[lldb][TypeSystemClang] Pass ClangASTMetadata around by value (#102161)

This patch changes the return type of `GetMetadata` from a
`ClangASTMetadata*` to a `std::optional<ClangASTMetadata>`. Except for
one call-site (`SetDeclIsForcefullyCompleted`), we never actually make
use of the mutability of the returned metadata. And we never make use of
the pointer-identity. By passing `ClangASTMetadata` by-value (the type
is fairly small, size of 2 64-bit pointers) we'll avoid some questions
surrounding the lifetimes/ownership/mutability of this metadata.

For consistency, we also change the parameter to `SetMetadata` from
`ClangASTMetadata&` to `ClangASTMetadata` (which is an NFC since we copy
the data anyway).

This came up during some changes we plan to make where we [create
redeclaration chains for decls in the LLDB
AST](https://github.com/llvm/llvm-project/pull/95100). We want to avoid
having to dig out the canonical decl of the declaration chain for
retrieving/setting the metadata. It should just be copied across all
decls in the chain. This is easier to guarantee when everything is done
by-value.

lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.cpp

@@ -84,8 +84,7 @@ clang::Decl *ClangASTImporter::CopyDecl(clang::ASTContext *dst_ast,
     LLDB_LOG_ERROR(log, result.takeError(), "Couldn't import decl: {0}");
     if (log) {
       lldb::user_id_t user_id = LLDB_INVALID_UID;
-      ClangASTMetadata *metadata = GetDeclMetadata(decl);
-      if (metadata)
+      if (std::optional<ClangASTMetadata> metadata = GetDeclMetadata(decl))
         user_id = metadata->GetUserID();
 
       if (NamedDecl *named_decl = dyn_cast<NamedDecl>(decl))
@@ -950,7 +949,8 @@ bool ClangASTImporter::RequireCompleteType(clang::QualType type) {
   return true;
 }
 
-ClangASTMetadata *ClangASTImporter::GetDeclMetadata(const clang::Decl *decl) {
+std::optional<ClangASTMetadata>
+ClangASTImporter::GetDeclMetadata(const clang::Decl *decl) {
   DeclOrigin decl_origin = GetDeclOrigin(decl);
 
   if (decl_origin.Valid()) {
@@ -1105,7 +1105,7 @@ ClangASTImporter::ASTImporterDelegate::ImportImpl(Decl *From) {
 
   // If we have a forcefully completed type, try to find an actual definition
   // for it in other modules.
-  const ClangASTMetadata *md = m_main.GetDeclMetadata(From);
+  std::optional<ClangASTMetadata> md = m_main.GetDeclMetadata(From);
   auto *td = dyn_cast<TagDecl>(From);
   if (td && md && md->IsForcefullyCompleted()) {
     Log *log = GetLog(LLDBLog::Expressions);
@@ -1284,8 +1284,7 @@ void ClangASTImporter::ASTImporterDelegate::Imported(clang::Decl *from,
   }
 
   lldb::user_id_t user_id = LLDB_INVALID_UID;
-  ClangASTMetadata *metadata = m_main.GetDeclMetadata(from);
-  if (metadata)
+  if (std::optional<ClangASTMetadata> metadata = m_main.GetDeclMetadata(from))
     user_id = metadata->GetUserID();
 
   if (log) {

lldb/source/Plugins/ExpressionParser/Clang/ClangASTImporter.h

@@ -189,7 +189,7 @@ public:
   /// is only a shallow clone that lacks any contents.
   void SetDeclOrigin(const clang::Decl *decl, clang::Decl *original_decl);
 
-  ClangASTMetadata *GetDeclMetadata(const clang::Decl *decl);
+  std::optional<ClangASTMetadata> GetDeclMetadata(const clang::Decl *decl);
 
   //
   // Namespace maps

lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp

@@ -219,9 +219,10 @@ void ClangUserExpression::ScanContext(ExecutionContext &exe_ctx, Status &err) {
     // whatever runtime the debug info says the object pointer belongs to.  Do
     // that here.
 
-    ClangASTMetadata *metadata =
-        TypeSystemClang::DeclContextGetMetaData(decl_context, function_decl);
-    if (metadata && metadata->HasObjectPtr()) {
+    if (std::optional<ClangASTMetadata> metadata =
+            TypeSystemClang::DeclContextGetMetaData(decl_context,
+                                                    function_decl);
+        metadata && metadata->HasObjectPtr()) {
       lldb::LanguageType language = metadata->GetObjectPtrLanguage();
       if (language == lldb::eLanguageTypeC_plus_plus) {
         if (m_enforce_valid_object) {

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCDeclVendor.cpp

@@ -398,9 +398,9 @@ bool AppleObjCDeclVendor::FinishDecl(clang::ObjCInterfaceDecl *interface_decl) {
   Log *log(
       GetLog(LLDBLog::Expressions)); // FIXME - a more appropriate log channel?
 
-  ClangASTMetadata *metadata = m_ast_ctx->GetMetadata(interface_decl);
   ObjCLanguageRuntime::ObjCISA objc_isa = 0;
-  if (metadata)
+  if (std::optional<ClangASTMetadata> metadata =
+          m_ast_ctx->GetMetadata(interface_decl))
     objc_isa = metadata->GetISAPtr();
 
   if (!objc_isa)
@@ -559,8 +559,8 @@ uint32_t AppleObjCDeclVendor::FindDecls(ConstString name, bool append,
               ast_ctx.getObjCInterfaceType(result_iface_decl);
 
           uint64_t isa_value = LLDB_INVALID_ADDRESS;
-          ClangASTMetadata *metadata = m_ast_ctx->GetMetadata(result_iface_decl);
-          if (metadata)
+          if (std::optional<ClangASTMetadata> metadata =
+                  m_ast_ctx->GetMetadata(result_iface_decl))
             isa_value = metadata->GetISAPtr();
 
           LLDB_LOGF(log,

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -1787,8 +1787,8 @@ bool TypeSystemClang::RecordHasFields(const RecordDecl *record_decl) {
   // -flimit-debug-info instead of just seeing nothing if this is a base class
   // (since we were hiding empty base classes), or nothing when you turn open
   // an valiable whose type was incomplete.
-  ClangASTMetadata *meta_data = GetMetadata(record_decl);
-  if (meta_data && meta_data->IsForcefullyCompleted())
+  if (std::optional<ClangASTMetadata> meta_data = GetMetadata(record_decl);
+      meta_data && meta_data->IsForcefullyCompleted())
     return true;
 
   return false;
@@ -2465,27 +2465,31 @@ void TypeSystemClang::SetMetadataAsUserID(const clang::Type *type,
 }
 
 void TypeSystemClang::SetMetadata(const clang::Decl *object,
-                                  ClangASTMetadata &metadata) {
+                                  ClangASTMetadata metadata) {
   m_decl_metadata[object] = metadata;
 }
 
 void TypeSystemClang::SetMetadata(const clang::Type *object,
-                                  ClangASTMetadata &metadata) {
+                                  ClangASTMetadata metadata) {
   m_type_metadata[object] = metadata;
 }
 
-ClangASTMetadata *TypeSystemClang::GetMetadata(const clang::Decl *object) {
+std::optional<ClangASTMetadata>
+TypeSystemClang::GetMetadata(const clang::Decl *object) {
   auto It = m_decl_metadata.find(object);
   if (It != m_decl_metadata.end())
-    return &It->second;
-  return nullptr;
+    return It->second;
+
+  return std::nullopt;
 }
 
-ClangASTMetadata *TypeSystemClang::GetMetadata(const clang::Type *object) {
+std::optional<ClangASTMetadata>
+TypeSystemClang::GetMetadata(const clang::Type *object) {
   auto It = m_type_metadata.find(object);
   if (It != m_type_metadata.end())
-    return &It->second;
-  return nullptr;
+    return It->second;
+
+  return std::nullopt;
 }
 
 void TypeSystemClang::SetCXXRecordDeclAccess(const clang::CXXRecordDecl *object,
@@ -2934,9 +2938,10 @@ bool TypeSystemClang::IsRuntimeGeneratedType(
   clang::ObjCInterfaceDecl *result_iface_decl =
       llvm::dyn_cast<clang::ObjCInterfaceDecl>(decl_ctx);
 
-  ClangASTMetadata *ast_metadata = GetMetadata(result_iface_decl);
+  std::optional<ClangASTMetadata> ast_metadata = GetMetadata(result_iface_decl);
   if (!ast_metadata)
     return false;
+
   return (ast_metadata->GetISAPtr() != 0);
 }
 
@@ -3622,8 +3627,8 @@ bool TypeSystemClang::IsPossibleDynamicType(lldb::opaque_compiler_type_t type,
             if (is_complete)
               success = cxx_record_decl->isDynamicClass();
             else {
-              ClangASTMetadata *metadata = GetMetadata(cxx_record_decl);
-              if (metadata)
+              if (std::optional<ClangASTMetadata> metadata =
+                      GetMetadata(cxx_record_decl))
                 success = metadata->GetIsDynamicCXXType();
               else {
                 is_complete = GetType(pointee_qual_type).GetCompleteType();
@@ -5326,7 +5331,8 @@ GetDynamicArrayInfo(TypeSystemClang &ast, SymbolFile *sym_file,
                     clang::QualType qual_type,
                     const ExecutionContext *exe_ctx) {
   if (qual_type->isIncompleteArrayType())
-    if (auto *metadata = ast.GetMetadata(qual_type.getTypePtr()))
+    if (std::optional<ClangASTMetadata> metadata =
+            ast.GetMetadata(qual_type.getTypePtr()))
       return sym_file->GetDynamicArrayInfoForUID(metadata->GetUserID(),
                                                  exe_ctx);
   return std::nullopt;
@@ -8868,8 +8874,7 @@ void TypeSystemClang::DumpTypeDescription(lldb::opaque_compiler_type_t type,
 
   CompilerType ct(weak_from_this(), type);
   const clang::Type *clang_type = ClangUtil::GetQualType(ct).getTypePtr();
-  ClangASTMetadata *metadata = GetMetadata(clang_type);
-  if (metadata) {
+  if (std::optional<ClangASTMetadata> metadata = GetMetadata(clang_type)) {
     metadata->Dump(&s);
   }
 }
@@ -9490,7 +9495,7 @@ bool TypeSystemClang::DeclContextIsClassMethod(void *opaque_decl_ctx) {
     return true;
   } else if (clang::FunctionDecl *fun_decl =
                  llvm::dyn_cast<clang::FunctionDecl>(decl_ctx)) {
-    if (ClangASTMetadata *metadata = GetMetadata(fun_decl))
+    if (std::optional<ClangASTMetadata> metadata = GetMetadata(fun_decl))
       return metadata->HasObjectPtr();
   }
 
@@ -9543,7 +9548,7 @@ TypeSystemClang::DeclContextGetLanguage(void *opaque_decl_ctx) {
   } else if (llvm::isa<clang::CXXMethodDecl>(decl_ctx)) {
     return eLanguageTypeC_plus_plus;
   } else if (auto *fun_decl = llvm::dyn_cast<clang::FunctionDecl>(decl_ctx)) {
-    if (ClangASTMetadata *metadata = GetMetadata(fun_decl))
+    if (std::optional<ClangASTMetadata> metadata = GetMetadata(fun_decl))
       return metadata->GetObjectPtrLanguage();
   }
 
@@ -9593,7 +9598,7 @@ TypeSystemClang::DeclContextGetAsNamespaceDecl(const CompilerDeclContext &dc) {
   return nullptr;
 }
 
-ClangASTMetadata *
+std::optional<ClangASTMetadata>
 TypeSystemClang::DeclContextGetMetaData(const CompilerDeclContext &dc,
                                         const Decl *object) {
   TypeSystemClang *ast = llvm::cast<TypeSystemClang>(dc.GetTypeSystem());
@@ -9827,8 +9832,7 @@ bool TypeSystemClang::IsForcefullyCompleted(lldb::opaque_compiler_type_t type) {
     if (record_type) {
       const clang::RecordDecl *record_decl = record_type->getDecl();
       assert(record_decl);
-      ClangASTMetadata *metadata = GetMetadata(record_decl);
-      if (metadata)
+      if (std::optional<ClangASTMetadata> metadata = GetMetadata(record_decl))
         return metadata->IsForcefullyCompleted();
     }
   }
@@ -9838,11 +9842,13 @@ bool TypeSystemClang::IsForcefullyCompleted(lldb::opaque_compiler_type_t type) {
 bool TypeSystemClang::SetDeclIsForcefullyCompleted(const clang::TagDecl *td) {
   if (td == nullptr)
     return false;
-  ClangASTMetadata *metadata = GetMetadata(td);
-  if (metadata == nullptr)
+  std::optional<ClangASTMetadata> metadata = GetMetadata(td);
+  if (!metadata)
     return false;
   m_has_forcefully_completed_types = true;
   metadata->SetIsForcefullyCompleted();
+  SetMetadata(td, *metadata);
+
   return true;
 }
 

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h

@@ -191,11 +191,11 @@ public:
   void SetMetadataAsUserID(const clang::Decl *decl, lldb::user_id_t user_id);
   void SetMetadataAsUserID(const clang::Type *type, lldb::user_id_t user_id);
 
-  void SetMetadata(const clang::Decl *object, ClangASTMetadata &meta_data);
+  void SetMetadata(const clang::Decl *object, ClangASTMetadata meta_data);
 
-  void SetMetadata(const clang::Type *object, ClangASTMetadata &meta_data);
-  ClangASTMetadata *GetMetadata(const clang::Decl *object);
-  ClangASTMetadata *GetMetadata(const clang::Type *object);
+  void SetMetadata(const clang::Type *object, ClangASTMetadata meta_data);
+  std::optional<ClangASTMetadata> GetMetadata(const clang::Decl *object);
+  std::optional<ClangASTMetadata> GetMetadata(const clang::Type *object);
 
   void SetCXXRecordDeclAccess(const clang::CXXRecordDecl *object,
                               clang::AccessSpecifier access);
@@ -616,8 +616,9 @@ public:
   static clang::NamespaceDecl *
   DeclContextGetAsNamespaceDecl(const CompilerDeclContext &dc);
 
-  static ClangASTMetadata *DeclContextGetMetaData(const CompilerDeclContext &dc,
-                                                  const clang::Decl *object);
+  static std::optional<ClangASTMetadata>
+  DeclContextGetMetaData(const CompilerDeclContext &dc,
+                         const clang::Decl *object);
 
   static clang::ASTContext *
   DeclContextGetTypeSystemClang(const CompilerDeclContext &dc);

lldb/unittests/Symbol/TestClangASTImporter.cpp

@@ -188,7 +188,7 @@ TEST_F(TestClangASTImporter, MetadataPropagation) {
   ASSERT_NE(nullptr, imported);
 
   // Check that we got the same Metadata.
-  ASSERT_NE(nullptr, importer.GetDeclMetadata(imported));
+  ASSERT_NE(std::nullopt, importer.GetDeclMetadata(imported));
   EXPECT_EQ(metadata, importer.GetDeclMetadata(imported)->GetUserID());
 }
 
@@ -219,7 +219,7 @@ TEST_F(TestClangASTImporter, MetadataPropagationIndirectImport) {
   ASSERT_NE(nullptr, imported);
 
   // Check that we got the same Metadata.
-  ASSERT_NE(nullptr, importer.GetDeclMetadata(imported));
+  ASSERT_NE(std::nullopt, importer.GetDeclMetadata(imported));
   EXPECT_EQ(metadata, importer.GetDeclMetadata(imported)->GetUserID());
 }
 
@@ -244,7 +244,7 @@ TEST_F(TestClangASTImporter, MetadataPropagationAfterCopying) {
   source.ast->SetMetadataAsUserID(source.record_decl, metadata);
 
   // Check that we got the same Metadata.
-  ASSERT_NE(nullptr, importer.GetDeclMetadata(imported));
+  ASSERT_NE(std::nullopt, importer.GetDeclMetadata(imported));
   EXPECT_EQ(metadata, importer.GetDeclMetadata(imported)->GetUserID());
 }