[NFC] Move DeclID from serialization/ASTBitCodes.h to AST/DeclID.h

Previously, the DeclID is defined in serialization/ASTBitCodes.h under
clang::serialization namespace. However, actually the DeclID is not
purely used in serialization part. The DeclID is already widely used in
AST and all around the clang project via classes like `LazyPtrDecl` or
calling `ExternalASTSource::getExernalDecl()`. All such uses are via the
raw underlying type of `DeclID` as `uint32_t`. This is not pretty good.

This patch moves the DeclID class family to a new header `AST/DeclID.h`
so that the whole project can use the wrapped class `DeclID`,
`GlobalDeclID` and `LocalDeclID` instead of the raw underlying type.
This can improve the readability and the type safety.

Author: ChuanqiXu9

clang/include/clang/AST/ASTContext.h

@@ -455,7 +455,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// initialization of another module).
   struct PerModuleInitializers {
     llvm::SmallVector<Decl*, 4> Initializers;
-    llvm::SmallVector<Decl::DeclID, 4> LazyInitializers;
+    llvm::SmallVector<DeclID, 4> LazyInitializers;
 
     void resolve(ASTContext &Ctx);
   };
@@ -1059,7 +1059,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// or an ImportDecl nominating another module that has initializers.
   void addModuleInitializer(Module *M, Decl *Init);
 
-  void addLazyModuleInitializers(Module *M, ArrayRef<Decl::DeclID> IDs);
+  void addLazyModuleInitializers(Module *M, ArrayRef<DeclID> IDs);
 
   /// Get the initializations to perform when importing a module, if any.
   ArrayRef<Decl*> getModuleInitializers(Module *M);

clang/include/clang/AST/DeclBase.h

@@ -15,6 +15,7 @@
 
 #include "clang/AST/ASTDumperUtils.h"
 #include "clang/AST/AttrIterator.h"
+#include "clang/AST/DeclID.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/SelectorLocationsKind.h"
 #include "clang/Basic/IdentifierTable.h"
@@ -239,9 +240,6 @@ class alignas(8) Decl {
     ModulePrivate
   };
 
-  /// An ID number that refers to a declaration in an AST file.
-  using DeclID = uint32_t;
-
 protected:
   /// The next declaration within the same lexical
   /// DeclContext. These pointers form the linked list that is

clang/include/clang/AST/DeclID.h

@@ -0,0 +1,175 @@
+//===--- DeclID.h - ID number for deserialized declarations  ----*- 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 DeclID class family to describe the deserialized
+// declarations. The DeclID is widely used in AST via LazyDeclPtr, or calls to
+// `ExternalASTSource::getExternalDecl`. It will be helpful for type safety to
+// require the use of `DeclID` to explicit.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_DECLID_H
+#define LLVM_CLANG_AST_DECLID_H
+
+namespace clang {
+
+/// Predefined declaration IDs.
+///
+/// These declaration IDs correspond to predefined declarations in the AST
+/// context, such as the NULL declaration ID. Such declarations are never
+/// actually serialized, since they will be built by the AST context when
+/// it is created.
+enum PredefinedDeclIDs {
+  /// The NULL declaration.
+  PREDEF_DECL_NULL_ID = 0,
+
+  /// The translation unit.
+  PREDEF_DECL_TRANSLATION_UNIT_ID = 1,
+
+  /// The Objective-C 'id' type.
+  PREDEF_DECL_OBJC_ID_ID = 2,
+
+  /// The Objective-C 'SEL' type.
+  PREDEF_DECL_OBJC_SEL_ID = 3,
+
+  /// The Objective-C 'Class' type.
+  PREDEF_DECL_OBJC_CLASS_ID = 4,
+
+  /// The Objective-C 'Protocol' type.
+  PREDEF_DECL_OBJC_PROTOCOL_ID = 5,
+
+  /// The signed 128-bit integer type.
+  PREDEF_DECL_INT_128_ID = 6,
+
+  /// The unsigned 128-bit integer type.
+  PREDEF_DECL_UNSIGNED_INT_128_ID = 7,
+
+  /// The internal 'instancetype' typedef.
+  PREDEF_DECL_OBJC_INSTANCETYPE_ID = 8,
+
+  /// The internal '__builtin_va_list' typedef.
+  PREDEF_DECL_BUILTIN_VA_LIST_ID = 9,
+
+  /// The internal '__va_list_tag' struct, if any.
+  PREDEF_DECL_VA_LIST_TAG = 10,
+
+  /// The internal '__builtin_ms_va_list' typedef.
+  PREDEF_DECL_BUILTIN_MS_VA_LIST_ID = 11,
+
+  /// The predeclared '_GUID' struct.
+  PREDEF_DECL_BUILTIN_MS_GUID_ID = 12,
+
+  /// The extern "C" context.
+  PREDEF_DECL_EXTERN_C_CONTEXT_ID = 13,
+
+  /// The internal '__make_integer_seq' template.
+  PREDEF_DECL_MAKE_INTEGER_SEQ_ID = 14,
+
+  /// The internal '__NSConstantString' typedef.
+  PREDEF_DECL_CF_CONSTANT_STRING_ID = 15,
+
+  /// The internal '__NSConstantString' tag type.
+  PREDEF_DECL_CF_CONSTANT_STRING_TAG_ID = 16,
+
+  /// The internal '__type_pack_element' template.
+  PREDEF_DECL_TYPE_PACK_ELEMENT_ID = 17,
+};
+
+/// The number of declaration IDs that are predefined.
+///
+/// For more information about predefined declarations, see the
+/// \c PredefinedDeclIDs type and the PREDEF_DECL_*_ID constants.
+const unsigned int NUM_PREDEF_DECL_IDS = 18;
+
+/// An ID number that refers to a declaration in an AST file.
+///
+/// The ID numbers of declarations are consecutive (in order of
+/// discovery), with values below NUM_PREDEF_DECL_IDS being reserved.
+/// At the start of a chain of precompiled headers, declaration ID 1 is
+/// used for the translation unit declaration.
+using DeclID = uint32_t;
+
+class LocalDeclID {
+public:
+  explicit LocalDeclID(DeclID ID) : ID(ID) {}
+
+  DeclID get() const { return ID; }
+
+private:
+  DeclID ID;
+};
+
+/// Wrapper class for DeclID. This is helpful to not mix the use of LocalDeclID
+/// and GlobalDeclID to improve the type safety.
+class GlobalDeclID {
+public:
+  GlobalDeclID() : ID(PREDEF_DECL_NULL_ID) {}
+  explicit GlobalDeclID(DeclID ID) : ID(ID) {}
+
+  DeclID get() const { return ID; }
+
+  explicit operator DeclID() const { return ID; }
+
+  friend bool operator==(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID == RHS.ID;
+  }
+  friend bool operator!=(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID != RHS.ID;
+  }
+  // We may sort the global decl ID.
+  friend bool operator<(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID < RHS.ID;
+  }
+  friend bool operator>(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID > RHS.ID;
+  }
+  friend bool operator<=(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID <= RHS.ID;
+  }
+  friend bool operator>=(const GlobalDeclID &LHS, const GlobalDeclID &RHS) {
+    return LHS.ID >= RHS.ID;
+  }
+
+private:
+  DeclID ID;
+};
+
+/// A helper iterator adaptor to convert the iterators to `SmallVector<DeclID>`
+/// to the iterators to `SmallVector<GlobalDeclID>`.
+class GlobalDeclIDIterator
+    : public llvm::iterator_adaptor_base<GlobalDeclIDIterator, const DeclID *,
+                                         std::forward_iterator_tag,
+                                         GlobalDeclID> {
+public:
+  GlobalDeclIDIterator() : iterator_adaptor_base(nullptr) {}
+
+  GlobalDeclIDIterator(const DeclID *ID) : iterator_adaptor_base(ID) {}
+
+  value_type operator*() const { return GlobalDeclID(*I); }
+
+  bool operator==(const GlobalDeclIDIterator &RHS) const { return I == RHS.I; }
+};
+
+/// A helper iterator adaptor to convert the iterators to
+/// `SmallVector<GlobalDeclID>` to the iterators to `SmallVector<DeclID>`.
+class DeclIDIterator
+    : public llvm::iterator_adaptor_base<DeclIDIterator, const GlobalDeclID *,
+                                         std::forward_iterator_tag, DeclID> {
+public:
+  DeclIDIterator() : iterator_adaptor_base(nullptr) {}
+
+  DeclIDIterator(const GlobalDeclID *ID) : iterator_adaptor_base(ID) {}
+
+  value_type operator*() const { return DeclID(*I); }
+
+  bool operator==(const DeclIDIterator &RHS) const { return I == RHS.I; }
+};
+
+} // namespace clang
+
+#endif

clang/include/clang/AST/DeclTemplate.h

@@ -797,7 +797,7 @@ class RedeclarableTemplateDecl : public TemplateDecl,
     ///
     /// The first value in the array is the number of specializations/partial
     /// specializations that follow.
-    Decl::DeclID *LazySpecializations = nullptr;
+    DeclID *LazySpecializations = nullptr;
 
     /// The set of "injected" template arguments used within this
     /// template.

clang/include/clang/AST/ExternalASTSource.h

@@ -99,7 +99,7 @@ class ExternalASTSource : public RefCountedBase<ExternalASTSource> {
   /// passes back decl sets as VisibleDeclaration objects.
   ///
   /// The default implementation of this method is a no-op.
-  virtual Decl *GetExternalDecl(Decl::DeclID ID);
+  virtual Decl *GetExternalDecl(DeclID ID);
 
   /// Resolve a selector ID into a selector.
   ///
@@ -579,7 +579,7 @@ using LazyDeclStmtPtr =
 
 /// A lazy pointer to a declaration.
 using LazyDeclPtr =
-    LazyOffsetPtr<Decl, Decl::DeclID, &ExternalASTSource::GetExternalDecl>;
+    LazyOffsetPtr<Decl, DeclID, &ExternalASTSource::GetExternalDecl>;
 
 /// A lazy pointer to a set of CXXCtorInitializers.
 using LazyCXXCtorInitializersPtr =

clang/include/clang/Frontend/ASTUnit.h

@@ -241,7 +241,7 @@ class ASTUnit {
 
   /// A list of the serialization ID numbers for each of the top-level
   /// declarations parsed within the precompiled preamble.
-  std::vector<serialization::DeclID> TopLevelDeclsInPreamble;
+  std::vector<DeclID> TopLevelDeclsInPreamble;
 
   /// Whether we should be caching code-completion results.
   bool ShouldCacheCodeCompletionResults : 1;

clang/include/clang/Frontend/MultiplexConsumer.h

@@ -35,7 +35,7 @@ class MultiplexASTDeserializationListener : public ASTDeserializationListener {
   void IdentifierRead(serialization::IdentID ID, IdentifierInfo *II) override;
   void MacroRead(serialization::MacroID ID, MacroInfo *MI) override;
   void TypeRead(serialization::TypeIdx Idx, QualType T) override;
-  void DeclRead(serialization::DeclID ID, const Decl *D) override;
+  void DeclRead(DeclID ID, const Decl *D) override;
   void SelectorRead(serialization::SelectorID iD, Selector Sel) override;
   void MacroDefinitionRead(serialization::PreprocessedEntityID,
                            MacroDefinitionRecord *MD) override;

clang/include/clang/Sema/MultiplexExternalSemaSource.h

@@ -65,7 +65,7 @@ class MultiplexExternalSemaSource : public ExternalSemaSource {
 
   /// Resolve a declaration ID into a declaration, potentially
   /// building a new declaration.
-  Decl *GetExternalDecl(Decl::DeclID ID) override;
+  Decl *GetExternalDecl(DeclID ID) override;
 
   /// Complete the redeclaration chain if it's been extended since the
   /// previous generation of the AST source.