ClangImporter: Test that PCHs are actually cached in memory

Follow up 236fc19121 with a ClangImporter
test that we get the PCH written by this process even if it has been
replaced on disk.

unittests/CMakeLists.txt

@@ -6,6 +6,7 @@ if(SWIFT_INCLUDE_TOOLS)
 
   add_subdirectory(AST)
   add_subdirectory(Basic)
+  add_subdirectory(ClangImporter)
   add_subdirectory(Driver)
   add_subdirectory(FrontendTool)
   add_subdirectory(IDE)

unittests/ClangImporter/CMakeLists.txt

@@ -0,0 +1,10 @@
+add_swift_unittest(SwiftClangImporterTests
+  ClangImporterTests.cpp
+)
+
+target_link_libraries(SwiftClangImporterTests
+    PRIVATE
+    swiftClangImporter
+    swiftParse
+    swiftAST
+)

unittests/ClangImporter/ClangImporterTests.cpp

@@ -0,0 +1,89 @@
+#include "swift/AST/ASTContext.h"
+#include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/SearchPathOptions.h"
+#include "swift/Basic/LLVMInitialize.h"
+#include "swift/Basic/LangOptions.h"
+#include "swift/Basic/SourceManager.h"
+#include "swift/ClangImporter/ClangImporter.h"
+#include "swift/ClangImporter/ClangImporterOptions.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+
+using namespace swift;
+
+static std::string createFilename(StringRef base, StringRef name) {
+  SmallString<256> path = base;
+  llvm::sys::path::append(path, name);
+  return llvm::Twine(path).str();
+}
+
+static bool emitFileWithContents(StringRef path, StringRef contents,
+                                 std::string *pathOut = nullptr) {
+  int FD;
+  if (llvm::sys::fs::openFileForWrite(path, FD))
+    return true;
+  if (pathOut)
+    *pathOut = path;
+  llvm::raw_fd_ostream file(FD, /*shouldClose=*/true);
+  file << contents;
+  return false;
+}
+
+static bool emitFileWithContents(StringRef base, StringRef name,
+                                 StringRef contents,
+                                 std::string *pathOut = nullptr) {
+  return emitFileWithContents(createFilename(base, name), contents, pathOut);
+}
+
+TEST(ClangImporterTest, emitPCHInMemory) {
+  // Create a temporary cache on disk and clean it up at the end.
+  ClangImporterOptions options;
+  SmallString<256> temp;
+  ASSERT_FALSE(llvm::sys::fs::createUniqueDirectory(
+      "ClangImporterTest.emitPCHInMemory", temp));
+  SWIFT_DEFER { llvm::sys::fs::remove_directories(temp); };
+
+  // Create a cache subdirectory for the modules and PCH.
+  std::string cache = createFilename(temp, "cache");
+  ASSERT_FALSE(llvm::sys::fs::create_directory(cache));
+  options.ModuleCachePath = cache;
+  options.PrecompiledHeaderOutputDir = cache;
+
+  // Create the includes.
+  std::string include = createFilename(temp, "include");
+  ASSERT_FALSE(llvm::sys::fs::create_directory(include));
+  options.ExtraArgs.emplace_back("-nosysteminc");
+  options.ExtraArgs.emplace_back((llvm::Twine("-I") + include).str());
+  ASSERT_FALSE(emitFileWithContents(include, "module.modulemap",
+                                    "module A {\n"
+                                    "  header \"A.h\"\n"
+                                    "}\n"));
+  ASSERT_FALSE(emitFileWithContents(include, "A.h", "int foo(void);\n"));
+
+  // Create a bridging header.
+  ASSERT_FALSE(emitFileWithContents(temp, "bridging.h", "#import <A.h>\n",
+                                    &options.BridgingHeader));
+
+  // Set up the importer and emit a bridging PCH.
+  swift::LangOptions langOpts;
+  langOpts.Target = llvm::Triple("x86_64", "apple", "darwin");
+  INITIALIZE_LLVM();
+  swift::SearchPathOptions searchPathOpts;
+  swift::SourceManager sourceMgr;
+  swift::DiagnosticEngine diags(sourceMgr);
+  std::unique_ptr<ASTContext> context(
+      ASTContext::get(langOpts, searchPathOpts, sourceMgr, diags));
+  auto importer = ClangImporter::create(*context, options);
+
+  std::string PCH = createFilename(cache, "bridging.h.pch");
+  ASSERT_FALSE(importer->canReadPCH(PCH));
+  ASSERT_FALSE(importer->emitBridgingPCH(options.BridgingHeader, PCH));
+  ASSERT_TRUE(importer->canReadPCH(PCH));
+
+  // Overwrite the PCH with garbage.  We should still be able to read it from
+  // the in-memory cache.
+  ASSERT_FALSE(emitFileWithContents(PCH, "garbage"));
+  ASSERT_TRUE(importer->canReadPCH(PCH));
+}