Add -driver-force-response-files to enable testing.

Author: David Ungar

include/swift/Option/Options.td

@@ -120,6 +120,10 @@ def driver_force_one_batch_repartition : Flag<["-"], "driver-force-one-batch-rep
   InternalDebugOpt,
   HelpText<"Force one batch repartitioning for testing">;
 
+def driver_force_response_files : Flag<["-"], "driver-force-response-files">,
+  InternalDebugOpt,
+  HelpText<"Force the use of response files for testing">;
+
 def driver_always_rebuild_dependents :
   Flag<["-"], "driver-always-rebuild-dependents">, InternalDebugOpt,
   HelpText<"Always rebuild dependents of files that have been modified">;

lib/Driver/ToolChain.cpp

@@ -116,9 +116,15 @@ std::unique_ptr<Job> ToolChain::constructJob(
 
   const char *responseFilePath = nullptr;
   const char *responseFileArg = nullptr;
-  if (invocationInfo.allowsResponseFiles &&
-      !llvm::sys::commandLineFitsWithinSystemLimits(
-          executablePath, invocationInfo.Arguments)) {
+
+  const bool forceResponseFiles =
+      C.getArgs().hasArg(options::OPT_driver_force_response_files);
+  assert((invocationInfo.allowsResponseFiles || !forceResponseFiles) &&
+         "Cannot force response file if platform does not allow it");
+
+  if (forceResponseFiles || (invocationInfo.allowsResponseFiles &&
+                             !llvm::sys::commandLineFitsWithinSystemLimits(
+                                 executablePath, invocationInfo.Arguments))) {
     responseFilePath = context.getTemporaryFilePath("arguments", "resp");
     responseFileArg = C.getArgs().MakeArgString(Twine("@") + responseFilePath);
   }

test/Driver/force-response-files.swift

@@ -0,0 +1,6 @@
+// Ensure that -driver-force-response-files works.
+
+
+// RUN: %swiftc_driver -driver-force-response-files -typecheck %S/../Inputs/empty.swift -### 2>&1 | %FileCheck %s
+// CHECK: @
+// CHECK: .resp