[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, 53 insertions, 0 deletions

diff --git a/clang/unittests/Frontend/CompilerInvocationTest.cpp b/clang/unittests/Frontend/CompilerInvocationTest.cpp
index b7f9ced..89e024d 100644
--- a/clang/unittests/Frontend/CompilerInvocationTest.cpp
+++ b/clang/unittests/Frontend/CompilerInvocationTest.cpp
@@ -11,6 +11,7 @@
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/TextDiagnosticBuffer.h"
 #include "clang/Lex/PreprocessorOptions.h"
+#include "clang/Serialization/ModuleFileExtension.h"
 #include "llvm/Support/Host.h"
 
 #include "gmock/gmock.h"
@@ -743,6 +744,58 @@ TEST_F(CommandLineTest, DigraphsEnabled) {
   ASSERT_THAT(GeneratedArgs, Contains(StrEq("-fdigraphs")));
 }
 
+struct DummyModuleFileExtension
+    : public llvm::RTTIExtends<DummyModuleFileExtension, ModuleFileExtension> {
+  static char ID;
+
+  ModuleFileExtensionMetadata getExtensionMetadata() const override {
+    return {};
+  };
+
+  llvm::hash_code hashExtension(llvm::hash_code Code) const override {
+    return {};
+  }
+
+  std::unique_ptr<ModuleFileExtensionWriter>
+  createExtensionWriter(ASTWriter &Writer) override {
+    return {};
+  }
+
+  std::unique_ptr<ModuleFileExtensionReader>
+  createExtensionReader(const ModuleFileExtensionMetadata &Metadata,
+                        ASTReader &Reader, serialization::ModuleFile &Mod,
+                        const llvm::BitstreamCursor &Stream) override {
+    return {};
+  }
+};
+
+char DummyModuleFileExtension::ID = 0;
+
+TEST_F(CommandLineTest, TestModuleFileExtension) {
+  const char *Args[] = {"-ftest-module-file-extension=first:2:1:0:first",
+                        "-ftest-module-file-extension=second:3:2:1:second"};
+
+  ASSERT_TRUE(CompilerInvocation::CreateFromArgs(Invocation, Args, *Diags));
+  ASSERT_THAT(Invocation.getFrontendOpts().ModuleFileExtensions.size(), 2);
+
+  // Exercise the check that only serializes instances of
+  // TestModuleFileExtension by providing an instance of another
+  // ModuleFileExtension subclass.
+  Invocation.getFrontendOpts().ModuleFileExtensions.push_back(
+      std::make_shared<DummyModuleFileExtension>());
+
+  Invocation.generateCC1CommandLine(GeneratedArgs, *this);
+
+  ASSERT_THAT(GeneratedArgs,
+              ContainsN(HasSubstr("-ftest-module-file-extension="), 2));
+  ASSERT_THAT(
+      GeneratedArgs,
+      Contains(StrEq("-ftest-module-file-extension=first:2:1:0:first")));
+  ASSERT_THAT(
+      GeneratedArgs,
+      Contains(StrEq("-ftest-module-file-extension=second:3:2:1:second")));
+}
+
 TEST_F(CommandLineTest, RoundTrip) {
   // Testing one marshalled and one manually generated option from each
   // CompilerInvocation member.