[clang][modules] Use extensible RTTI for ModuleFileExtension

Clang exposes an interface for extending the PCM/PCH file format: `ModuleFileExtension`.

Clang itself has only a single implementation of the interface: `TestModuleFileExtension` that can be instantiated via the `-ftest-module-file_extension=` command line argument (and is stored in `FrontendOptions::ModuleFileExtensions`).

Clients of the Clang library can extend the PCM/PCH file format by pushing an instance of their extension class to the `FrontendOptions::ModuleFileExtensions` vector.

When generating the `-ftest-module-file_extension=` command line argument from `FrontendOptions`, a downcast is used to distinguish between the Clang's testing extension and other (client) extensions.

This functionality is enabled by LLVM-style RTTI. However, this style of RTTI is hard to extend, as it requires patching Clang (adding new case to the `ModuleFileExtensionKind` enum).

This patch switches to the LLVM RTTI for open class hierarchies, which allows libClang users (e.g. Swift) to create implementations of `ModuleFileExtension` without patching Clang. (Documentation of the feature: https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html#rtti-for-open-class-hierarchies)

Reviewed By: artemcm

Differential Revision: https://reviews.llvm.org/D97702

1 files changed, 2 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ModuleFileExtension.cpp b/clang/lib/Serialization/ModuleFileExtension.cpp
index e1ae8a4..6b7fd1d 100644
--- a/clang/lib/Serialization/ModuleFileExtension.cpp
+++ b/clang/lib/Serialization/ModuleFileExtension.cpp
@@ -9,6 +9,8 @@
 #include "llvm/ADT/Hashing.h"
 using namespace clang;
 
+char ModuleFileExtension::ID = 0;
+
 ModuleFileExtension::~ModuleFileExtension() { }
 
 llvm::hash_code ModuleFileExtension::hashExtension(llvm::hash_code Code) const {