[ASTUnit] Use createPhysicalFileSystem if no VFS was provided

Use `createPhysicalFileSystem` instead of `getRealFileSystem` to
avoid affecting the process' current working directory, and set it
at the top of `ASTUnit::LoadFromCommandLine` such that the driver
used for argument parsing and the ASTUnit share the same VFS. This
ensures that '-working-directory' correctly sets the VFS working
directory in addition to the FileManager working directory.

Author: hamishknight

clang/lib/Frontend/ASTUnit.cpp

@@ -1752,6 +1752,12 @@ ASTUnit *ASTUnit::LoadFromCommandLine(
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
   assert(Diags.get() && "no DiagnosticsEngine was provided");
 
+  // If no VFS was provided, create one that tracks the physical file system.
+  // If '-working-directory' was passed as an argument, 'createInvocation' will
+  // set this as the current working directory of the VFS.
+  if (!VFS)
+    VFS = llvm::vfs::createPhysicalFileSystem();
+
   SmallVector<StoredDiagnostic, 4> StoredDiagnostics;
 
   std::shared_ptr<CompilerInvocation> CI;
@@ -1801,8 +1807,6 @@ ASTUnit *ASTUnit::LoadFromCommandLine(
   ConfigureDiags(Diags, *AST, CaptureDiagnostics);
   AST->Diagnostics = Diags;
   AST->FileSystemOpts = CI->getFileSystemOpts();
-  if (!VFS)
-    VFS = llvm::vfs::getRealFileSystem();
   VFS = createVFSFromCompilerInvocation(*CI, *Diags, VFS);
   AST->FileMgr = new FileManager(AST->FileSystemOpts, VFS);
   AST->ModuleCache = new InMemoryModuleCache;

clang/unittests/Frontend/ASTUnitTest.cpp

@@ -178,4 +178,36 @@ TEST_F(ASTUnitTest, LoadFromCommandLineEarlyError) {
   ASSERT_NE(ErrUnit->stored_diag_size(), 0U);
 }
 
+TEST_F(ASTUnitTest, LoadFromCommandLineWorkingDirectory) {
+  EXPECT_FALSE(
+      llvm::sys::fs::createTemporaryFile("bar", "c", FD, InputFileName));
+  auto Input = std::make_unique<ToolOutputFile>(InputFileName, FD);
+  Input->os() << "";
+
+  SmallString<128> WorkingDir;
+  ASSERT_FALSE(sys::fs::createUniqueDirectory("foo", WorkingDir));
+  const char *Args[] = {"clang", "-working-directory", WorkingDir.c_str(),
+                        InputFileName.c_str()};
+
+  auto Diags = CompilerInstance::createDiagnostics(new DiagnosticOptions());
+  auto PCHContainerOps = std::make_shared<PCHContainerOperations>();
+  std::unique_ptr<clang::ASTUnit> ErrUnit;
+
+  auto *AST = ASTUnit::LoadFromCommandLine(
+      &Args[0], &Args[4], PCHContainerOps, Diags, "", false,
+      CaptureDiagsKind::All, None, true, 0, TU_Complete, false, false, false,
+      SkipFunctionBodiesScope::None, false, true, false, false, None, &ErrUnit,
+      nullptr);
+
+  ASSERT_NE(AST, nullptr);
+  ASSERT_FALSE(Diags->hasErrorOccurred());
+
+  // Make sure '-working-directory' sets both the FileSystemOpts and underlying
+  // VFS working directory.
+  const auto &FM = AST->getFileManager();
+  const auto &VFS = FM.getVirtualFileSystem();
+  ASSERT_EQ(*VFS.getCurrentWorkingDirectory(), WorkingDir.str());
+  ASSERT_EQ(FM.getFileSystemOpts().WorkingDir, WorkingDir.str());
+}
+
 } // anonymous namespace