ModuleInterfaceSupport.cpp

//===------- ModuleInterfaceSupport.cpp - swiftinterface files ------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 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 "swift/AST/ASTContext.h"
#include "swift/AST/ASTPrinter.h"
#include "swift/AST/Decl.h"
#include "swift/AST/DiagnosticsFrontend.h"
#include "swift/AST/DiagnosticsSema.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/FileSystem.h"
#include "swift/AST/Module.h"
#include "swift/AST/ModuleNameLookup.h"
#include "swift/AST/NameLookupRequests.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/TypeCheckRequests.h"
#include "swift/AST/TypeRepr.h"
#include "swift/Basic/STLExtras.h"
#include "swift/Frontend/Frontend.h"
#include "swift/Frontend/ModuleInterfaceSupport.h"
#include "swift/Frontend/PrintingDiagnosticConsumer.h"
#include "swift/SILOptimizer/PassManager/Passes.h"
#include "swift/Serialization/SerializationOptions.h"
#include "swift/Serialization/Validation.h"
#include "clang/Basic/Module.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/StringSet.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/Regex.h"
#include "llvm/Support/StringSaver.h"

using namespace swift;

// MARK: Module interface header comments

version::Version swift::InterfaceFormatVersion({1, 0});

/// Prints to \p out a comment containing a format version number, tool version
/// string as well as any relevant command-line flags in \p Opts used to
/// construct \p M.
static void printToolVersionAndFlagsComment(raw_ostream &out,
                                            ModuleInterfaceOptions const &Opts,
                                            ModuleDecl *M,
                                            llvm::SmallSet<StringRef, 4>
                                              &AliasModuleNamesTargets) {
  auto &Ctx = M->getASTContext();
  auto ToolsVersion =
      getSwiftInterfaceCompilerVersionForCurrentCompiler(Ctx);
  out << "// " SWIFT_INTERFACE_FORMAT_VERSION_KEY ": "
      << InterfaceFormatVersion << "\n";
  out << "// " SWIFT_COMPILER_VERSION_KEY ": "
      << ToolsVersion << "\n";
  out << "// " SWIFT_MODULE_FLAGS_KEY ": "
      << Opts.Flags;

  // Insert additional -module-alias flags
  if (Opts.AliasModuleNames) {
    StringRef moduleName = M->getNameStr();
    AliasModuleNamesTargets.insert(M->getNameStr());
    out << " -module-alias " << MODULE_DISAMBIGUATING_PREFIX <<
           moduleName << "=" << moduleName;

    ModuleDecl::ImportFilter filter = {ModuleDecl::ImportFilterKind::Default,
                                       ModuleDecl::ImportFilterKind::Exported};
    if (!Opts.printPublicInterface())
      filter |= ModuleDecl::ImportFilterKind::SPIOnly;
    if (Opts.printPackageInterface())
      filter |= ModuleDecl::ImportFilterKind::PackageOnly;

    SmallVector<ImportedModule> imports;
    M->getImportedModules(imports, filter);
    M->getMissingImportedModules(imports);

    for (ImportedModule import: imports) {
      StringRef importedName = import.importedModule->getNameStr();
      // Skip Swift as it's commonly used in inlinable code,
      // and Builtin as it's imported implicitly by name.
      if (importedName == STDLIB_NAME ||
          importedName == BUILTIN_NAME)
        continue;

      if (AliasModuleNamesTargets.insert(importedName).second) {
        out << " -module-alias " << MODULE_DISAMBIGUATING_PREFIX <<
               importedName << "=" << importedName;
      }
    }
  }
  out << "\n";

  if (!Opts.IgnorableFlags.empty()) {
    out << "// " SWIFT_MODULE_FLAGS_IGNORABLE_KEY ": "
        << Opts.IgnorableFlags << "\n";
  }

  auto hasPrivateIgnorableFlags = !Opts.printPublicInterface() && !Opts.IgnorablePrivateFlags.empty();
  if (hasPrivateIgnorableFlags) {
    out << "// " SWIFT_MODULE_FLAGS_IGNORABLE_PRIVATE_KEY ": "
        << Opts.IgnorablePrivateFlags << "\n";
  }
}

std::string
swift::getSwiftInterfaceCompilerVersionForCurrentCompiler(ASTContext &ctx) {
  return swift::version::getSwiftFullVersion(
             ctx.LangOpts.EffectiveLanguageVersion);
}

llvm::Regex swift::getSwiftInterfaceFormatVersionRegex() {
  return llvm::Regex("^// " SWIFT_INTERFACE_FORMAT_VERSION_KEY
                     ": ([0-9\\.]+)$", llvm::Regex::Newline);
}

llvm::Regex swift::getSwiftInterfaceCompilerVersionRegex() {
  return llvm::Regex("^// " SWIFT_COMPILER_VERSION_KEY
                     ": (.+)$", llvm::Regex::Newline);
}

llvm::Regex swift::getSwiftInterfaceCompilerToolsVersionRegex() {
  return llvm::Regex("Swift version ([0-9\\.]+)", llvm::Regex::Newline);
}

// MARK(https://github.com/apple/swift/issues/43510): Module name shadowing warnings
//
// When swiftc emits a module interface, it qualifies most types with their
// module name. This usually makes the interface less ambiguous, but if a type
// exists with the same name as a module, then references to that module will
// incorrectly look inside the type instead. This breakage is not obvious until
// someone tries to load the module interface, and may sometimes only occur in
// clients' module interfaces.
//
// Truly fixing this will require a new module-qualification syntax which
// completely ignores shadowing. In lieu of that, we detect and warn about three
// common examples which are relatively actionable:
//
// 1. An `import` statement written into the module interface will
//    (transitively) import a type with the module interface's name.
//
// 2. The module interface declares a type with the same name as the module the
//    interface is for.
//
// 3. The module interface declares a type with the same name as a module it has
//     (transitively) imported without `@_implementationOnly`.
//
// We do not check for shadowing between imported module names and imported
// declarations; this is both much rarer and much more difficult to solve.
// We silence these warnings if you use the temporary workaround flag,
// '-module-interface-preserve-types-as-written'.

/// Emit a warning explaining that \p shadowingDecl will interfere with
/// references to types in \p shadowedModule in the module interfaces of
/// \p brokenModule and its clients.
static void
diagnoseDeclShadowsModule(ModuleInterfaceOptions const &Opts,
                          TypeDecl *shadowingDecl, ModuleDecl *shadowedModule,
                          ModuleDecl *brokenModule) {
  if (Opts.PreserveTypesAsWritten || shadowingDecl == shadowedModule)
    return;

  shadowingDecl->diagnose(
      diag::warning_module_shadowing_may_break_module_interface,
      shadowingDecl->getDescriptiveKind(),
      FullyQualified<Type>(shadowingDecl->getDeclaredInterfaceType()),
      shadowedModule, brokenModule);
}

/// Check whether importing \p importedModule will bring in any declarations
/// that will shadow \p importingModule, and diagnose them if so.
static void
diagnoseIfModuleImportsShadowingDecl(ModuleInterfaceOptions const &Opts,
                                     ModuleDecl *importedModule,
                                     ModuleDecl *importingModule) {
  using namespace namelookup;

  SmallVector<ValueDecl *, 4> decls;
  lookupInModule(importedModule, importingModule->getName(), decls,
                 NLKind::UnqualifiedLookup, ResolutionKind::TypesOnly,
                 importedModule, SourceLoc(),
                 NL_UnqualifiedDefault | NL_IncludeUsableFromInline);
  for (auto decl : decls)
    diagnoseDeclShadowsModule(Opts, cast<TypeDecl>(decl), importingModule,
                              importingModule);
}

/// Check whether \p D will shadow any modules imported by \p M, and diagnose
/// them if so.
static void diagnoseIfDeclShadowsKnownModule(ModuleInterfaceOptions const &Opts,
                                             Decl *D, ModuleDecl *M) {
  ASTContext &ctx = M->getASTContext();

  // We only care about types (and modules, which are a subclass of TypeDecl);
  // when the grammar expects a type name, it ignores non-types during lookup.
  TypeDecl *TD = dyn_cast<TypeDecl>(D);
  if (!TD)
    return;

  ModuleDecl *shadowedModule = ctx.getLoadedModule(TD->getName());
  if (!shadowedModule || M->isImportedImplementationOnly(shadowedModule))
    return;

  diagnoseDeclShadowsModule(Opts, TD, shadowedModule, M);
}

// MARK: Import statements

/// Diagnose any scoped imports in \p imports, i.e. those with a non-empty
/// access path. These are not yet supported by module interfaces, since the
/// information about the declaration kind is not preserved through the binary
/// serialization that happens as an intermediate step in non-whole-module
/// builds.
///
/// These come from declarations like `import class FooKit.MainFooController`.
static void diagnoseScopedImports(DiagnosticEngine &diags,
                                  ArrayRef<ImportedModule> imports){
  for (const ImportedModule &importPair : imports) {
    if (importPair.accessPath.empty())
      continue;
    diags.diagnose(importPair.accessPath.front().Loc,
                   diag::module_interface_scoped_import_unsupported);
  }
}

/// Prints the imported modules in \p M to \p out in the form of \c import
/// source declarations.
static void printImports(raw_ostream &out,
                         ModuleInterfaceOptions const &Opts,
                         ModuleDecl *M,
                         const llvm::SmallSet<StringRef, 4>
                           &AliasModuleNamesTargets) {
  auto &ctx = M->getASTContext();
  // FIXME: This is very similar to what's in Serializer::writeInputBlock, but
  // it's not obvious what higher-level optimization would be factored out here.
  ModuleDecl::ImportFilter allImportFilter = {
      ModuleDecl::ImportFilterKind::Exported,
      ModuleDecl::ImportFilterKind::Default};

  // With -experimental-spi-imports:
  // When printing the private or package swiftinterface file, print implementation-only
  // imports only if they are also SPI. First, list all implementation-only imports and
  // filter them later.
  llvm::SmallSet<ImportedModule, 4, ImportedModule::Order> ioiImportSet;
  if (!Opts.printPublicInterface() && Opts.ExperimentalSPIImports) {

    SmallVector<ImportedModule, 4> ioiImports, allImports;
    M->getImportedModules(ioiImports,
                          ModuleDecl::ImportFilterKind::ImplementationOnly);

    // Only consider modules imported consistently as implementation-only.
    M->getImportedModules(allImports,
                          allImportFilter);
    llvm::SmallSet<ImportedModule, 8, ImportedModule::Order> allImportSet;
    allImportSet.insert(allImports.begin(), allImports.end());

    for (auto import: ioiImports)
      if (allImportSet.count(import) == 0)
        ioiImportSet.insert(import);

    allImportFilter |= ModuleDecl::ImportFilterKind::ImplementationOnly;
  }

  /// Collect @_spiOnly imports that are not imported elsewhere publicly.
  llvm::SmallSet<ImportedModule, 4, ImportedModule::Order> spiOnlyImportSet;
  if (!Opts.printPublicInterface()) {
    SmallVector<ImportedModule, 4> spiOnlyImports, otherImports;
    M->getImportedModules(spiOnlyImports,
                          ModuleDecl::ImportFilterKind::SPIOnly);

    M->getImportedModules(otherImports,
                          allImportFilter);
    llvm::SmallSet<ImportedModule, 8, ImportedModule::Order> otherImportsSet;
    otherImportsSet.insert(otherImports.begin(), otherImports.end());

    // Rule out inconsistent imports.
    for (auto import: spiOnlyImports)
      if (otherImportsSet.count(import) == 0)
        spiOnlyImportSet.insert(import);

    allImportFilter |= ModuleDecl::ImportFilterKind::SPIOnly;
  }

  SmallVector<ImportedModule, 8> allImports;
  M->getImportedModules(allImports, allImportFilter);

  if (Opts.PrintMissingImports)
    M->getMissingImportedModules(allImports);

  ImportedModule::removeDuplicates(allImports);
  diagnoseScopedImports(ctx.Diags, allImports);

  // Collect the public imports as a subset so that we can mark them with
  // '@_exported'.
  SmallVector<ImportedModule, 8> publicImports;
  M->getImportedModules(publicImports, ModuleDecl::ImportFilterKind::Exported);
  llvm::SmallSet<ImportedModule, 8, ImportedModule::Order> publicImportSet;
  publicImportSet.insert(publicImports.begin(), publicImports.end());

  for (auto import : allImports) {
    auto importedModule = import.importedModule;
    if (importedModule->isOnoneSupportModule()) {
      continue;
    }

    // Unless '-enable-builtin-module' /
    // '-enable-experimental-feature BuiltinModule' was passed, do not print
    // 'import Builtin' in the interface. '-parse-stdlib' still implicitly
    // imports it however...
    if (importedModule->isBuiltinModule() &&
        !ctx.LangOpts.hasFeature(Feature::BuiltinModule)) {
      continue;
    }

    if (llvm::count(Opts.ModulesToSkipInPublicInterface,
                    importedModule->getName().str())) {
      continue;
    }

    llvm::SmallSetVector<Identifier, 4> spis;
    M->lookupImportedSPIGroups(importedModule, spis);

    // Only print implementation-only imports which have an SPI import.
    if (ioiImportSet.count(import)) {
      if (spis.empty())
        continue;
      out << "@_implementationOnly ";
    }

    if (publicImportSet.count(import))
      out << "@_exported ";

    if (!Opts.printPublicInterface()) {
      // An import visible in the private or package swiftinterface only.
      //
      // In the long term, we want to print this attribute for consistency and
      // to enforce exportability analysis of generated code.
      // For now, not printing the attribute allows us to have backwards
      // compatible swiftinterfaces and we can live without
      // checking the generate code for a while.
      if (spiOnlyImportSet.count(import))
        out << "@_spiOnly ";

      // List of imported SPI groups for local use.
      for (auto spiName : spis)
        out << "@_spi(" << spiName << ") ";
    }

    if (ctx.LangOpts.hasFeature(Feature::InternalImportsByDefault)) {
      out << "public ";
    }

    out << "import ";
    if (Opts.AliasModuleNames &&
        AliasModuleNamesTargets.contains(importedModule->getName().str()))
      out << MODULE_DISAMBIGUATING_PREFIX;
    importedModule->getReverseFullModuleName().printForward(out);

    // Write the access path we should be honoring but aren't.
    // (See diagnoseScopedImports above.)
    if (!import.accessPath.empty()) {
      out << "/*";
      for (const auto &accessPathElem : import.accessPath)
        out << "." << accessPathElem.Item;
      out << "*/";
    }

    out << "\n";

    diagnoseIfModuleImportsShadowingDecl(Opts, importedModule, M);
  }
}

// MARK: Dummy protocol conformances

// FIXME: Copied from ASTPrinter.cpp...
static bool isPublicOrUsableFromInline(const ValueDecl *VD) {
  AccessScope scope =
      VD->getFormalAccessScope(/*useDC*/nullptr,
                               /*treatUsableFromInlineAsPublic*/true);
  return scope.isPublic();
}

static bool isPublicOrUsableFromInline(Type ty) {
  // Note the double negative here: we're looking for any referenced decls that
  // are *not* public-or-usableFromInline.
  return !ty.findIf([](Type typePart) -> bool {
    // FIXME: If we have an internal typealias for a non-internal type, we ought
    // to be able to print it by desugaring.
    if (auto *aliasTy = dyn_cast<TypeAliasType>(typePart.getPointer()))
      return !isPublicOrUsableFromInline(aliasTy->getDecl());
    if (auto *nominal = typePart->getAnyNominal())
      return !isPublicOrUsableFromInline(nominal);
    return false;
  });
}

namespace {
/// Collects protocols that are conformed to by a particular nominal. Since
/// ASTPrinter will only print the public ones, the non-public ones get left by
/// the wayside. This is a problem when a non-public protocol inherits from a
/// public protocol; the generated module interface still needs to make that
/// dependency public.
///
/// The solution implemented here is to generate synthetic extensions that
/// declare the extra conformances. This isn't perfect (it loses the sugared
/// spelling of the protocol type, as well as the locality in the file), but it
/// does work.
class InheritedProtocolCollector {
  static const StringLiteral DummyProtocolName;

  using AvailableAttrList = TinyPtrVector<const AvailableAttr *>;
  using OriginallyDefinedInAttrList =
      TinyPtrVector<const OriginallyDefinedInAttr *>;
  using ProtocolAndAvailability =
      std::tuple<ProtocolDecl *, AvailableAttrList, bool /*isUnchecked*/,
                 OriginallyDefinedInAttrList>;

  /// Protocols that will be included by the ASTPrinter without any extra work.
  SmallVector<ProtocolDecl *, 8> IncludedProtocols;
  /// Protocols that will not be printed by the ASTPrinter, along with the
  /// availability they were declared with.
  SmallVector<ProtocolAndAvailability, 8> ExtraProtocols;
  /// Protocols that can be printed, but whose conformances are constrained with
  /// something that \e can't be printed.
  SmallVector<const ProtocolType *, 8> ConditionalConformanceProtocols;

  /// Helper to extract the `@available` attributes on a decl.
  static AvailableAttrList
  getAvailabilityAttrs(const Decl *D, std::optional<AvailableAttrList> &cache) {
    if (cache.has_value())
      return cache.value();

    cache.emplace();
    while (D) {
      for (auto *nextAttr : D->getAttrs().getAttributes<AvailableAttr>()) {
        // FIXME: This is just approximating the effects of nested availability
        // attributes for the same platform; formally they'd need to be merged.
        bool alreadyHasMoreSpecificAttrForThisPlatform =
            llvm::any_of(*cache, [nextAttr](const AvailableAttr *existingAttr) {
          return existingAttr->Platform == nextAttr->Platform;
        });
        if (alreadyHasMoreSpecificAttrForThisPlatform)
          continue;
        cache->push_back(nextAttr);
      }
      D = D->getDeclContext()->getAsDecl();
    }

    return cache.value();
  }

  static OriginallyDefinedInAttrList
  getOriginallyDefinedInAttrList(const Decl *D) {
    OriginallyDefinedInAttrList results;
    while (D) {
      for (auto *result :
           D->getAttrs().getAttributes<OriginallyDefinedInAttr>()) {
        results.push_back(result);
      }
      D = D->getDeclContext()->getAsDecl();
    }
    return results;
  }

  static bool canPrintProtocolTypeNormally(Type type, const Decl *D) {
    return isPublicOrUsableFromInline(type);
  }

  static bool isUncheckedConformance(ProtocolConformance *conformance) {
    if (auto normal = dyn_cast<NormalProtocolConformance>(conformance->getRootConformance()))
      return normal->isUnchecked();
    return false;
  }

  /// For each type in \p directlyInherited, classify the protocols it refers to
  /// as included for printing or not, and record them in the appropriate
  /// vectors.
  ///
  /// If \p skipExtra is true then avoid recording any extra protocols to
  /// print, such as synthesized conformances or conformances to non-public
  /// protocols.
  void recordProtocols(InheritedTypes directlyInherited, const Decl *D,
                       bool skipExtra = false) {
    std::optional<AvailableAttrList> availableAttrs;

    for (int i : directlyInherited.getIndices()) {
      Type inheritedTy = directlyInherited.getResolvedType(i);
      if (!inheritedTy || !inheritedTy->isExistentialType())
        continue;

      bool canPrintNormally = canPrintProtocolTypeNormally(inheritedTy, D);
      if (!canPrintNormally && skipExtra)
        continue;

      auto inherited = directlyInherited.getEntry(i);
      ExistentialLayout layout = inheritedTy->getExistentialLayout();
      for (ProtocolDecl *protoDecl : layout.getProtocols()) {
        if (canPrintNormally)
          IncludedProtocols.push_back(protoDecl);
        else
          ExtraProtocols.push_back(ProtocolAndAvailability(
              protoDecl, getAvailabilityAttrs(D, availableAttrs),
              inherited.isUnchecked(), getOriginallyDefinedInAttrList(D)));
      }
      // FIXME: This ignores layout constraints, but currently we don't support
      // any of those besides 'AnyObject'.
    }

    if (skipExtra)
      return;

    // Check for synthesized protocols, like Hashable on enums.
    if (auto *nominal = dyn_cast<NominalTypeDecl>(D)) {
      SmallVector<ProtocolConformance *, 4> localConformances =
          nominal->getLocalConformances(ConformanceLookupKind::NonInherited);

      for (auto *conf : localConformances) {
        if (conf->getSourceKind() != ConformanceEntryKind::Synthesized)
          continue;
        ExtraProtocols.push_back(ProtocolAndAvailability(
            conf->getProtocol(), getAvailabilityAttrs(D, availableAttrs),
            isUncheckedConformance(conf), getOriginallyDefinedInAttrList(D)));
      }
    }
  }

  /// For each type directly inherited by \p extension, record any protocols
  /// that we would have printed in ConditionalConformanceProtocols.
  void recordConditionalConformances(const ExtensionDecl *extension) {
    auto inheritedTypes = extension->getInherited();
    for (unsigned i : inheritedTypes.getIndices()) {
      Type inheritedTy = inheritedTypes.getResolvedType(i);
      if (!inheritedTy || !inheritedTy->isExistentialType())
        continue;

      ExistentialLayout layout = inheritedTy->getExistentialLayout();
      for (ProtocolDecl *protoDecl : layout.getProtocols()) {
        auto protoTy = protoDecl->getDeclaredInterfaceType()->castTo<ProtocolType>();
        if (!isPublicOrUsableFromInline(protoTy))
          continue;
        ConditionalConformanceProtocols.push_back(protoTy);
      }
      // FIXME: This ignores layout constraints, but currently we don't support
      // any of those besides 'AnyObject'.
    }
  }

public:
  using PerTypeMap = llvm::MapVector<const NominalTypeDecl *,
                                     InheritedProtocolCollector>;

  /// Given that we're about to print \p D, record its protocols in \p map.
  ///
  /// \sa recordProtocols
  static void collectProtocols(PerTypeMap &map, const Decl *D) {
    InheritedTypes directlyInherited = InheritedTypes(D);
    const NominalTypeDecl *nominal;
    const IterableDeclContext *memberContext;

    auto shouldInclude = [](const ExtensionDecl *extension) {
      if (extension->isConstrainedExtension()) {
        // Conditional conformances never apply to inherited protocols, nor
        // can they provide unconditional conformances that might be used in
        // other extensions.
        return false;
      }
      return true;
    };
    if ((nominal = dyn_cast<NominalTypeDecl>(D))) {
      memberContext = nominal;

    } else if (auto *extension = dyn_cast<ExtensionDecl>(D)) {
      if (!shouldInclude(extension)) {
        return;
      }
      nominal = extension->getExtendedNominal();
      memberContext = extension;
    } else {
      return;
    }

    if (!isPublicOrUsableFromInline(nominal))
      return;

    map[nominal].recordProtocols(directlyInherited, D);
    // Collect protocols inherited from super classes
    if (auto *CD = dyn_cast<ClassDecl>(D)) {
      for (auto *SD = CD->getSuperclassDecl(); SD;
           SD = SD->getSuperclassDecl()) {
        map[nominal].recordProtocols(SD->getInherited(), SD,
                                     /*skipExtra=*/true);
        for (auto *Ext: SD->getExtensions()) {
          if (shouldInclude(Ext)) {
            map[nominal].recordProtocols(Ext->getInherited(), Ext,
                                         /*skipExtra=*/true);
          }
        }
      }
    }

    // Recurse to find any nested types.
    for (const Decl *member : memberContext->getMembers())
      collectProtocols(map, member);
  }

  /// If \p D is an extension providing conditional conformances, record those
  /// in \p map.
  ///
  /// \sa recordConditionalConformances
  static void collectSkippedConditionalConformances(
                                            PerTypeMap &map,
                                            const Decl *D,
                                            const PrintOptions &printOptions) {
    auto *extension = dyn_cast<ExtensionDecl>(D);
    if (!extension || !extension->isConstrainedExtension())
      return;

    // Skip SPI extensions in the public interface.
    if (printOptions.printPublicInterface() && extension->isSPI())
      return;

    const NominalTypeDecl *nominal = extension->getExtendedNominal();
    if (!isPublicOrUsableFromInline(nominal))
      return;

    map[nominal].recordConditionalConformances(extension);
    // No recursion here because extensions are never nested.
  }

  /// Returns true if the conformance of \p nominal to \p proto is declared in
  /// module \p M.
  static bool conformanceDeclaredInModule(ModuleDecl *M,
                                          const NominalTypeDecl *nominal,
                                          ProtocolDecl *proto) {
    SmallVector<ProtocolConformance *, 4> conformances;
    nominal->lookupConformance(proto, conformances);
    return llvm::all_of(conformances,
                        [M](const ProtocolConformance *conformance) -> bool {
      return M == conformance->getDeclContext()->getParentModule();
    });
  }

  /// If there were any public protocols that need to be printed (i.e. they
  /// weren't conformed to explicitly or inherited by another printed protocol),
  /// do so now by printing a dummy extension on \p nominal to \p out.
  void
  printSynthesizedExtensionIfNeeded(raw_ostream &out,
                                    const PrintOptions &printOptions,
                                    ModuleDecl *M,
                                    const NominalTypeDecl *nominal) const {
    if (ExtraProtocols.empty())
      return;

    if (!printOptions.shouldPrint(nominal))
      return;

    /// is this nominal specifically an 'actor'?
    bool actorClass = false;
    if (auto klass = dyn_cast<ClassDecl>(nominal))
      actorClass = klass->isActor();

    SmallPtrSet<ProtocolDecl *, 16> handledProtocols;

    // First record all protocols that have already been handled.
    for (ProtocolDecl *proto : IncludedProtocols) {
      handledProtocols.insert(proto);
      auto allInherited = proto->getAllInheritedProtocols();
      handledProtocols.insert(allInherited.begin(), allInherited.end());
    }

    // Preserve the behavior of previous implementations which formatted of
    // empty extensions compactly with '{}' on the same line.
    PrintOptions extensionPrintOptions = printOptions;
    extensionPrintOptions.PrintEmptyMembersOnSameLine = true;

    // Then walk the remaining ones, and see what we need to print.
    // FIXME: This will pick the availability attributes from the first sight
    // of a protocol rather than the maximally available case.
    for (const auto &protoAndAvailability : ExtraProtocols) {
      auto proto = std::get<0>(protoAndAvailability);
      auto availability = std::get<1>(protoAndAvailability);
      auto isUnchecked = std::get<2>(protoAndAvailability);
      auto otherAttrs = std::get<3>(protoAndAvailability);
      proto->walkInheritedProtocols(
          [&](ProtocolDecl *inherited) -> TypeWalker::Action {
        if (!handledProtocols.insert(inherited).second)
          return TypeWalker::Action::SkipNode;

        // If 'nominal' is an actor, we do not synthesize its conformance
        // to the Actor protocol through a dummy extension.
        // There is a special restriction on the Actor protocol in that
        // it is only valid to conform to Actor on an 'actor' decl,
        // not extensions of that 'actor'.
        if (actorClass &&
            inherited->isSpecificProtocol(KnownProtocolKind::Actor))
          return TypeWalker::Action::SkipNode;

        // Do not synthesize an extension to print a conformance to an
        // invertible protocol, as their conformances are always re-inferred
        // using the interface itself.
        if (auto kp = inherited->getKnownProtocolKind())
          if (getInvertibleProtocolKind(*kp))
            return TypeWalker::Action::SkipNode;

        if (inherited->isSPI() && printOptions.printPublicInterface())
          return TypeWalker::Action::Continue;

        if (isPublicOrUsableFromInline(inherited) &&
            conformanceDeclaredInModule(M, nominal, inherited) &&
            !M->isImportedImplementationOnly(inherited->getParentModule())) {
          auto protoAndAvailability = ProtocolAndAvailability(
              inherited, availability, isUnchecked, otherAttrs);
          printSynthesizedExtension(out, extensionPrintOptions, M, nominal,
                                    protoAndAvailability);
          return TypeWalker::Action::SkipNode;
        }

        return TypeWalker::Action::Continue;
      });
    }
  }

  /// Prints a dummy extension on \p nominal to \p out for a public conformance
  /// to the protocol contained by \p protoAndAvailability.
  static void
  printSynthesizedExtension(raw_ostream &out, const PrintOptions &printOptions,
                            ModuleDecl *M, const NominalTypeDecl *nominal,
                            ProtocolAndAvailability &protoAndAvailability) {
    StreamPrinter printer(out);

    auto proto = std::get<0>(protoAndAvailability);
    auto availability = std::get<1>(protoAndAvailability);
    auto isUnchecked = std::get<2>(protoAndAvailability);
    auto originallyDefinedInAttrs = std::get<3>(protoAndAvailability);

    // Create a synthesized ExtensionDecl for the conformance.
    ASTContext &ctx = M->getASTContext();
    auto inherits = ctx.AllocateCopy(llvm::ArrayRef(InheritedEntry(
        TypeLoc::withoutLoc(proto->getDeclaredInterfaceType()), isUnchecked,
        /*isRetroactive=*/false,
        /*isPreconcurrency=*/false)));
    auto extension =
        ExtensionDecl::create(ctx, SourceLoc(), nullptr, inherits,
                              nominal->getModuleScopeContext(), nullptr);
    extension->setImplicit();

    // Build up synthesized DeclAttributes for the extension.
    TinyPtrVector<const DeclAttribute *> clonedAttrs;
    for (auto *attr : availability) {
      clonedAttrs.push_back(attr->clone(ctx, /*implicit*/ true));
    }
    for (auto *attr : proto->getAttrs().getAttributes<SPIAccessControlAttr>()) {
      clonedAttrs.push_back(attr->clone(ctx, /*implicit*/ true));
    }
    for (auto *attr : originallyDefinedInAttrs) {
      clonedAttrs.push_back(attr->clone(ctx, /*implicit*/ true));
    }

    // Since DeclAttributes is a linked list where each added attribute becomes
    // the head, we need to add these attributes in reverse order to reproduce
    // the order in which previous implementations printed these attributes.
    for (auto attr = clonedAttrs.rbegin(), end = clonedAttrs.rend();
         attr != end; ++attr) {
      extension->getAttrs().add(const_cast<DeclAttribute *>(*attr));
    }

    ctx.evaluator.cacheOutput(ExtendedTypeRequest{extension},
                              nominal->getDeclaredType());
    ctx.evaluator.cacheOutput(ExtendedNominalRequest{extension},
                              const_cast<NominalTypeDecl *>(nominal));

    extension->print(printer, printOptions);
    printer << "\n";
  }

  /// If there were any conditional conformances that couldn't be printed,
  /// make a dummy extension that conforms to all of them, constrained by a
  /// fake protocol.
  bool printInaccessibleConformanceExtensionIfNeeded(
      raw_ostream &out, const PrintOptions &printOptions,
      const NominalTypeDecl *nominal) const {
    if (ConditionalConformanceProtocols.empty())
      return false;
    assert(nominal->isGenericContext());

    if (!printOptions.printPublicInterface())
      out << "@_spi(" << DummyProtocolName << ")\n";
    out << "@available(*, unavailable)\nextension ";
    nominal->getDeclaredType().print(out, printOptions);
    out << " : ";
    llvm::interleave(
        ConditionalConformanceProtocols,
        [&out, &printOptions](const ProtocolType *protoTy) {
          protoTy->print(out, printOptions);
        },
        [&out] { out << ", "; });
    out << " where "
        << nominal->getGenericSignature().getGenericParams().front()->getName()
        << " : " << DummyProtocolName << " {}\n";
    return true;
  }

  /// Print a fake protocol declaration for use by
  /// #printInaccessibleConformanceExtensionIfNeeded.
  static void printDummyProtocolDeclaration(raw_ostream &out) {
    out << "\n@usableFromInline\ninternal protocol " << DummyProtocolName
        << " {}\n";
  }
};

const StringLiteral InheritedProtocolCollector::DummyProtocolName =
    "_ConstraintThatIsNotPartOfTheAPIOfThisLibrary";
} // end anonymous namespace

// MARK: Interface emission

bool swift::emitSwiftInterface(raw_ostream &out,
                               ModuleInterfaceOptions const &Opts,
                               ModuleDecl *M) {
  assert(M);

  llvm::SmallSet<StringRef, 4> aliasModuleNamesTargets;
  printToolVersionAndFlagsComment(out, Opts, M, aliasModuleNamesTargets);

  printImports(out, Opts, M, aliasModuleNamesTargets);

  bool useExportedModuleNames = Opts.printPublicInterface();

  const PrintOptions printOptions = PrintOptions::printSwiftInterfaceFile(
      M, Opts.PreserveTypesAsWritten, Opts.PrintFullConvention,
      Opts.InterfaceContentMode,
      useExportedModuleNames,
      Opts.AliasModuleNames, &aliasModuleNamesTargets);
  InheritedProtocolCollector::PerTypeMap inheritedProtocolMap;

  SmallVector<Decl *, 16> topLevelDecls;
  M->getTopLevelDeclsWithAuxiliaryDecls(topLevelDecls);
  for (const Decl *D : topLevelDecls) {
    InheritedProtocolCollector::collectProtocols(inheritedProtocolMap, D);

    if (!D->shouldPrintInContext(printOptions) ||
        !printOptions.shouldPrint(D)) {

      InheritedProtocolCollector::collectSkippedConditionalConformances(
          inheritedProtocolMap, D, printOptions);
      continue;
    }

    D->print(out, printOptions);
    out << "\n";

    diagnoseIfDeclShadowsKnownModule(Opts, const_cast<Decl *>(D), M);
  }

  // Print dummy extensions for any protocols that were indirectly conformed to.
  bool needDummyProtocolDeclaration = false;
  for (const auto &nominalAndCollector : inheritedProtocolMap) {
    const NominalTypeDecl *nominal = nominalAndCollector.first;
    const InheritedProtocolCollector &collector = nominalAndCollector.second;
    collector.printSynthesizedExtensionIfNeeded(out, printOptions, M, nominal);
    needDummyProtocolDeclaration |=
        collector.printInaccessibleConformanceExtensionIfNeeded(out,
                                                                printOptions,
                                                                nominal);
  }
  if (needDummyProtocolDeclaration)
    InheritedProtocolCollector::printDummyProtocolDeclaration(out);

  if (Opts.DebugPrintInvalidSyntax)
    out << "#__debug_emit_invalid_swiftinterface_syntax__\n";

  return false;
}