle.
          if (auto *proto = dyn_cast<ProtocolDecl>(container)) {
            if (!hasBeenRequested(proto) || !hasBeenRequested(TD)) {
              if (!require(TD))
                hadAnyDelayedMembers = true;
              return;
            }
          }

          // Otherwise, we have a cyclic dependency. Give up and continue with
          // regular forward-declarations even though this will lead to an
          // error; there's nothing we can do here.
          // FIXME: It would be nice to diagnose this.
        }

        forwardDeclareType(TD);
      });

      if (needsToBeIndividuallyDelayed) {
        assert(isa<ClassDecl>(container));
        objcDelayedMembers.insert(VD);
      }
    }

    declsToWrite.insert(declsToWrite.end()-1, nestedTypes.rbegin(),
                        nestedTypes.rend());

    // Separate forward declarations from the class itself.
    return !hadAnyDelayedMembers;
  }

  bool writeClass(const ClassDecl *CD) {
    if (addImport(CD))
      return true;

    if (seenTypes[CD].first == EmissionState::Defined)
      return true;

    bool allRequirementsSatisfied = true;

    const ClassDecl *superclass = nullptr;
    if ((superclass = CD->getSuperclassDecl())) {
      allRequirementsSatisfied &= require(superclass);
    }
    if (outputLangMode != OutputLanguageMode::Cxx) {
      for (auto proto :
           CD->getLocalProtocols(ConformanceLookupKind::OnlyExplicit))
        if (printer.shouldInclude(proto))
          allRequirementsSatisfied &= require(proto);
    }
    if (!allRequirementsSatisfied)
      return false;

    (void)forwardDeclareMemberTypes(CD->getMembers(), CD);
    seenTypes[CD] = { EmissionState::Defined, true };
    os << '\n';
    printer.print(CD);
    return true;
  }

  bool writeFunc(const FuncDecl *FD) {
    if (addImport(FD))
      return true;

    PrettyStackTraceDecl entry(
        "printing forward declarations needed by function", FD);
    ReferencedTypeFinder::walk(
        FD->getInterfaceType(),
        [&](ReferencedTypeFinder &finder, const TypeDecl *TD) {
          PrettyStackTraceDecl entry("walking its interface type, currently at",
                                     TD);
          forwardDeclareType(TD);
        });

    os << '\n';
    printer.print(FD);
    return true;
  }

  bool writeStruct(const StructDecl *SD) {
    if (addImport(SD))
      return true;
    if (outputLangMode == OutputLanguageMode::Cxx) {
      (void)forwardDeclareMemberTypes(SD->getMembers(), SD);
      for (const auto *ed :
           printer.getInteropContext().getExtensionsForNominalType(SD)) {
        (void)forwardDeclareMemberTypes(ed->getMembers(), SD);
      }
      forwardDeclareCxxValueTypeIfNeeded(SD);
    }
    printer.print(SD);
    return true;
  }

  bool writeProtocol(const ProtocolDecl *PD) {
    if (addImport(PD))
      return true;

    if (seenTypes[PD].first == EmissionState::Defined)
      return true;

    bool allRequirementsSatisfied = true;

    for (auto proto : PD->getInheritedProtocols()) {
      if (printer.shouldInclude(proto)) {
        assert(proto->isObjC());
        allRequirementsSatisfied &= require(proto);
      }
    }

    if (!allRequirementsSatisfied)
      return false;

    if (!forwardDeclareMemberTypes(PD->getMembers(), PD))
      return false;

    seenTypes[PD] = { EmissionState::Defined, true };
    os << '\n';
    printer.print(PD);
    return true;
  }

  bool writeExtension(const ExtensionDecl *ED) {
    if (printer.isEmptyExtensionDecl(ED))
      return true;

    bool allRequirementsSatisfied = true;

    const ClassDecl *CD = ED->getSelfClassDecl();
    allRequirementsSatisfied &= require(CD);
    for (auto proto : ED->getLocalProtocols())
      if (printer.shouldInclude(proto))
        allRequirementsSatisfied &= require(proto);

    if (!allRequirementsSatisfied)
      return false;

    // This isn't rolled up into the previous set of requirements because
    // it /also/ prints forward declarations, and the header is a little
    // prettier if those are as close as possible to the necessary extension.
    if (!forwardDeclareMemberTypes(ED->getMembers(), ED))
      return false;

    os << '\n';
    printer.print(ED);
    return true;
  }
  
  bool writeEnum(const EnumDecl *ED) {
    if (addImport(ED))
      return true;

    if (outputLangMode == OutputLanguageMode::Cxx) {
      forwardDeclareMemberTypes(ED->getMembers(), ED);
      forwardDeclareCxxValueTypeIfNeeded(ED);
    }

    if (seenTypes[ED].first == EmissionState::Defined)
      return true;
    
    seenTypes[ED] = {EmissionState::Defined, true};
    printer.print(ED);

    ASTContext &ctx = M.getASTContext();

    SmallVector<ProtocolConformance *, 1> conformances;
    auto errorTypeProto = ctx.getProtocol(KnownProtocolKind::Error);
    if (outputLangMode != OutputLanguageMode::Cxx
        && ED->lookupConformance(errorTypeProto, conformances)) {
      bool hasDomainCase = std::any_of(ED->getAllElements().begin(),
                                       ED->getAllElements().end(),
                                       [](const EnumElementDecl *elem) {
        return elem->getBaseIdentifier().str() == "Domain";
      });
      if (!hasDomainCase) {
        os << "static NSString * _Nonnull const " << getNameForObjC(ED)
           << "Domain = @\"" << getErrorDomainStringForObjC(ED) << "\";\n";
      }
    }

    return true;
  }

  void write() {
    SmallVector<Decl *, 64> decls;
    M.getTopLevelDecls(decls);
    llvm::DenseSet<const ValueDecl *> removedValueDecls;

    auto newEnd =
        std::remove_if(decls.begin(), decls.end(),
                       [this, &removedValueDecls](const Decl *D) -> bool {
                         if (auto VD = dyn_cast<ValueDecl>(D)) {
                           auto shouldRemove = !printer.shouldInclude(VD);
                           if (shouldRemove)
                             removedValueDecls.insert(VD);
                           return shouldRemove;
                         }

                         if (auto ED = dyn_cast<ExtensionDecl>(D)) {
                           if (outputLangMode == OutputLanguageMode::Cxx)
                             return false;
                           auto baseClass = ED->getSelfClassDecl();
                           return !baseClass ||
                                  !printer.shouldInclude(baseClass) ||
                                  baseClass->isForeign();
                         }
                         return true;
                       });
    decls.erase(newEnd, decls.end());

    if (M.isStdlibModule()) {
      llvm::SmallVector<Decl *, 2> nestedAdds;
      for (const auto *d : decls) {
        auto *ext = dyn_cast<ExtensionDecl>(d);
        if (!ext ||
            ext->getExtendedNominal() != M.getASTContext().getStringDecl())
          continue;
        for (auto *m : ext->getMembers()) {
          if (auto *sd = dyn_cast<StructDecl>(m)) {
            if (sd->getBaseIdentifier().str() == "UTF8View" ||
                sd->getBaseIdentifier().str() == "Index") {
              nestedAdds.push_back(sd);
            }
          }
        }
      }
      decls.append(nestedAdds);
    }

    // REVERSE sort the decls, since we are going to copy them onto a stack.
    llvm::array_pod_sort(decls.begin(), decls.end(),
                         [](Decl * const *lhs, Decl * const *rhs) -> int {
      enum : int {
        Ascending = -1,
        Equivalent = 0,
        Descending = 1,
      };

      assert(*lhs != *rhs && "duplicate top-level decl");

      auto getSortName = [](const Decl *D) -> StringRef {
        if (auto VD = dyn_cast<ValueDecl>(D))
          return VD->getBaseName().userFacingName();

        if (auto ED = dyn_cast<ExtensionDecl>(D)) {
          auto baseClass = ED->getSelfClassDecl();
          if (!baseClass)
              return ED->getExtendedNominal()->getName().str();
          return baseClass->getName().str();
        }
        llvm_unreachable("unknown top-level ObjC decl");
      };

      // Sort by names.
      int result = getSortName(*rhs).compare(getSortName(*lhs));
      if (result != 0)
        return result;
      // Two overloaded functions can have the same name when emitting C++.
      if (isa<AbstractFunctionDecl>(*rhs) && isa<AbstractFunctionDecl>(*lhs)) {
        // Sort top level functions with the same C++ name by their location to
        // have stable sorting that depends on users source but not on the
        // compiler invocation.
        if ((*rhs)->getLoc().isValid() && (*lhs)->getLoc().isValid()) {
          std::string rhsLoc, lhsLoc;
          auto getLocText = [](const AbstractFunctionDecl *afd) {
            std::string res;
            llvm::raw_string_ostream os(res);
            afd->getLoc().print(os, afd->getASTContext().SourceMgr);
            return std::move(os.str());
          };
          if (getLocText(cast<AbstractFunctionDecl>(*lhs)) <
              getLocText(cast<AbstractFunctionDecl>(*rhs)))
            return Descending;
          return Ascending;
        }
        return result;
      }

      // A function and a global variable can have the same name in C++,
      // even when the variable might not actually be emitted by the emitter.
      // In that case, order the function before the variable.
      if (isa<AbstractFunctionDecl>(*rhs) && isa<VarDecl>(*lhs))
        return 1;
      if (isa<AbstractFunctionDecl>(*lhs) && isa<VarDecl>(*rhs))
        return -1;

      // Prefer value decls to extensions.
      assert(!(isa<ValueDecl>(*lhs) && isa<ValueDecl>(*rhs)));
      if (isa<ValueDecl>(*lhs) && !isa<ValueDecl>(*rhs))
        return Descending;
      if (!isa<ValueDecl>(*lhs) && isa<ValueDecl>(*rhs))
        return Ascending;

      // Break ties in extensions by putting smaller extensions last (in reverse
      // order).
      // FIXME: This will end up taking linear time.
      auto lhsMembers = cast<ExtensionDecl>(*lhs)->getMembers();
      auto rhsMembers = cast<ExtensionDecl>(*rhs)->getMembers();
      unsigned numLHSMembers = std::distance(lhsMembers.begin(),
                                             lhsMembers.end());
      unsigned numRHSMembers = std::distance(rhsMembers.begin(),
                                             rhsMembers.end());
      if (numLHSMembers != numRHSMembers)
        return numLHSMembers < numRHSMembers ? Descending : Ascending;

      // Or the extension with fewer protocols.
      auto lhsProtos = cast<ExtensionDecl>(*lhs)->getLocalProtocols();
      auto rhsProtos = cast<ExtensionDecl>(*rhs)->getLocalProtocols();
      if (lhsProtos.size() != rhsProtos.size())
        return lhsProtos.size() < rhsProtos.size() ? Descending : Ascending;

      // If that fails, arbitrarily pick the extension whose protocols are
      // alphabetically first.
      auto mismatch =
        std::mismatch(lhsProtos.begin(), lhsProtos.end(), rhsProtos.begin(),
                      [] (const ProtocolDecl *nextLHSProto,
                          const ProtocolDecl *nextRHSProto) {
        return nextLHSProto->getName() != nextRHSProto->getName();
      });
      if (mismatch.first == lhsProtos.end())
        return Equivalent;
      StringRef lhsProtoName = (*mismatch.first)->getName().str();
      return lhsProtoName.compare((*mismatch.second)->getName().str());
    });

    assert(declsToWrite.empty());
    declsToWrite.assign(decls.begin(), decls.end());

    if (outputLangMode == OutputLanguageMode::Cxx) {
      for (const Decl *D : declsToWrite) {
        if (auto *ED = dyn_cast<ExtensionDecl>(D)) {
          const auto *type = ED->getExtendedNominal();
          if (isa<StructDecl>(type) || isa<EnumDecl>(type))
            printer.getInteropContext().recordExtensions(type, ED);
        }
      }
    }

    while (!declsToWrite.empty()) {
      const Decl *D = declsToWrite.back();
      bool success = true;

      if (auto ED = dyn_cast<EnumDecl>(D)) {
        success = writeEnum(ED);
      } else if (auto CD = dyn_cast<ClassDecl>(D)) {
        success = writeClass(CD);
      } else if (outputLangMode == OutputLanguageMode::Cxx) {
        if (auto FD = dyn_cast<FuncDecl>(D))
          success = writeFunc(FD);
        else if (auto SD = dyn_cast<StructDecl>(D))
          success = writeStruct(SD);
        else if (auto *vd = dyn_cast<ValueDecl>(D))
          topLevelEmissionScope.additionalUnrepresentableDeclarations.push_back(
              vd);
      } else if (isa<ValueDecl>(D)) {
        if (auto PD = dyn_cast<ProtocolDecl>(D))
          success = writeProtocol(PD);
        else if (auto ED = dyn_cast<FuncDecl>(D))
          success = writeFunc(ED);
        else
          llvm_unreachable("unknown top-level ObjC value decl");

      } else if (auto ED = dyn_cast<ExtensionDecl>(D)) {
        success = writeExtension(ED);

      } else {
        llvm_unreachable("unknown top-level ObjC decl");
      }

      if (success) {
        assert(declsToWrite.back() == D);
        os << "\n";
        declsToWrite.pop_back();
      }
    }

    if (outputLangMode == OutputLanguageMode::ObjC)
      if (!objcDelayedMembers.empty()) {
        auto groupBegin = objcDelayedMembers.begin();
        for (auto i = groupBegin, e = objcDelayedMembers.end(); i != e; ++i) {
          if ((*i)->getDeclContext() != (*groupBegin)->getDeclContext()) {
            printer.printAdHocCategory(make_range(groupBegin, i));
            groupBegin = i;
          }
        }
        printer.printAdHocCategory(
            make_range(groupBegin, objcDelayedMembers.end()));
      }

    // Print any out of line definitions.
    os << outOfLineDefinitionsOS.str();

    // In C++ section, emit unavailable stubs for top value level
    // declarations that couldn't be represented in C++.
    if (outputLangMode != OutputLanguageMode::Cxx)
      return;
    auto &emissionScope = topLevelEmissionScope;
    auto removedVDList = std::vector<const ValueDecl *>(
        removedValueDecls.begin(), removedValueDecls.end());
    for (const auto *removedVD :
         emissionScope.additionalUnrepresentableDeclarations)
      removedVDList.push_back(removedVD);

    // Do not report internal/private decls as unavailable.
    // @objc declarations are emitted in the Objective-C section, so do not
    // report them as unavailable. Also skip underscored decls from the standard
    // library. Also skip structs from the standard library, they can cause
    // ambiguities because of the arithmetic types that conflict with types we
    // already have in `swift::` namespace. Also skip `Error` protocol from
    // stdlib, we have experimental support for it.
    removedVDList.erase(
        llvm::remove_if(
            removedVDList,
            [&](const ValueDecl *vd) {
              return !printer.isVisible(vd) || vd->isObjC() ||
                     (vd->isStdlibDecl() && !vd->getName().isSpecial() &&
                      vd->getBaseIdentifier().hasUnderscoredNaming()) ||
                     (vd->isStdlibDecl() && isa<StructDecl>(vd)) ||
                     (vd->isStdlibDecl() &&
                      vd->getASTContext().getErrorDecl() == vd);
            }),
        removedVDList.end());
    // Sort the unavaiable decls by their name and kind.
    llvm::sort(removedVDList, [](const ValueDecl *lhs, const ValueDecl *rhs) {
      auto getSortKey = [](const ValueDecl *vd) {
        std::string sortKey;
        llvm::raw_string_ostream os(sortKey);
        vd->getName().print(os);
        os << ' ' << (unsigned)vd->getDescriptiveKind();
        return std::move(os.str());
      };
      return getSortKey(lhs) < getSortKey(rhs);
    });

    for (const auto *vd : removedVDList) {
      assert(!vd->isObjC());
      os << "\n";
      auto emitStubComment = [&]() {
        // Emit a generic comment for an handled declaration.
        os << "// Unavailable in C++: Swift "
           << vd->getDescriptiveKindName(vd->getDescriptiveKind()) << " '";
        vd->getName().print(os);
        os << "'.\n";
      };

      // Do not emit a C++ declaration with a specific C++ name more than once.
      auto cxxName = cxx_translation::getNameForCxx(vd);
      if (emissionScope.emittedDeclarationNames.contains(cxxName)) {
        emitStubComment();
        continue;
      }
      emissionScope.emittedDeclarationNames.insert(cxxName);

      // Emit an unavailable stub for a Swift type.
      if (auto *nmtd = dyn_cast<NominalTypeDecl>(vd)) {
        auto representation = cxx_translation::getDeclRepresentation(vd);
        os << "class ";
        ClangSyntaxPrinter(os).printBaseName(vd);
        os << " { } SWIFT_UNAVAILABLE_MSG(\"";

        auto diag =
            representation.isUnsupported() && representation.error.has_value()
                ? cxx_translation::diagnoseRepresenationError(
                      *representation.error, const_cast<ValueDecl *>(vd))
                : Diagnostic(
                      vd->isStdlibDecl() ? diag::unexposed_other_decl_in_cxx
                                         : diag::unsupported_other_decl_in_cxx,
                      const_cast<ValueDecl *>(vd));
        // Emit a specific unavailable message when we know why a decl can't be
        // exposed, or a generic message otherwise.
        auto diagString = M.getASTContext().Diags.diagnosticStringFor(
            diag.getID(), /*PrintDiagnosticNames=*/false);
        DiagnosticEngine::formatDiagnosticText(os, diagString, diag.getArgs(),
                                               DiagnosticFormatOptions());
        os << "\");\n";
        continue;
      }

      // FIXME: Emit an unavailable stub for a function / function overload set
      // / variable.
      // FIXME: Note unrepresented type aliases too.
      emitStubComment();
    }
  }
};
} // end anonymous namespace

static AccessLevel getRequiredAccess(const ModuleDecl &M) {
  return M.isExternallyConsumed() ? AccessLevel::Public : AccessLevel::Internal;
}

void swift::printModuleContentsAsObjC(
    raw_ostream &os, llvm::SmallPtrSetImpl<ImportModuleTy> &imports,
    ModuleDecl &M, SwiftToClangInteropContext &interopContext) {
  llvm::raw_null_ostream prologueOS;
  llvm::StringSet<> exposedModules;
  ModuleWriter(os, prologueOS, imports, M, interopContext, getRequiredAccess(M),
               /*requiresExposedAttribute=*/false, exposedModules,
               OutputLanguageMode::ObjC)
      .write();
}

EmittedClangHeaderDependencyInfo swift::printModuleContentsAsCxx(
    raw_ostream &os, ModuleDecl &M, SwiftToClangInteropContext &interopContext,
    bool requiresExposedAttribute, llvm::StringSet<> &exposedModules) {
  std::string moduleContentsBuf;
  llvm::raw_string_ostream moduleOS{moduleContentsBuf};
  std::string modulePrologueBuf;
  llvm::raw_string_ostream prologueOS{modulePrologueBuf};
  EmittedClangHeaderDependencyInfo info;

  // Define the `SWIFT_SYMBOL` macro.
  os << "#ifdef SWIFT_SYMBOL\n";
  os << "#undef SWIFT_SYMBOL\n";
  os << "#endif\n";
  os << "#define SWIFT_SYMBOL(usrValue) SWIFT_SYMBOL_MODULE_USR(\"";
  ClangSyntaxPrinter(os).printBaseName(&M);
  os << "\", usrValue)\n";

  // FIXME: Use getRequiredAccess once @expose is supported.
  ModuleWriter writer(moduleOS, prologueOS, info.imports, M, interopContext,
                      AccessLevel::Public, requiresExposedAttribute,
                      exposedModules, OutputLanguageMode::Cxx);
  writer.write();
  info.dependsOnStandardLibrary = writer.isStdlibRequired();
  if (M.isStdlibModule()) {
    // Embed additional STL includes.
    os << "#ifndef SWIFT_CXX_INTEROP_HIDE_STL_OVERLAY\n";
    os << "#include <string>\n";
    os << "#endif\n";
    os << "#include <new>\n";
    // Embed an overlay for the standard library.
    ClangSyntaxPrinter(moduleOS).printIncludeForShimHeader(
        "_SwiftStdlibCxxOverlay.h");
    // Ignore typos in Swift stdlib doc comments.
    os << "#pragma clang diagnostic push\n";
    os << "#pragma clang diagnostic ignored \"-Wdocumentation\"\n";
  }

  os << "#ifndef SWIFT_PRINTED_CORE\n";
  os << "#define SWIFT_PRINTED_CORE\n";
  printSwiftToClangCoreScaffold(interopContext, M.getASTContext(),
                                writer.getTypeMapping(), os);
  os << "#endif\n";

  // FIXME: refactor.
  if (!prologueOS.str().empty()) {
    // FIXME: This is a workaround for prologue being emitted outside of
    // __cplusplus.
    if (!M.isStdlibModule())
      os << "#endif\n";
    os << "#ifdef __cplusplus\n";
    os << "namespace ";
    ClangSyntaxPrinter(os).printBaseName(&M);
    os << " SWIFT_PRIVATE_ATTR";
    ClangSyntaxPrinter(os).printSymbolUSRAttribute(&M);
    os << " {\n";
    os << "namespace " << cxx_synthesis::getCxxImplNamespaceName() << " {\n";
    os << "extern \"C\" {\n";
    os << "#endif\n\n";

    os << prologueOS.str();

    if (!M.isStdlibModule())
      os << "\n#ifdef __cplusplus\n";
    os << "}\n";
    os << "}\n";
    os << "}\n";
  }

  // Construct a C++ namespace for the module.
  ClangSyntaxPrinter(os).printNamespace(
      [&](raw_ostream &os) { ClangSyntaxPrinter(os).printBaseName(&M); },
      [&](raw_ostream &os) { os << moduleOS.str(); },
      ClangSyntaxPrinter::NamespaceTrivia::AttributeSwiftPrivate, &M);

  if (M.isStdlibModule()) {
    os << "#pragma clang diagnostic pop\n";
  }
  os << "#undef SWIFT_SYMBOL\n";
  return info;
}