Module.cpp

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//===- Module.cpp - Describe a module -------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the Module class, which describes a module in the source
// code.
//
//===----------------------------------------------------------------------===//
 
#include "clang/Basic/Module.h"
#include "clang/Basic/CharInfo.h"
#include "clang/Basic/FileManager.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Basic/SourceLocation.h"
#include "clang/Basic/TargetInfo.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <functional>
#include <string>
#include <utility>
#include <vector>
 
using namespace clang;
 
std::optional<ModuleFileKey>
ModuleFileName::makeKey(FileManager &FileMgr) const {
  if (ImplicitModuleSuffixLength) {
    StringRef ModuleCachePath =
        StringRef(Path).drop_back(ImplicitModuleSuffixLength);
    StringRef ImplicitModuleSuffix =
        StringRef(Path).take_back(ImplicitModuleSuffixLength);
    if (auto ModuleCache = FileMgr.getOptionalDirectoryRef(
            ModuleCachePath, /*CacheFailure=*/false))
      return ModuleFileKey(*ModuleCache, ImplicitModuleSuffix);
  } else {
    if (auto ModuleFile = FileMgr.getOptionalFileRef(Path, /*OpenFile=*/true,
                                                     /*CacheFailure=*/false,
                                                     /*IsText=*/false))
      return ModuleFileKey(*ModuleFile);
  }
 
  return std::nullopt;
}
 
Module::Module(ModuleConstructorTag, StringRef Name,
               SourceLocation DefinitionLoc, Module *Parent, bool IsFramework,
               bool IsExplicit, unsigned VisibilityID)
    : Name(Name), DefinitionLoc(DefinitionLoc), Parent(Parent),
      VisibilityID(VisibilityID), IsUnimportable(false),
      HasIncompatibleModuleFile(false), IsAvailable(true),
      IsFromModuleFile(false), IsFramework(IsFramework), IsExplicit(IsExplicit),
      IsSystem(false), IsExternC(false), IsInferred(false),
      InferSubmodules(false), InferExplicitSubmodules(false),
      InferExportWildcard(false), ConfigMacrosExhaustive(false),
      NoUndeclaredIncludes(false), ModuleMapIsPrivate(false),
      NamedModuleHasInit(true), NameVisibility(Hidden) {
  if (Parent) {
    IsAvailable = Parent->isAvailable();
    IsUnimportable = Parent->isUnimportable();
    IsSystem = Parent->IsSystem;
    IsExternC = Parent->IsExternC;
    NoUndeclaredIncludes = Parent->NoUndeclaredIncludes;
    ModuleMapIsPrivate = Parent->ModuleMapIsPrivate;
 
    Parent->SubModules.push_back(this);
  }
}
 
Module::~Module() = default;
 
static bool isPlatformEnvironment(const TargetInfo &Target, StringRef Feature) {
  StringRef Platform = Target.getPlatformName();
  StringRef Env = Target.getTriple().getEnvironmentName();
 
  // Attempt to match platform and environment.
  if (Platform == Feature || Target.getTriple().getOSName() == Feature ||
      Env == Feature)
    return true;
 
  auto CmpPlatformEnv = [](StringRef LHS, StringRef RHS) {
    auto Pos = LHS.find('-');
    if (Pos == StringRef::npos)
      return false;
    SmallString<128> NewLHS = LHS.slice(0, Pos);
    NewLHS += LHS.slice(Pos+1, LHS.size());
    return NewLHS == RHS;
  };
 
  SmallString<128> PlatformEnv = Target.getTriple().getOSAndEnvironmentName();
  // Darwin has different but equivalent variants for simulators, example:
  //   1. x86_64-apple-ios-simulator
  //   2. x86_64-apple-iossimulator
  // where both are valid examples of the same platform+environment but in the
  // variant (2) the simulator is hardcoded as part of the platform name. Both
  // forms above should match for "iossimulator" requirement.
  if (Target.getTriple().isOSDarwin() && PlatformEnv.ends_with("simulator"))
    return PlatformEnv == Feature || CmpPlatformEnv(PlatformEnv, Feature);
 
  return PlatformEnv == Feature;
}
 
/// Determine whether a translation unit built using the current
/// language options has the given feature.
static bool hasFeature(StringRef Feature, const LangOptions &LangOpts,
                       const TargetInfo &Target) {
  bool HasFeature = llvm::StringSwitch<bool>(Feature)
                        .Case("altivec", LangOpts.AltiVec)
                        .Case("blocks", LangOpts.Blocks)
                        .Case("coroutines", LangOpts.Coroutines)
                        .Case("cplusplus", LangOpts.CPlusPlus)
                        .Case("cplusplus11", LangOpts.CPlusPlus11)
                        .Case("cplusplus14", LangOpts.CPlusPlus14)
                        .Case("cplusplus17", LangOpts.CPlusPlus17)
                        .Case("cplusplus20", LangOpts.CPlusPlus20)
                        .Case("cplusplus23", LangOpts.CPlusPlus23)
                        .Case("cplusplus26", LangOpts.CPlusPlus26)
                        .Case("c99", LangOpts.C99)
                        .Case("c11", LangOpts.C11)
                        .Case("c17", LangOpts.C17)
                        .Case("c23", LangOpts.C23)
                        .Case("freestanding", LangOpts.Freestanding)
                        .Case("gnuinlineasm", LangOpts.GNUAsm)
                        .Case("objc", LangOpts.ObjC)
                        .Case("objc_arc", LangOpts.ObjCAutoRefCount)
                        .Case("opencl", LangOpts.OpenCL)
                        .Case("tls", Target.isTLSSupported())
                        .Case("zvector", LangOpts.ZVector)
                        .Default(Target.hasFeature(Feature) ||
                                 isPlatformEnvironment(Target, Feature));
  if (!HasFeature)
    HasFeature = llvm::is_contained(LangOpts.ModuleFeatures, Feature);
  return HasFeature;
}
 
bool Module::isUnimportable(const LangOptions &LangOpts,
                            const TargetInfo &Target, Requirement &Req,
                            Module *&ShadowingModule) const {
  if (!IsUnimportable)
    return false;
 
  for (const Module *Current = this; Current; Current = Current->Parent) {
    if (Current->ShadowingModule) {
      ShadowingModule = Current->ShadowingModule;
      return true;
    }
    for (unsigned I = 0, N = Current->Requirements.size(); I != N; ++I) {
      if (hasFeature(Current->Requirements[I].FeatureName, LangOpts, Target) !=
          Current->Requirements[I].RequiredState) {
        Req = Current->Requirements[I];
        return true;
      }
    }
  }
 
  llvm_unreachable("could not find a reason why module is unimportable");
}
 
// The -fmodule-name option tells the compiler to textually include headers in
// the specified module, meaning Clang won't build the specified module. This
// is useful in a number of situations, for instance, when building a library
// that vends a module map, one might want to avoid hitting intermediate build
// products containing the module map or avoid finding the system installed
// modulemap for that library.
bool Module::isForBuilding(const LangOptions &LangOpts) const {
  StringRef TopLevelName = getTopLevelModuleName();
  StringRef CurrentModule = LangOpts.CurrentModule;
 
  // When building the implementation of framework Foo, we want to make sure
  // that Foo *and* Foo_Private are textually included and no modules are built
  // for either.
  if (!LangOpts.isCompilingModule() && getTopLevelModule()->IsFramework &&
      CurrentModule == LangOpts.ModuleName &&
      !CurrentModule.ends_with("_Private") &&
      TopLevelName.ends_with("_Private"))
    TopLevelName = TopLevelName.drop_back(8);
 
  return TopLevelName == CurrentModule;
}
 
bool Module::isAvailable(const LangOptions &LangOpts, const TargetInfo &Target,
                         Requirement &Req,
                         UnresolvedHeaderDirective &MissingHeader,
                         Module *&ShadowingModule) const {
  if (IsAvailable)
    return true;
 
  if (isUnimportable(LangOpts, Target, Req, ShadowingModule))
    return false;
 
  // FIXME: All missing headers are listed on the top-level module. Should we
  // just look there?
  for (const Module *Current = this; Current; Current = Current->Parent) {
    if (!Current->MissingHeaders.empty()) {
      MissingHeader = Current->MissingHeaders.front();
      return false;
    }
  }
 
  llvm_unreachable("could not find a reason why module is unavailable");
}
 
bool Module::isSubModuleOf(const Module *Other) const {
  for (auto *Parent = this; Parent; Parent = Parent->Parent) {
    if (Parent == Other)
      return true;
  }
  return false;
}
 
const Module *Module::getTopLevelModule() const {
  const Module *Result = this;
  while (Result->Parent)
    Result = Result->Parent;
 
  return Result;
}
 
static StringRef getModuleNameFromComponent(
    const std::pair<std::string, SourceLocation> &IdComponent) {
  return IdComponent.first;
}
 
static StringRef getModuleNameFromComponent(StringRef R) { return R; }
 
template<typename InputIter>
static void printModuleId(raw_ostream &OS, InputIter Begin, InputIter End,
                          bool AllowStringLiterals = true) {
  for (InputIter It = Begin; It != End; ++It) {
    if (It != Begin)
      OS << ".";
 
    StringRef Name = getModuleNameFromComponent(*It);
    if (!AllowStringLiterals || isValidAsciiIdentifier(Name))
      OS << Name;
    else {
      OS << '"';
      OS.write_escaped(Name);
      OS << '"';
    }
  }
}
 
template<typename Container>
static void printModuleId(raw_ostream &OS, const Container &C) {
  return printModuleId(OS, C.begin(), C.end());
}
 
std::string Module::getFullModuleName(bool AllowStringLiterals) const {
  SmallVector<StringRef, 2> Names;
 
  // Build up the set of module names (from innermost to outermost).
  for (const Module *M = this; M; M = M->Parent)
    Names.push_back(M->Name);
 
  std::string Result;
 
  llvm::raw_string_ostream Out(Result);
  printModuleId(Out, Names.rbegin(), Names.rend(), AllowStringLiterals);
 
  return Result;
}
 
bool Module::fullModuleNameIs(ArrayRef<StringRef> nameParts) const {
  for (const Module *M = this; M; M = M->Parent) {
    if (nameParts.empty() || M->Name != nameParts.back())
      return false;
    nameParts = nameParts.drop_back();
  }
  return nameParts.empty();
}
 
OptionalDirectoryEntryRef Module::getEffectiveUmbrellaDir() const {
  if (const auto *Hdr = std::get_if<FileEntryRef>(&Umbrella))
    return Hdr->getDir();
  if (const auto *Dir = std::get_if<DirectoryEntryRef>(&Umbrella))
    return *Dir;
  return std::nullopt;
}
 
void Module::addTopHeader(FileEntryRef File) {
  assert(File);
  TopHeaders.insert(File);
}
 
ArrayRef<FileEntryRef> Module::getTopHeaders(FileManager &FileMgr) {
  if (!TopHeaderNames.empty()) {
    for (StringRef TopHeaderName : TopHeaderNames)
      if (auto FE = FileMgr.getOptionalFileRef(TopHeaderName))
        TopHeaders.insert(*FE);
    TopHeaderNames.clear();
  }
 
  return llvm::ArrayRef(TopHeaders.begin(), TopHeaders.end());
}
 
bool Module::directlyUses(const Module *Requested) {
  auto *Top = getTopLevelModule();
 
  // A top-level module implicitly uses itself.
  if (Requested->isSubModuleOf(Top))
    return true;
 
  for (auto *Use : Top->DirectUses)
    if (Requested->isSubModuleOf(Use))
      return true;
 
  // Anyone is allowed to use our builtin stddef.h and its accompanying modules.
  if (Requested->fullModuleNameIs({"_Builtin_stddef", "max_align_t"}) ||
      Requested->fullModuleNameIs({"_Builtin_stddef_wint_t"}))
    return true;
  // Darwin is allowed is to use our builtin 'ptrauth.h' and its accompanying
  // module.
  if (!Requested->Parent && Requested->Name == "ptrauth")
    return true;
 
  if (NoUndeclaredIncludes)
    UndeclaredUses.insert(Requested);
 
  return false;
}
 
void Module::addRequirement(StringRef Feature, bool RequiredState,
                            const LangOptions &LangOpts,
                            const TargetInfo &Target) {
  Requirements.push_back(Requirement{std::string(Feature), RequiredState});
 
  // If this feature is currently available, we're done.
  if (hasFeature(Feature, LangOpts, Target) == RequiredState)
    return;
 
  markUnavailable(/*Unimportable*/true);
}
 
void Module::markUnavailable(bool Unimportable) {
  auto needUpdate = [Unimportable](Module *M) {
    return M->IsAvailable || (!M->IsUnimportable && Unimportable);
  };
 
  if (!needUpdate(this))
    return;
 
  SmallVector<Module *, 2> Stack;
  Stack.push_back(this);
  while (!Stack.empty()) {
    Module *Current = Stack.pop_back_val();
 
    if (!needUpdate(Current))
      continue;
 
    Current->IsAvailable = false;
    Current->IsUnimportable |= Unimportable;
    for (auto *Submodule : Current->submodules()) {
      if (needUpdate(Submodule))
        Stack.push_back(Submodule);
    }
  }
}
 
Module *Module::findSubmodule(StringRef Name) const {
  // Add new submodules into the index.
  for (unsigned I = SubModuleIndex.size(), E = SubModules.size(); I != E; ++I)
    SubModuleIndex[SubModules[I]->Name] = I;
 
  if (auto It = SubModuleIndex.find(Name); It != SubModuleIndex.end())
    return SubModules[It->second];
 
  return nullptr;
}
 
Module *Module::getGlobalModuleFragment() const {
  assert(isNamedModuleUnit() && "We should only query the global module "
                                "fragment from the C++20 Named modules");
 
  for (auto *SubModule : SubModules)
    if (SubModule->isExplicitGlobalModule())
      return SubModule;
 
  return nullptr;
}
 
Module *Module::getPrivateModuleFragment() const {
  assert(isNamedModuleUnit() && "We should only query the private module "
                                "fragment from the C++20 Named modules");
 
  for (auto *SubModule : SubModules)
    if (SubModule->isPrivateModule())
      return SubModule;
 
  return nullptr;
}
 
void Module::getExportedModules(SmallVectorImpl<Module *> &Exported) const {
  // All non-explicit submodules are exported.
  for (std::vector<Module *>::const_iterator I = SubModules.begin(),
                                             E = SubModules.end();
       I != E; ++I) {
    Module *Mod = *I;
    if (!Mod->IsExplicit)
      Exported.push_back(Mod);
  }
 
  // Find re-exported modules by filtering the list of imported modules.
  bool AnyWildcard = false;
  bool UnrestrictedWildcard = false;
  SmallVector<Module *, 4> WildcardRestrictions;
  for (unsigned I = 0, N = Exports.size(); I != N; ++I) {
    Module *Mod = Exports[I].getPointer();
    if (!Exports[I].getInt()) {
      // Export a named module directly; no wildcards involved.
      Exported.push_back(Mod);
 
      continue;
    }
 
    // Wildcard export: export all of the imported modules that match
    // the given pattern.
    AnyWildcard = true;
    if (UnrestrictedWildcard)
      continue;
 
    if (Module *Restriction = Exports[I].getPointer())
      WildcardRestrictions.push_back(Restriction);
    else {
      WildcardRestrictions.clear();
      UnrestrictedWildcard = true;
    }
  }
 
  // If there were any wildcards, push any imported modules that were
  // re-exported by the wildcard restriction.
  if (!AnyWildcard)
    return;
 
  for (unsigned I = 0, N = Imports.size(); I != N; ++I) {
    Module *Mod = Imports[I];
    bool Acceptable = UnrestrictedWildcard;
    if (!Acceptable) {
      // Check whether this module meets one of the restrictions.
      for (unsigned R = 0, NR = WildcardRestrictions.size(); R != NR; ++R) {
        Module *Restriction = WildcardRestrictions[R];
        if (Mod == Restriction || Mod->isSubModuleOf(Restriction)) {
          Acceptable = true;
          break;
        }
      }
    }
 
    if (!Acceptable)
      continue;
 
    Exported.push_back(Mod);
  }
}
 
void Module::buildVisibleModulesCache() const {
  assert(VisibleModulesCache.empty() && "cache does not need building");
 
  // This module is visible to itself.
  VisibleModulesCache.insert(this);
 
  // Every imported module is visible.
  SmallVector<Module *, 16> Stack(Imports.begin(), Imports.end());
  while (!Stack.empty()) {
    Module *CurrModule = Stack.pop_back_val();
 
    // Every module transitively exported by an imported module is visible.
    if (VisibleModulesCache.insert(CurrModule).second)
      CurrModule->getExportedModules(Stack);
  }
}
 
void Module::print(raw_ostream &OS, unsigned Indent, bool Dump) const {
  OS.indent(Indent);
  if (IsFramework)
    OS << "framework ";
  if (IsExplicit)
    OS << "explicit ";
  OS << "module ";
  printModuleId(OS, &Name, &Name + 1);
 
  if (IsSystem || IsExternC) {
    OS.indent(Indent + 2);
    if (IsSystem)
      OS << " [system]";
    if (IsExternC)
      OS << " [extern_c]";
  }
 
  OS << " {\n";
 
  if (!Requirements.empty()) {
    OS.indent(Indent + 2);
    OS << "requires ";
    for (unsigned I = 0, N = Requirements.size(); I != N; ++I) {
      if (I)
        OS << ", ";
      if (!Requirements[I].RequiredState)
        OS << "!";
      OS << Requirements[I].FeatureName;
    }
    OS << "\n";
  }
 
  if (std::optional<Header> H = getUmbrellaHeaderAsWritten()) {
    OS.indent(Indent + 2);
    OS << "umbrella header \"";
    OS.write_escaped(H->NameAsWritten);
    OS << "\"\n";
  } else if (std::optional<DirectoryName> D = getUmbrellaDirAsWritten()) {
    OS.indent(Indent + 2);
    OS << "umbrella \"";
    OS.write_escaped(D->NameAsWritten);
    OS << "\"\n";
  }
 
  if (!ConfigMacros.empty() || ConfigMacrosExhaustive) {
    OS.indent(Indent + 2);
    OS << "config_macros ";
    if (ConfigMacrosExhaustive)
      OS << "[exhaustive]";
    for (unsigned I = 0, N = ConfigMacros.size(); I != N; ++I) {
      if (I)
        OS << ", ";
      OS << ConfigMacros[I];
    }
    OS << "\n";
  }
 
  struct {
    StringRef Prefix;
    HeaderKind Kind;
  } Kinds[] = {{"", HK_Normal},
               {"textual ", HK_Textual},
               {"private ", HK_Private},
               {"private textual ", HK_PrivateTextual},
               {"exclude ", HK_Excluded}};
 
  for (auto &K : Kinds) {
    assert(&K == &Kinds[K.Kind] && "kinds in wrong order");
    for (auto &H : getHeaders(K.Kind)) {
      OS.indent(Indent + 2);
      OS << K.Prefix << "header \"";
      OS.write_escaped(H.NameAsWritten);
      OS << "\" { size " << H.Entry.getSize()
         << " mtime " << H.Entry.getModificationTime() << " }\n";
    }
  }
  for (auto *Unresolved : {&UnresolvedHeaders, &MissingHeaders}) {
    for (auto &U : *Unresolved) {
      OS.indent(Indent + 2);
      OS << Kinds[U.Kind].Prefix << "header \"";
      OS.write_escaped(U.FileName);
      OS << "\"";
      if (U.Size || U.ModTime) {
        OS << " {";
        if (U.Size)
          OS << " size " << *U.Size;
        if (U.ModTime)
          OS << " mtime " << *U.ModTime;
        OS << " }";
      }
      OS << "\n";
    }
  }
 
  if (!ExportAsModule.empty()) {
    OS.indent(Indent + 2);
    OS << "export_as" << ExportAsModule << "\n";
  }
 
  for (auto *Submodule : submodules())
    // Print inferred subframework modules so that we don't need to re-infer
    // them (requires expensive directory iteration + stat calls) when we build
    // the module. Regular inferred submodules are OK, as we need to look at all
    // those header files anyway.
    if (!Submodule->IsInferred || Submodule->IsFramework)
      Submodule->print(OS, Indent + 2, Dump);
 
  for (unsigned I = 0, N = Exports.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "export ";
    if (Module *Restriction = Exports[I].getPointer()) {
      OS << Restriction->getFullModuleName(true);
      if (Exports[I].getInt())
        OS << ".*";
    } else {
      OS << "*";
    }
    OS << "\n";
  }
 
  for (unsigned I = 0, N = UnresolvedExports.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "export ";
    printModuleId(OS, UnresolvedExports[I].Id);
    if (UnresolvedExports[I].Wildcard)
      OS << (UnresolvedExports[I].Id.empty() ? "*" : ".*");
    OS << "\n";
  }
 
  if (Dump) {
    for (Module *M : Imports) {
      OS.indent(Indent + 2);
      llvm::errs() << "import " << M->getFullModuleName() << "\n";
    }
  }
 
  for (unsigned I = 0, N = DirectUses.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "use ";
    OS << DirectUses[I]->getFullModuleName(true);
    OS << "\n";
  }
 
  for (unsigned I = 0, N = UnresolvedDirectUses.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "use ";
    printModuleId(OS, UnresolvedDirectUses[I]);
    OS << "\n";
  }
 
  for (unsigned I = 0, N = LinkLibraries.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "link ";
    if (LinkLibraries[I].IsFramework)
      OS << "framework ";
    OS << "\"";
    OS.write_escaped(LinkLibraries[I].Library);
    OS << "\"";
  }
 
  for (unsigned I = 0, N = UnresolvedConflicts.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "conflict ";
    printModuleId(OS, UnresolvedConflicts[I].Id);
    OS << ", \"";
    OS.write_escaped(UnresolvedConflicts[I].Message);
    OS << "\"\n";
  }
 
  for (unsigned I = 0, N = Conflicts.size(); I != N; ++I) {
    OS.indent(Indent + 2);
    OS << "conflict ";
    OS << Conflicts[I].Other->getFullModuleName(true);
    OS << ", \"";
    OS.write_escaped(Conflicts[I].Message);
    OS << "\"\n";
  }
 
  if (InferSubmodules) {
    OS.indent(Indent + 2);
    if (InferExplicitSubmodules)
      OS << "explicit ";
    OS << "module * {\n";
    if (InferExportWildcard) {
      OS.indent(Indent + 4);
      OS << "export *\n";
    }
    OS.indent(Indent + 2);
    OS << "}\n";
  }
 
  OS.indent(Indent);
  OS << "}\n";
}
 
LLVM_DUMP_METHOD void Module::dump() const {
  print(llvm::errs(), 0, true);
}
 
void VisibleModuleSet::setVisible(Module *M, SourceLocation Loc,
                                  bool IncludeExports, VisibleCallback Vis,
                                  ConflictCallback Cb) {
  // We can't import a global module fragment so the location can be invalid.
  assert((M->isGlobalModule() || Loc.isValid()) &&
         "setVisible expects a valid import location");
  if (isVisible(M))
    return;
 
  ++Generation;
 
  struct Visiting {
    Module *M;
    Visiting *ExportedBy;
  };
 
  std::function<void(Visiting)> VisitModule = [&](Visiting V) {
    // Nothing to do for a module that's already visible.
    unsigned ID = V.M->getVisibilityID();
    if (ImportLocs.size() <= ID)
      ImportLocs.resize(ID + 1);
    else if (ImportLocs[ID].isValid())
      return;
 
    ImportLocs[ID] = Loc;
    Vis(V.M);
 
    // Make any exported modules visible.
    if (IncludeExports) {
      SmallVector<Module *, 16> Exports;
      V.M->getExportedModules(Exports);
      for (Module *E : Exports) {
        // Don't import non-importable modules.
        if (!E->isUnimportable())
          VisitModule({E, &V});
      }
    }
 
    for (auto &C : V.M->Conflicts) {
      if (isVisible(C.Other)) {
        llvm::SmallVector<Module*, 8> Path;
        for (Visiting *I = &V; I; I = I->ExportedBy)
          Path.push_back(I->M);
        Cb(Path, C.Other, C.Message);
      }
    }
  };
  VisitModule({M, nullptr});
}