feat(cxx_indexer): add alias-taking semantic (#5540)

* feat(cxx_indexer): add alias-taking semantic

kythe/cxx/common/kythe_metadata_file.cc

@@ -141,6 +141,9 @@ absl::optional<MetadataFile::Rule> MetadataFile::LoadMetaElement(
       case MappingRule::SEMA_READ_WRITE:
         sema = Semantic::kReadWrite;
         break;
+      case MappingRule::SEMA_TAKE_ALIAS:
+        sema = Semantic::kTakeAlias;
+        break;
       default:
         sema = Semantic::kNone;
     }

kythe/cxx/common/kythe_metadata_file.h

@@ -35,6 +35,7 @@ class MetadataFile {
     kNone,       ///< No special semantics.
     kWrite,      ///< Write semantics.
     kReadWrite,  ///< Read+write semantics.
+    kTakeAlias,  ///< Alias-taking semantics.
   };
 
   /// \brief A single metadata rule.

kythe/cxx/common/protobuf_metadata_file.cc

@@ -100,6 +100,7 @@ std::unique_ptr<kythe::MetadataFile> ProtobufMetadataSupport::ParseFile(
           absl::StartsWith(token, "release_")) {
         rule.semantic = kythe::MetadataFile::Semantic::kWrite;
       } else if (absl::StartsWith(token, "mutable_")) {
+        // TODO(zarko): kTakeAlias in a PR after #5538
         rule.semantic = kythe::MetadataFile::Semantic::kReadWrite;
       }
     }

kythe/cxx/indexer/cxx/GraphObserver.h

@@ -313,6 +313,8 @@ class GraphObserver {
     kWrite,
     /// This use site is both a read and a write.
     kReadWrite,
+    /// This use site takes an alias.
+    kTakeAlias,
   };
 
   GraphObserver() {}

kythe/cxx/indexer/cxx/IndexerASTHooks.cc

@@ -5905,6 +5905,9 @@ void IndexerASTVisitor::PrepareAlternateSemanticCache() {
         case MetadataFile::Semantic::kReadWrite:
           kind = GraphObserver::UseKind::kReadWrite;
           break;
+        case MetadataFile::Semantic::kTakeAlias:
+          kind = GraphObserver::UseKind::kTakeAlias;
+          break;
         default:
           break;
       }

kythe/cxx/indexer/cxx/IndexerASTHooks.h

@@ -1020,6 +1020,15 @@ class IndexerASTVisitor : public RecursiveTypeVisitor<IndexerASTVisitor> {
     return stmt;
   }
 
+  /// \brief Marks that `expr` was used as an alias target.
+  /// \return `expr` as passed.
+  const clang::Expr* UsedAsAlias(const clang::Expr* expr) {
+    if (expr != nullptr)
+      use_kinds_[SkipTrivialAliasing(expr)] =
+          GraphObserver::UseKind::kTakeAlias;
+    return expr;
+  }
+
   /// \brief Maps known Decls to their NodeIds.
   llvm::DenseMap<const clang::Decl*, GraphObserver::NodeId> DeclToNodeId;
 

kythe/proto/metadata.proto

@@ -67,6 +67,7 @@ message MappingRule {
     SEMA_NONE = 0;
     SEMA_WRITE = 1;
     SEMA_READ_WRITE = 2;
+    SEMA_TAKE_ALIAS = 3;
   };
 
   Semantic semantic = 11;