Index: include/clang/Tooling/AllTUsExecution.h
===================================================================
--- include/clang/Tooling/AllTUsExecution.h
+++ include/clang/Tooling/AllTUsExecution.h
@@ -45,6 +45,8 @@
 
   StringRef getExecutorName() const override { return ExecutorName; }
 
+  bool isSingleProcess() const override { return true; }
+
   using ToolExecutor::execute;
 
   llvm::Error
Index: include/clang/Tooling/Execution.h
===================================================================
--- include/clang/Tooling/Execution.h
+++ include/clang/Tooling/Execution.h
@@ -114,6 +114,13 @@
   /// Returns the name of a specific executor.
   virtual StringRef getExecutorName() const = 0;
 
+  /// Should return true iff executor runs all actions in a single process.
+  /// Clients can use this signal to find out if they can collect results
+  /// in-memory (e.g. to avoid serialization costs of using ToolResults).
+  /// The single-process executors can still run multiple threads, but all
+  /// executions are guaranteed to share the same memory.
+  virtual bool isSingleProcess() const = 0;
+
   /// Executes each action with a corresponding arguments adjuster.
   virtual llvm::Error
   execute(llvm::ArrayRef<
Index: include/clang/Tooling/StandaloneExecution.h
===================================================================
--- include/clang/Tooling/StandaloneExecution.h
+++ include/clang/Tooling/StandaloneExecution.h
@@ -52,6 +52,8 @@
 
   StringRef getExecutorName() const override { return ExecutorName; }
 
+  bool isSingleProcess() const override { return true; }
+
   using ToolExecutor::execute;
 
   llvm::Error
Index: unittests/Tooling/ExecutionTest.cpp
===================================================================
--- unittests/Tooling/ExecutionTest.cpp
+++ unittests/Tooling/ExecutionTest.cpp
@@ -96,6 +96,8 @@
 
   StringRef getExecutorName() const override { return ExecutorName; }
 
+  bool isSingleProcess() const override { return true; }
+
   llvm::Error
   execute(llvm::ArrayRef<std::pair<std::unique_ptr<FrontendActionFactory>,
                                    ArgumentsAdjuster>>) override {