swift-mirror/lib/AST/Module.cpp

//===--- Module.cpp - Swift Language Module Implementation ----------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
//  This file implements the Module class and subclasses.
//
//===----------------------------------------------------------------------===//

#include "swift/AST/ASTWalker.h"
#include "swift/AST/Diagnostics.h"
#include "swift/AST/LazyResolver.h"
#include "swift/AST/LinkLibrary.h"
#include "swift/AST/Module.h"
#include "swift/AST/ModuleLoader.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/AST.h"
#include "swift/AST/PrintOptions.h"
#include "swift/Basic/SourceManager.h"
#include "clang/Basic/Module.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/TinyPtrVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/SaveAndRestore.h"

using namespace swift;

//===----------------------------------------------------------------------===//
// Builtin Module Name lookup
//===----------------------------------------------------------------------===//

namespace {
  /// BuiltinModuleCache - This is the type of the cache for the BuiltinModule.
  /// This is lazily created on its first use an hangs off
  /// Module::LookupCachePimpl.
  class BuiltinModuleCache {
    /// The cache of identifiers we've already looked up.  We use a
    /// single hashtable for both types and values as a minor
    /// optimization; this prevents us from having both a builtin type
    /// and a builtin value with the same name, but that's okay.
    llvm::DenseMap<Identifier, ValueDecl*> Cache;
  public:

    void lookupValue(Identifier Name, NLKind LookupKind, BuiltinModule &M, 
                     SmallVectorImpl<ValueDecl*> &Result);
  };
} // end anonymous namespace.

static BuiltinModuleCache &getBuiltinCachePimpl(void *&Ptr) {
  // FIXME: This leaks.  Sticking this into ASTContext isn't enough because then
  // the DenseMap will leak.
  if (Ptr == 0)
    Ptr = new BuiltinModuleCache();
  return *(BuiltinModuleCache*)Ptr;
}

void BuiltinModuleCache::lookupValue(Identifier Name, NLKind LookupKind,
                                     BuiltinModule &M,
                                     SmallVectorImpl<ValueDecl*> &Result) {
  // Only qualified lookup ever finds anything in the builtin module.
  if (LookupKind != NLKind::QualifiedLookup) return;
  
  ValueDecl *&Entry = Cache[Name];

  if (Entry == 0)
    if (Type Ty = getBuiltinType(M.Ctx, Name.str()))
      Entry = new (M.Ctx) TypeAliasDecl(SourceLoc(), Name, SourceLoc(),
                                        TypeLoc::withoutLoc(Ty),
                                        M.Ctx.TheBuiltinModule,
                                        MutableArrayRef<TypeLoc>());

  if (Entry == 0)
    Entry = getBuiltinValue(M.Ctx, Name);

  if (Entry)
    Result.push_back(Entry);
}
                       
//===----------------------------------------------------------------------===//
// Normal Module Name Lookup
//===----------------------------------------------------------------------===//

namespace {
  /// This is the type of the cache for the TranslationUnit.
  ///
  /// This is lazily created on its first use and hangs off
  /// Module::LookupCachePimpl.
  class TUModuleCache {
    llvm::DenseMap<Identifier, TinyPtrVector<ValueDecl*>> TopLevelValues;
    llvm::DenseMap<Identifier, TinyPtrVector<ValueDecl*>> ClassMembers;
    bool MemberCachePopulated = false;
    void doPopulateCache(ArrayRef<Decl*> decls, bool onlyOperators);
    void addToMemberCache(ArrayRef<Decl*> decls);
    void populateMemberCache(const TranslationUnit &TU);
  public:
    typedef Module::AccessPathTy AccessPathTy;
    
    TUModuleCache(const TranslationUnit &TU);
    
    void lookupValue(AccessPathTy AccessPath, Identifier Name, 
                     NLKind LookupKind, TranslationUnit &TU, 
                     SmallVectorImpl<ValueDecl*> &Result);
    
    void lookupVisibleDecls(AccessPathTy AccessPath,
                            VisibleDeclConsumer &Consumer,
                            NLKind LookupKind,
                            const TranslationUnit &TU);
    
    void lookupClassMembers(AccessPathTy AccessPath,
                            VisibleDeclConsumer &consumer,
                            const TranslationUnit &TU);
                            
    void lookupClassMember(AccessPathTy accessPath,
                           Identifier name,
                           SmallVectorImpl<ValueDecl*> &results,
                           const TranslationUnit &TU);

    SmallVector<ValueDecl *, 0> AllVisibleValues;
  };
} // end anonymous namespace.

static TUModuleCache &getTUCachePimpl(void *&Ptr, const TranslationUnit &TU) {
  // FIXME: This leaks.  Sticking this into ASTContext isn't enough because then
  // the DenseMap will leak.
  if (Ptr == 0)
    Ptr = new TUModuleCache(TU);
  return *(TUModuleCache*)Ptr;
}

static void freeTUCachePimpl(void *&Ptr) {
  delete (TUModuleCache*)Ptr;
  Ptr = 0;
}

void TUModuleCache::doPopulateCache(ArrayRef<Decl*> decls, bool onlyOperators) {
  for (Decl *D : decls) {
    if (ValueDecl *VD = dyn_cast<ValueDecl>(D))
      if (onlyOperators ? VD->getName().isOperator() : !VD->getName().empty())
        TopLevelValues[VD->getName()].push_back(VD);
    if (NominalTypeDecl *NTD = dyn_cast<NominalTypeDecl>(D))
      doPopulateCache(NTD->getMembers(), true);
    if (ExtensionDecl *ED = dyn_cast<ExtensionDecl>(D))
      doPopulateCache(ED->getMembers(), true);
  }
}

void TUModuleCache::populateMemberCache(const TranslationUnit &TU) {
  for (const Decl *D : TU.Decls) {
    if (const NominalTypeDecl *NTD = dyn_cast<NominalTypeDecl>(D)) {
      addToMemberCache(NTD->getMembers());
    } else if (const ExtensionDecl *ED = dyn_cast<ExtensionDecl>(D)) {
      addToMemberCache(ED->getMembers());
    }
  }
}

void TUModuleCache::addToMemberCache(ArrayRef<Decl*> decls) {
  for (Decl *D : decls) {
    auto VD = dyn_cast<ValueDecl>(D);
    if (!VD)
      continue;

    if (auto NTD = dyn_cast<NominalTypeDecl>(VD)) {
      assert(!VD->canBeAccessedByDynamicLookup() &&
             "inner types cannot be accessed by dynamic lookup");
      addToMemberCache(NTD->getMembers());
    } else if (VD->canBeAccessedByDynamicLookup()) {
      ClassMembers[VD->getName()].push_back(VD);
    }
  }
}

/// Populate our cache on the first name lookup.
TUModuleCache::TUModuleCache(const TranslationUnit &TU) {
  doPopulateCache(TU.Decls, false);
}


void TUModuleCache::lookupValue(AccessPathTy AccessPath, Identifier Name, 
                                NLKind LookupKind, TranslationUnit &TU, 
                                SmallVectorImpl<ValueDecl*> &Result) {
  assert(AccessPath.size() <= 1 && "can only refer to top-level decls");
  
  // If this import is specific to some named type or decl ("import swift.int")
  // then filter out any lookups that don't match.
  if (AccessPath.size() == 1 && AccessPath.front().first != Name)
    return;
  
  auto I = TopLevelValues.find(Name);
  if (I == TopLevelValues.end()) return;
  
  Result.reserve(I->second.size());
  for (ValueDecl *Elt : I->second)
    Result.push_back(Elt);
}

void TUModuleCache::lookupVisibleDecls(AccessPathTy AccessPath,
                                       VisibleDeclConsumer &Consumer,
                                       NLKind LookupKind,
                                       const TranslationUnit &TU) {
  assert(AccessPath.size() <= 1 && "can only refer to top-level decls");

  if (!AccessPath.empty()) {
    auto I = TopLevelValues.find(AccessPath.front().first);
    if (I == TopLevelValues.end()) return;

    for (auto vd : I->second)
      Consumer.foundDecl(vd, DeclVisibilityKind::VisibleAtTopLevel);
    return;
  }

  for (auto &tlv : TopLevelValues) {
    for (ValueDecl *vd : tlv.second)
      Consumer.foundDecl(vd, DeclVisibilityKind::VisibleAtTopLevel);
  }
}

void TUModuleCache::lookupClassMembers(AccessPathTy accessPath,
                                       VisibleDeclConsumer &consumer,
                                       const TranslationUnit &TU) {
  if (!MemberCachePopulated)
    populateMemberCache(TU);
  
  assert(accessPath.size() <= 1 && "can only refer to top-level decls");
  
  if (!accessPath.empty()) {
    for (auto &member : ClassMembers) {
      for (ValueDecl *vd : member.second) {
        Type ty = vd->getDeclContext()->getDeclaredTypeOfContext();
        if (auto nominal = ty->getAnyNominal())
          if (nominal->getName() == accessPath.front().first)
            consumer.foundDecl(vd, DeclVisibilityKind::DynamicLookup);
      }
    }
    return;
  }

  for (auto &member : ClassMembers) {
    for (ValueDecl *vd : member.second)
      consumer.foundDecl(vd, DeclVisibilityKind::DynamicLookup);
  }
}

void TUModuleCache::lookupClassMember(AccessPathTy accessPath,
                                      Identifier name,
                                      SmallVectorImpl<ValueDecl*> &results,
                                      const TranslationUnit &TU) {
  if (!MemberCachePopulated)
    populateMemberCache(TU);
  
  assert(accessPath.size() <= 1 && "can only refer to top-level decls");
  
  auto iter = ClassMembers.find(name);
  if (iter == ClassMembers.end())
    return;
  
  if (!accessPath.empty()) {
    for (ValueDecl *vd : iter->second) {
      Type ty = vd->getDeclContext()->getDeclaredTypeOfContext();
      if (auto nominal = ty->getAnyNominal())
        if (nominal->getName() == accessPath.front().first)
          results.push_back(vd);
    }
    return;
  }

  results.append(iter->second.begin(), iter->second.end());
}

//===----------------------------------------------------------------------===//
// Module Implementation
//===----------------------------------------------------------------------===//

void Module::lookupValue(AccessPathTy AccessPath, Identifier Name,
                         NLKind LookupKind, 
                         SmallVectorImpl<ValueDecl*> &Result) {
  if (BuiltinModule *BM = dyn_cast<BuiltinModule>(this)) {
    assert(AccessPath.empty() && "builtin module's access path always empty!");
    return getBuiltinCachePimpl(LookupCachePimpl)
      .lookupValue(Name, LookupKind, *BM, Result);
  }

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    // Look in the translation unit.
    return getTUCachePimpl(LookupCachePimpl, *TU)
      .lookupValue(AccessPath, Name, LookupKind, *TU, Result);
  }

  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.lookupValue(this, AccessPath, Name, LookupKind, Result);
}

void Module::lookupVisibleDecls(AccessPathTy AccessPath,
                                VisibleDeclConsumer &Consumer,
                                NLKind LookupKind) const {
  if (auto BM = dyn_cast<BuiltinModule>(this)) {
    // TODO Look through the Builtin module.
    (void)BM;
    return;
  }

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    return getTUCachePimpl(LookupCachePimpl, *TU)
      .lookupVisibleDecls(AccessPath, Consumer, LookupKind, *TU);
  }

  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.lookupVisibleDecls(this, AccessPath, Consumer, LookupKind);
}

void Module::lookupClassMembers(AccessPathTy accessPath,
                                VisibleDeclConsumer &consumer) const {
  if (isa<BuiltinModule>(this)) {
    // The Builtin module defines no classes.
    return;
  }

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    return getTUCachePimpl(LookupCachePimpl, *TU)
      .lookupClassMembers(accessPath, consumer, *TU);
  }

  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.lookupClassMembers(this, accessPath, consumer);
}

void Module::lookupClassMember(AccessPathTy accessPath,
                               Identifier name,
                               SmallVectorImpl<ValueDecl*> &results) const {
  if (isa<BuiltinModule>(this)) {
    // The Builtin module defines no classes.
    return;
  }
  
  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    return getTUCachePimpl(LookupCachePimpl, *TU)
      .lookupClassMember(accessPath, name, results, *TU);
  }
  
  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.lookupClassMember(this, accessPath, name, results);
}

void Module::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) {
  if (isa<BuiltinModule>(this)) {
    return;
  }

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    Results.append(TU->Decls.begin(), TU->Decls.end());
    return;
  }

  ModuleLoader &Owner = cast<LoadedModule>(this)->getOwner();
  return Owner.getTopLevelDecls(this, Results);
}

ArrayRef<Substitution> BoundGenericType::getSubstitutions(
                                           Module *module,
                                           LazyResolver *resolver) {
  // FIXME: If there is no module, infer one. This is a hack for callers that
  // don't have access to the module. It will have to go away once we're
  // properly differentiating bound generic types based on the protocol
  // conformances visible from a given module.
  if (!module) {
    module = getDecl()->getParentModule();
  }

  // If we already have a cached copy of the substitutions, return them.
  auto *canon = getCanonicalType()->castTo<BoundGenericType>();
  const ASTContext &ctx = canon->getASTContext();
  if (auto known = ctx.getSubstitutions(canon))
    return *known;

  // Compute the set of substitutions.
  llvm::SmallPtrSet<ArchetypeType *, 8> knownArchetypes;
  SmallVector<ArchetypeType *, 8> archetypeStack;
  TypeSubstitutionMap substitutions;
  auto genericParams = canon->getDecl()->getGenericParams();
  unsigned index = 0;
  for (Type arg : canon->getGenericArgs()) {
    auto gp = genericParams->getParams()[index++];
    auto archetype = gp.getAsTypeParam()->getArchetype();
    substitutions[archetype] = arg;
  }

  // Collect all of the archetypes.
  SmallVector<ArchetypeType *, 2> allArchetypesList;
  ArrayRef<ArchetypeType *> allArchetypes = genericParams->getAllArchetypes();
  if (genericParams->getOuterParameters()) {
    SmallVector<const GenericParamList *, 2> allGenericParams;
    unsigned numArchetypes = 0;
    for (; genericParams; genericParams = genericParams->getOuterParameters()) {
      allGenericParams.push_back(genericParams);
      numArchetypes += genericParams->getAllArchetypes().size();
    }
    allArchetypesList.reserve(numArchetypes);
    for (auto gp = allGenericParams.rbegin(), gpEnd = allGenericParams.rend();
         gp != gpEnd; ++gp) {
      allArchetypesList.append((*gp)->getAllArchetypes().begin(),
                               (*gp)->getAllArchetypes().end());
    }
    allArchetypes = allArchetypesList;
  }

  // For each of the archetypes, compute the substitution.
  bool hasTypeVariables = canon->hasTypeVariable();
  SmallVector<Substitution, 4> resultSubstitutions;
  resultSubstitutions.resize(allArchetypes.size());
  index = 0;
  for (auto archetype : allArchetypes) {
    // Substitute into the type.
    auto type = Type(archetype).subst(module, substitutions,
                                      /*ignoreMissing=*/hasTypeVariables,
                                      resolver);
    assert(type && "Unable to perform type substitution");

    SmallVector<ProtocolConformance *, 4> conformances;
    if (type->hasTypeVariable()) {
      // If the type involves a type variable, just fill in null conformances.
      // FIXME: It seems like we should record these as requirements (?).
      conformances.assign(archetype->getConformsTo().size(), nullptr);
    } else {
      // Find the conformances.
      for (auto proto : archetype->getConformsTo()) {
        auto conforms = module->lookupConformance(type, proto, resolver);
        switch (conforms.getInt()) {
        case ConformanceKind::Conforms:
          conformances.push_back(conforms.getPointer());
          break;

        case ConformanceKind::DoesNotConform:
        case ConformanceKind::UncheckedConforms:
          llvm_unreachable("Couldn't find conformance");
        }
      }
    }

    // Record this substitution.
    resultSubstitutions[index].Archetype = archetype;
    resultSubstitutions[index].Replacement = type;
    resultSubstitutions[index].Conformance = ctx.AllocateCopy(conformances);
    ++index;
  }

  // Copy and record the substitutions.
  auto permanentSubs = ctx.AllocateCopy(resultSubstitutions,
                                        hasTypeVariables
                                          ? AllocationArena::ConstraintSolver
                                          : AllocationArena::Permanent);
  ctx.setSubstitutions(canon, permanentSubs);
  return permanentSubs;
}

/// Gather the set of substitutions required to map from the generic form of
/// the given type to the specialized form.
static ArrayRef<Substitution> gatherSubstitutions(Module *module, Type type,
                                                  LazyResolver *resolver) {
  assert(type->isSpecialized() && "Type is not specialized");
  SmallVector<ArrayRef<Substitution>, 2> allSubstitutions;

  while (type) {
    // Record the substitutions in a bound generic type.
    if (auto boundGeneric = type->getAs<BoundGenericType>()) {
      allSubstitutions.push_back(boundGeneric->getSubstitutions(module,
                                                                resolver));
      type = boundGeneric->getParent();
      continue;
    }

    // Skip to the parent of a nominal type.
    if (auto nominal = type->getAs<NominalType>()) {
      type = nominal->getParent();
      continue;
    }

    llvm_unreachable("Not a nominal or bound generic type");
  }
  assert(!allSubstitutions.empty() && "No substitutions?");

  // If there is only one list of substitutions, return it. There's no
  // need to copy it.
  if (allSubstitutions.size() == 1)
    return allSubstitutions.front();

  SmallVector<Substitution, 4> flatSubstitutions;
  for (auto substitutions : allSubstitutions)
    flatSubstitutions.append(substitutions.begin(), substitutions.end());
  auto &ctx = module->getASTContext();
  return ctx.AllocateCopy(flatSubstitutions);
}

/// Given a type witness map and a set of substitutions, produce the specialized
/// type witness map by applying the substitutions to each type witness.
static TypeWitnessMap
specializeTypeWitnesses(ASTContext &ctx,
                        Module *module,
                        const TypeWitnessMap &witnesses,
                        ArrayRef<Substitution> substitutions,
                        LazyResolver *resolver) {
  // Compute the substitution map, which is needed for substType().
  TypeSubstitutionMap substitutionMap;
  for (const auto &substitution : substitutions) {
    substitutionMap[substitution.Archetype] = substitution.Replacement;
  }

  // Substitute into each of the type witnesses.
  TypeWitnessMap result;
  for (const auto &genericWitness : witnesses) {
    // Substitute into the type witness to produce the type witness for
    // the specialized type.
    auto specializedType
      = genericWitness.second.Replacement.subst(module, substitutionMap,
                                                /*ignoreMissing=*/false,
                                                resolver);

    // If the type witness was unchanged, just copy it directly.
    if (specializedType.getPointer() ==
          genericWitness.second.Replacement.getPointer()) {
      result.insert(genericWitness);
      continue;
    }

    // Gather the conformances for the type witness. These should never fail.
    SmallVector<ProtocolConformance *, 4> conformances;
    auto archetype = genericWitness.second.Archetype;
    for (auto proto : archetype->getConformsTo()) {
      auto conforms = module->lookupConformance(specializedType, proto,
                                                resolver);
      switch (conforms.getInt()) {
      case ConformanceKind::Conforms:
        conformances.push_back(conforms.getPointer());
        break;

      case ConformanceKind::DoesNotConform:
      case ConformanceKind::UncheckedConforms:
        // FIXME: Signal errors in a more sane way.
        return TypeWitnessMap();
      }
    }

    result[genericWitness.first]
      = Substitution{archetype, specializedType,
                     ctx.AllocateCopy(conformances)};
  }

  return result;
}

/// Retrieve the explicit conformance of the given nominal type declaration
/// to the given protocol.
static std::tuple<NominalTypeDecl *, Decl *, ProtocolConformance *>
findExplicitConformance(NominalTypeDecl *nominal, ProtocolDecl *protocol,
                        LazyResolver *resolver) {
  // FIXME: Introduce a cache/lazy lookup structure to make this more efficient?

  // Walk the nominal type, its extensions, superclasses, and so on.
  llvm::SmallPtrSet<ProtocolDecl *, 4> visitedProtocols;
  SmallVector<std::tuple<NominalTypeDecl *, NominalTypeDecl *, Decl *>,4> stack;
  NominalTypeDecl *owningNominal = nullptr;
  Decl *declaresConformance = nullptr;
  ProtocolConformance *nominalConformance = nullptr;

  // Local function that checks for our protocol in the given array of
  // protocols.
  auto isProtocolInList
    = [&](NominalTypeDecl *currentNominal,
          Decl *currentOwner,
          ArrayRef<ProtocolDecl *> protocols,
          ArrayRef<ProtocolConformance *> conformances) -> bool {
      for (unsigned i = 0, n = protocols.size(); i != n; ++i) {
        auto testProto = protocols[i];
        if (testProto == protocol) {
          owningNominal = currentNominal;
          declaresConformance = currentOwner;
          if (i < conformances.size())
            nominalConformance = conformances[i];
          return true;
        }

        if (visitedProtocols.insert(testProto))
          stack.push_back({testProto, currentNominal, currentOwner});
      }

      return false;
    };

  resolver->resolveDeclSignature(nominal);

  // Walk the stack of types to find a conformance.
  stack.push_back({nominal, nominal, nominal});
  while (!stack.empty()) {
    NominalTypeDecl *current;
    NominalTypeDecl *currentNominal;
    Decl *currentOwner;
    std::tie(current, currentNominal, currentOwner) = stack.back();
    stack.pop_back();

    // Visit the superclass of a class.
    if (auto classDecl = dyn_cast<ClassDecl>(current)) {
      if (auto superclassTy = classDecl->getSuperclass()) {
        auto nominal = superclassTy->getAnyNominal();
        stack.push_back({nominal, nominal, nominal});
      }
    }

    // Visit the protocols this type conforms to directly.
    if (isProtocolInList(currentNominal, currentOwner,
                         current->getProtocols(),
                         current->getConformances()))
      break;

    // Visit the extensions of this type.
    for (auto ext : current->getExtensions()) {
      if (isProtocolInList(currentNominal, ext, ext->getProtocols(),
                           ext->getConformances()))
        break;
    }
  }

  // If we didn't find the protocol, we don't conform. Cache the negative result
  // and return.
  if (!owningNominal) {
    return { nullptr, nullptr, nullptr };
  }

  // If we don't have a nominal conformance, but we do have a resolver, try
  // to resolve the nominal conformance now.
  if (!nominalConformance && resolver) {
    nominalConformance = resolver->resolveConformance(
                           owningNominal,
                           protocol,
                           dyn_cast<ExtensionDecl>(declaresConformance));
  }

  // If we have a nominal conformance, we're done.
  if (nominalConformance) {
    return { owningNominal, declaresConformance, nominalConformance };
  }

  return { nullptr, nullptr, nullptr };
}

LookupConformanceResult Module::lookupConformance(Type type,
                                                  ProtocolDecl *protocol,
                                                  LazyResolver *resolver) {
  ASTContext &ctx = getASTContext();

  // An archetype conforms to a protocol if the protocol is listed in the
  // archetype's list of conformances.
  if (auto archetype = type->getAs<ArchetypeType>()) {
    for (auto ap : archetype->getConformsTo()) {
      if (ap == protocol || ap->inheritsFrom(protocol))
        return { nullptr, ConformanceKind::Conforms };
    }

    return { nullptr, ConformanceKind::DoesNotConform };
  }

  // An archetype conforms to a protocol if the protocol is listed in the
  // existential's list of conformances and the existential conforms to
  // itself.
  if (type->isExistentialType()) {
    // If the protocol doesn't conform to itself, there's no point in looking
    // further.
    auto known = protocol->existentialConformsToSelf();
    if (!known && resolver) {
      resolver->resolveExistentialConformsToItself(protocol);
      known = protocol->existentialConformsToSelf();
    }

    // If we know that protocol doesn't conform to itself, we're done.
    if (known && !*known)
      return { nullptr, ConformanceKind::DoesNotConform };

    // Look for this protocol within the existential's list of conformances.
    SmallVector<ProtocolDecl *, 4> protocols;
    type->isExistentialType(protocols);
    for (auto ap : protocols) {
      if (ap == protocol || ap->inheritsFrom(protocol)) {
        return { nullptr,
                 known? ConformanceKind::Conforms
                      : ConformanceKind::UncheckedConforms };
      }
    }

    // We didn't find our protocol in the existential's list; it doesn't
    // conform.
    return { nullptr, ConformanceKind::DoesNotConform };
  }

  // Check whether we have already cached an answer to this query.
  ASTContext::ConformsToMap::key_type key(type->getCanonicalType(), protocol);
  auto known = ctx.ConformsTo.find(key);
  if (known != ctx.ConformsTo.end()) {
    // If we conform, return the conformance.
    if (known->second.getInt()) {
      return { known->second.getPointer(), ConformanceKind::Conforms };
    }

    // We don't conform.
    return { nullptr, ConformanceKind::DoesNotConform };
  }

  auto nominal = type->getAnyNominal();

  // If we don't have a nominal type, there are no conformances.
  // FIXME: We may have implicit conformances for some cases. Handle those
  // here.
  if (!nominal) {
    return { nullptr, ConformanceKind::DoesNotConform };
  }

  // Find the explicit conformance.
  NominalTypeDecl *owningNominal = nullptr;
  Decl *declaresConformance = nullptr;
  ProtocolConformance *nominalConformance = nullptr;
  std::tie(owningNominal, declaresConformance, nominalConformance)
    = findExplicitConformance(nominal, protocol, resolver);

  // If we didn't find an owning nominal, we don't conform. Cache the negative
  // result and return.
  if (!owningNominal) {
    ctx.ConformsTo[key] = ConformanceEntry(nullptr, false);
    return { nullptr, ConformanceKind::DoesNotConform };
  }

  // If we found an owning nominal but didn't have a conformance, this is
  // an unchecked conformance.
  if (!nominalConformance) {
    return { nullptr, ConformanceKind::UncheckedConforms };
  }

  // If the nominal type in which we found the conformance is not the same
  // as the type we asked for, it's an inherited type.
  if (owningNominal != nominal) {
    // Find the superclass type
    Type superclassTy = type->getSuperclass(resolver);
    while (superclassTy->getAnyNominal() != owningNominal)
      superclassTy = superclassTy->getSuperclass(resolver);

    // Compute the conformance for the inherited type.
    auto inheritedConformance = lookupConformance(superclassTy, protocol,
                                                  resolver);
    switch (inheritedConformance.getInt()) {
    case ConformanceKind::DoesNotConform:
      llvm_unreachable("We already found the inherited conformance");

    case ConformanceKind::UncheckedConforms:
      return inheritedConformance;

    case ConformanceKind::Conforms:
      // Create inherited conformance below.
      break;
    }

    // Create the inherited conformance entry.
    auto result
      = ctx.getInheritedConformance(type, inheritedConformance.getPointer());
    ctx.ConformsTo[key] = ConformanceEntry(result, true);
    return { result, ConformanceKind::Conforms };
  }

  // If the type is specialized, find the conformance for the generic type.
  if (type->isSpecialized()) {
    // Figure out the type that's explicitly conforming to this protocol.
    Type explicitConformanceType;
    if (auto nominal = dyn_cast<NominalTypeDecl>(declaresConformance)) {
      explicitConformanceType = nominal->getDeclaredTypeInContext();
    } else {
      explicitConformanceType = cast<ExtensionDecl>(declaresConformance)
        ->getDeclaredTypeInContext();
    }

    // If the explicit conformance is associated with a type that is different
    // from the type we're checking, retrieve generic conformance.
    if (!explicitConformanceType->isEqual(type)) {
      // Gather the substitutions we need to map the generic conformance to
      // the specialized conformance.
      auto substitutions = gatherSubstitutions(this, type, resolver);

      // The type witnesses for the specialized conformance.
      TypeWitnessMap typeWitnesses
        = specializeTypeWitnesses(ctx, this,
                                  nominalConformance->getTypeWitnesses(),
                                  substitutions,
                                  resolver);

      // Create the specialized conformance entry.
      ctx.ConformsTo[key] = ConformanceEntry(nullptr, false);
      auto result = ctx.getSpecializedConformance(type, nominalConformance,
                                                  substitutions,
                                                  std::move(typeWitnesses));
      ctx.ConformsTo[key] = ConformanceEntry(result, true);
      return { result, ConformanceKind::Conforms };
    }
  }

  // Record and return the simple conformance.
  ctx.ConformsTo[key] = ConformanceEntry(nominalConformance, true);
  return { nominalConformance, ConformanceKind::Conforms };
}

void Module::getDisplayDecls(SmallVectorImpl<Decl*> &results) {
  if (isa<BuiltinModule>(this)) {
    // FIXME: The Builtin module isn't usually visible, but it would be nice
    // to have the option to display its decls. Unfortunately those decls are
    // lazily generated.
    return;
  }
  
  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    results.append(TU->Decls.begin(), TU->Decls.end());
    return;
  }
  
  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.getDisplayDecls(this, results);
}

namespace {
// Returns Nothing on error, Optional(nullptr) if no operator decl found, or
// Optional(decl) if decl was found.
template<typename OP_DECL>
Optional<OP_DECL *> lookupOperatorDeclForName(Module *M,
                          SourceLoc Loc,
                          Identifier Name,
                          llvm::StringMap<OP_DECL *> TranslationUnit::*OP_MAP)
{
  if (auto loadedModule = dyn_cast<LoadedModule>(M))
    return loadedModule->lookupOperator<OP_DECL>(Name);

  auto *TU = dyn_cast<TranslationUnit>(M);
  if (!TU)
    return Nothing;

  // Look for an operator declaration in the current module.
  auto found = (TU->*OP_MAP).find(Name.get());
  if (found != (TU->*OP_MAP).end())
    return found->getValue()? Optional<OP_DECL *>(found->getValue()) : Nothing;
  
  // Look for imported operator decls.
  
  llvm::DenseSet<OP_DECL*> importedOperators;
  for (auto &imported : TU->getImports()) {
    Optional<OP_DECL *> maybeOp
      = lookupOperatorDeclForName(imported.first.second, Loc, Name, OP_MAP);
    if (!maybeOp)
      return Nothing;
    
    if (OP_DECL *op = *maybeOp)
      importedOperators.insert(op);
  }
  
  // If we found a single import, use it.
  if (importedOperators.empty()) {
    // Cache the mapping so we don't need to troll imports next time.
    (TU->*OP_MAP)[Name.get()] = nullptr;
    return Nothing;
  }
  if (importedOperators.size() == 1) {
    // Cache the mapping so we don't need to troll imports next time.
    OP_DECL *result = *importedOperators.begin();
    (TU->*OP_MAP)[Name.get()] = result;
    return result;
  }
  
  // Otherwise, check for conflicts.
  auto i = importedOperators.begin(), end = importedOperators.end();
  OP_DECL *first = *i;
  for (++i; i != end; ++i) {
    if ((*i)->conflictsWith(first)) {
      if (Loc.isValid()) {
        ASTContext &C = M->getASTContext();
        C.Diags.diagnose(Loc, diag::ambiguous_operator_decls);
        C.Diags.diagnose(first->getLoc(), diag::found_this_operator_decl);
        C.Diags.diagnose((*i)->getLoc(), diag::found_this_operator_decl);
      }
      return Nothing;
    }
  }
  // Cache the mapping so we don't need to troll imports next time.
  (TU->*OP_MAP)[Name.get()] = first;
  return first;
}
} // end anonymous namespace

Optional<PrefixOperatorDecl *> Module::lookupPrefixOperator(Identifier name,
                                                            SourceLoc diagLoc) {
  return lookupOperatorDeclForName(this, diagLoc, name,
                                   &TranslationUnit::PrefixOperators);
}

Optional<PostfixOperatorDecl *> Module::lookupPostfixOperator(Identifier name,
                                                            SourceLoc diagLoc) {
  return lookupOperatorDeclForName(this, diagLoc, name,
                                   &TranslationUnit::PostfixOperators);
}

Optional<InfixOperatorDecl *> Module::lookupInfixOperator(Identifier name,
                                                          SourceLoc diagLoc) {
  return lookupOperatorDeclForName(this, diagLoc, name,
                                   &TranslationUnit::InfixOperators);
}

void
Module::getImportedModules(SmallVectorImpl<ImportedModule> &modules,
                           bool includePrivate) const {
  if (isa<BuiltinModule>(this))
    return;

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    for (auto importPair : TU->getImports())
      if (includePrivate || importPair.second)
        modules.push_back(importPair.first);
    return;
  }

  ModuleLoader &owner = cast<LoadedModule>(this)->getOwner();
  return owner.getImportedModules(this, modules, includePrivate);
}

namespace {
  /// Arbitrarily orders ImportedModule records, for inclusion in sets and such.
  class OrderImportedModules {
    using ImportedModule = Module::ImportedModule;
    using AccessPathTy = Module::AccessPathTy;
  public:
    bool operator()(const ImportedModule &lhs, const ImportedModule &rhs) {
      if (lhs.second != rhs.second)
        return std::less<const Module *>()(lhs.second, rhs.second);
      if (lhs.first.data() != rhs.first.data())
        return std::less<AccessPathTy::iterator>()(lhs.first.begin(),
                                                   rhs.first.begin());
      return lhs.first.size() < rhs.first.size();
    }
  };
}

bool Module::isSameAccessPath(AccessPathTy lhs, AccessPathTy rhs) {
  using AccessPathElem = std::pair<Identifier, SourceLoc>;
  if (lhs.size() != rhs.size())
    return false;
  auto iters = std::mismatch(lhs.begin(), lhs.end(), rhs.begin(),
                             [](const AccessPathElem &lElem,
                                const AccessPathElem &rElem) {
    return lElem.first == rElem.first;
  });
  return iters.first == lhs.end();
}

StringRef Module::getModuleFilename() const {
  if (isa<BuiltinModule>(this))
    return StringRef();

  if (auto TU = dyn_cast<TranslationUnit>(this)) {
    if (TU->getImportBufferID() == -1)
      return StringRef();
    return Ctx.SourceMgr->getMemoryBuffer(
                                TU->getImportBufferID())->getBufferIdentifier();
  }

  ModuleLoader &Owner = cast<LoadedModule>(this)->getOwner();
  return Owner.getModuleFilename(this);
}

bool Module::isStdlibModule() const {
  return !getParent() && Name == Ctx.StdlibModuleName;
}

template<bool respectVisibility, typename Callback>
static void forAllImportedModules(Module *topLevel,
                                  Optional<Module::AccessPathTy> thisPath,
                                  const Callback &fn) {
  using ImportedModule = Module::ImportedModule;
  using AccessPathTy = Module::AccessPathTy;
  
  llvm::SmallSet<ImportedModule, 32, OrderImportedModules> visited;
  SmallVector<ImportedModule, 32> queue;

  AccessPathTy overridingPath;
  if (thisPath.hasValue()) {
    if (respectVisibility)
      overridingPath = thisPath.getValue();
    queue.push_back(ImportedModule(overridingPath, topLevel));
  } else {
    visited.insert(ImportedModule({}, topLevel));
  }

  // Even if we're processing the top-level module like any other, we still want
  // to include non-exported modules.
  topLevel->getImportedModules(queue, true);

  while (!queue.empty()) {
    auto next = queue.pop_back_val();

    // Filter any whole-module imports, and skip specific-decl imports if the
    // import path doesn't match exactly.
    if (next.first.empty() || !respectVisibility)
      next.first = overridingPath;
    else if (!overridingPath.empty() &&
             !Module::isSameAccessPath(next.first, overridingPath)) {
      // If we ever allow importing non-top-level decls, it's possible the rule
      // above isn't what we want.
      assert(next.first.size() == 1 && "import of non-top-level decl");
      continue;
    }

    if (!visited.insert(next))
      continue;

    if (!fn(next))
      break;
    next.second->getImportedModules(queue, !respectVisibility);
  }
}

void Module::forAllVisibleModules(Optional<AccessPathTy> thisPath,
                                  std::function<bool(ImportedModule)> fn) {
  forAllImportedModules<true>(this, thisPath, fn);
}

void Module::collectLinkLibraries(LinkLibraryCallback callback) {
  forAllImportedModules<false>(this, AccessPathTy(),
                               [=](ImportedModule import) -> bool {
    Module *module = import.second;
    if (isa<BuiltinModule>(module)) {
      // The Builtin module requires no libraries.
      return true;
    }
    
    if (auto TU = dyn_cast<TranslationUnit>(module)) {
      for (auto lib : TU->getLinkLibraries())
        callback(lib);
      return true;
    }
    
    ModuleLoader &owner = cast<LoadedModule>(module)->getOwner();
    owner.getLinkLibraries(module, callback);
    return true;
  });
}


//===----------------------------------------------------------------------===//
// TranslationUnit Implementation
//===----------------------------------------------------------------------===//

void TranslationUnit::print(raw_ostream &os) {
  print(os, PrintOptions::printEverything());
}

void TranslationUnit::print(raw_ostream &os, const PrintOptions &options) {
  for (auto decl : Decls) {
    if (!decl->shouldPrintInContext())
      continue;

    decl->print(os, options);
    os << "\n";
  }
}

void TranslationUnit::clearLookupCache() {
  freeTUCachePimpl(LookupCachePimpl);
}

void
TranslationUnit::cacheVisibleDecls(SmallVectorImpl<ValueDecl*> &&globals) const{
  auto &cached = getTUCachePimpl(LookupCachePimpl, *this).AllVisibleValues;
  static_cast<SmallVectorImpl<ValueDecl*>&>(cached) = std::move(globals);
}

const SmallVectorImpl<ValueDecl *> &
TranslationUnit::getCachedVisibleDecls() const {
  return getTUCachePimpl(LookupCachePimpl, *this).AllVisibleValues;
}

bool TranslationUnit::walk(ASTWalker &Walker) {
  llvm::SaveAndRestore<ASTWalker::ParentTy> SAR(Walker.Parent, this);
  for (Decl *D : Decls) {
    if (D->walk(Walker))
      return true;
  }
  return false;
}

//===----------------------------------------------------------------------===//
// LoadedModule Implementation
//===----------------------------------------------------------------------===//

OperatorDecl *LoadedModule::lookupOperator(Identifier name, DeclKind fixity) {
  return getOwner().lookupOperator(this, name, fixity);
}

template<>
PrefixOperatorDecl *
LoadedModule::lookupOperator<PrefixOperatorDecl>(Identifier name) {
  auto result = lookupOperator(name, DeclKind::PrefixOperator);
  return cast_or_null<PrefixOperatorDecl>(result);
}

template<>
PostfixOperatorDecl *
LoadedModule::lookupOperator<PostfixOperatorDecl>(Identifier name) {
  auto result = lookupOperator(name, DeclKind::PostfixOperator);
  return cast_or_null<PostfixOperatorDecl>(result);
}

template<>
InfixOperatorDecl *
LoadedModule::lookupOperator<InfixOperatorDecl>(Identifier name) {
  auto result = lookupOperator(name, DeclKind::InfixOperator);
  return cast_or_null<InfixOperatorDecl>(result);
}


//===----------------------------------------------------------------------===//
// ModuleLoader Implementation
//===----------------------------------------------------------------------===//
ModuleLoader::~ModuleLoader() {}