Serialization: Consider C++ interop use in module cache hash, preventing collisions

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2880,7 +2880,11 @@ static std::string getContextHash(const CompilerInvocation &CI,
       //
       // If OSSA modules are enabled, we use a separate namespace of modules to
       // ensure that we compile all swift interface files with the option set.
-      unsigned(CI.getSILOptions().EnableOSSAModules));
+      unsigned(CI.getSILOptions().EnableOSSAModules),
+
+      // Is the C++ interop enabled?
+      unsigned(CI.getLangOptions().EnableCXXInterop)
+  );
 
   return llvm::toString(llvm::APInt(64, H), 36, /*Signed=*/false);
 }

test/ModuleInterface/ModuleCache/cxx-interop-caching.swift

@@ -0,0 +1,35 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t --leading-lines
+
+/// Build one library module
+// RUN: %target-swift-frontend -parse-stdlib -emit-module \
+// RUN:   -o %t/Lib.swiftmodule %t/Lib.swift \
+// RUN:   -emit-module-interface-path %t/Lib.swiftinterface \
+// RUN:   -swift-version 5 -enable-library-evolution -module-name Lib
+
+/// Distributed scenario with only the swiftinterface
+// RUN: rm %t/Lib.swiftmodule
+// RUN: %empty-directory(%t/ModuleCache)
+
+/// Rebuild from the swiftinterface just once for non-C++ users
+// RUN: %target-swift-frontend -parse-stdlib -typecheck %t/Rebuild.swift -I %t \
+// RUN:   -module-cache-path %t/ModuleCache -Rmodule-interface-rebuild -verify
+// RUN: %target-swift-frontend -parse-stdlib -typecheck %t/UseExisting.swift -I %t \
+// RUN:   -module-cache-path %t/ModuleCache -Rmodule-interface-rebuild -verify
+
+/// Rebuild from the swiftinterface just once for C++ users
+// RUN: %target-swift-frontend -parse-stdlib -typecheck %t/Rebuild.swift -I %t \
+// RUN:   -module-cache-path %t/ModuleCache -Rmodule-interface-rebuild -verify \
+// RUN:   -cxx-interoperability-mode=default
+// RUN: %target-swift-frontend -parse-stdlib -typecheck %t/UseExisting.swift -I %t \
+// RUN:   -module-cache-path %t/ModuleCache -Rmodule-interface-rebuild -verify \
+// RUN:   -cxx-interoperability-mode=default
+
+//--- Lib.swift
+public func publicFunction() {}
+
+//--- UseExisting.swift
+import Lib
+
+//--- Rebuild.swift
+import Lib // expected-remark {{rebuilding module 'Lib' from interface}}