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+//===- ModuleLoader.h - Module Loader Interface -----------------*- C++ -*-===//
+//
+// 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 ModuleLoader interface, which is responsible for
+//  loading named modules.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LEX_MODULELOADER_H
+#define LLVM_CLANG_LEX_MODULELOADER_H
+
+#include "clang/Basic/LLVM.h"
+#include "clang/Basic/Module.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/PointerIntPair.h"
+#include "llvm/ADT/StringRef.h"
+#include <utility>
+
+namespace clang {
+
+class GlobalModuleIndex;
+class IdentifierInfo;
+
+/// A sequence of identifier/location pairs used to describe a particular
+/// module or submodule, e.g., std.vector.
+using ModuleIdPath = ArrayRef<std::pair<IdentifierInfo *, SourceLocation>>;
+
+/// Describes the result of attempting to load a module.
+class ModuleLoadResult {
+public:
+  enum LoadResultKind {
+    // We either succeeded or failed to load the named module.
+    Normal,
+
+    // The module exists, but does not actually contain the named submodule.
+    // This should only happen if the named submodule was inferred from an
+    // umbrella directory, but not actually part of the umbrella header.
+    MissingExpected,
+
+    // The module exists but cannot be imported due to a configuration mismatch.
+    ConfigMismatch
+  };
+  llvm::PointerIntPair<Module *, 2, LoadResultKind> Storage;
+
+  ModuleLoadResult() = default;
+  ModuleLoadResult(Module *M) : Storage(M, Normal) {}
+  ModuleLoadResult(LoadResultKind Kind) : Storage(nullptr, Kind) {}
+
+  operator Module *() const { return Storage.getPointer(); }
+
+  /// Determines whether the module, which failed to load, was
+  /// actually a submodule that we expected to see (based on implying the
+  /// submodule from header structure), but didn't materialize in the actual
+  /// module.
+  bool isMissingExpected() const { return Storage.getInt() == MissingExpected; }
+
+  /// Determines whether the module failed to load due to a configuration
+  /// mismatch with an explicitly-named .pcm file from the command line.
+  bool isConfigMismatch() const { return Storage.getInt() == ConfigMismatch; }
+};
+
+/// Abstract interface for a module loader.
+///
+/// This abstract interface describes a module loader, which is responsible
+/// for resolving a module name (e.g., "std") to an actual module file, and
+/// then loading that module.
+class ModuleLoader {
+  // Building a module if true.
+  bool BuildingModule;
+
+public:
+  explicit ModuleLoader(bool BuildingModule = false)
+      : BuildingModule(BuildingModule) {}
+
+  virtual ~ModuleLoader();
+
+  /// Returns true if this instance is building a module.
+  bool buildingModule() const {
+    return BuildingModule;
+  }
+
+  /// Flag indicating whether this instance is building a module.
+  void setBuildingModule(bool BuildingModuleFlag) {
+    BuildingModule = BuildingModuleFlag;
+  }
+
+  /// Attempt to load the given module.
+  ///
+  /// This routine attempts to load the module described by the given
+  /// parameters.
+  ///
+  /// \param ImportLoc The location of the 'import' keyword.
+  ///
+  /// \param Path The identifiers (and their locations) of the module
+  /// "path", e.g., "std.vector" would be split into "std" and "vector".
+  ///
+  /// \param Visibility The visibility provided for the names in the loaded
+  /// module.
+  ///
+  /// \param IsInclusionDirective Indicates that this module is being loaded
+  /// implicitly, due to the presence of an inclusion directive. Otherwise,
+  /// it is being loaded due to an import declaration.
+  ///
+  /// \returns If successful, returns the loaded module. Otherwise, returns
+  /// NULL to indicate that the module could not be loaded.
+  virtual ModuleLoadResult loadModule(SourceLocation ImportLoc,
+                                      ModuleIdPath Path,
+                                      Module::NameVisibilityKind Visibility,
+                                      bool IsInclusionDirective) = 0;
+
+  /// Attempt to load the given module from the specified source buffer. Does
+  /// not make any submodule visible; for that, use loadModule or
+  /// makeModuleVisible.
+  ///
+  /// \param Loc The location at which the module was loaded.
+  /// \param ModuleName The name of the module to build.
+  /// \param Source The source of the module: a (preprocessed) module map.
+  virtual void loadModuleFromSource(SourceLocation Loc, StringRef ModuleName,
+                                    StringRef Source) = 0;
+
+  /// Make the given module visible.
+  virtual void makeModuleVisible(Module *Mod,
+                                 Module::NameVisibilityKind Visibility,
+                                 SourceLocation ImportLoc) = 0;
+
+  /// Load, create, or return global module.
+  /// This function returns an existing global module index, if one
+  /// had already been loaded or created, or loads one if it
+  /// exists, or creates one if it doesn't exist.
+  /// Also, importantly, if the index doesn't cover all the modules
+  /// in the module map, it will be update to do so here, because
+  /// of its use in searching for needed module imports and
+  /// associated fixit messages.
+  /// \param TriggerLoc The location for what triggered the load.
+  /// \returns Returns null if load failed.
+  virtual GlobalModuleIndex *loadGlobalModuleIndex(
+                                                SourceLocation TriggerLoc) = 0;
+
+  /// Check global module index for missing imports.
+  /// \param Name The symbol name to look for.
+  /// \param TriggerLoc The location for what triggered the load.
+  /// \returns Returns true if any modules with that symbol found.
+  virtual bool lookupMissingImports(StringRef Name,
+                                    SourceLocation TriggerLoc) = 0;
+
+  bool HadFatalFailure = false;
+};
+
+/// A module loader that doesn't know how to load modules.
+class TrivialModuleLoader : public ModuleLoader {
+public:
+  ModuleLoadResult loadModule(SourceLocation ImportLoc, ModuleIdPath Path,
+                              Module::NameVisibilityKind Visibility,
+                              bool IsInclusionDirective) override {
+    return {};
+  }
+
+  void loadModuleFromSource(SourceLocation ImportLoc, StringRef ModuleName,
+                            StringRef Source) override {}
+
+  void makeModuleVisible(Module *Mod, Module::NameVisibilityKind Visibility,
+                         SourceLocation ImportLoc) override {}
+
+  GlobalModuleIndex *loadGlobalModuleIndex(SourceLocation TriggerLoc) override {
+    return nullptr;
+  }
+
+  bool lookupMissingImports(StringRef Name,
+                            SourceLocation TriggerLoc) override {
+    return false;
+  }
+};
+
+} // namespace clang
+
+#endif // LLVM_CLANG_LEX_MODULELOADER_H