Driver: forwarding driver invocations to the new driver when SWIFT_US…

…E_NEW_DRIVER is defined

This allows us to experiment with the new swift-driver without changing build systems.

include/swift/AST/DiagnosticsDriver.def

@@ -190,5 +190,7 @@ WARNING(warn_drv_darwin_sdk_invalid_settings, none,
     "SDK settings were ignored because 'SDKSettings.json' could not be parsed",
     ())
 
+REMARK(remark_forwarding_to_new_driver, none, "new Swift driver will be used", ())
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

tools/driver/driver.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/DiagnosticsDriver.h"
 #include "swift/Basic/LLVMInitialize.h"
 #include "swift/Basic/PrettyStackTrace.h"
 #include "swift/Basic/Program.h"
@@ -159,6 +160,35 @@ static int run_driver(StringRef ExecName,
   DiagnosticEngine Diags(SM);
   Diags.addConsumer(PDC);
 
+  // Forwarding calls to the swift driver if the C++ driver is invoked as `swift`
+  // or `swiftc`, and an environment variable SWIFT_USE_NEW_DRIVER is defined.
+  if (llvm::sys::Process::GetEnv("SWIFT_USE_NEW_DRIVER") &&
+      (ExecName == "swift" || ExecName == "swiftc")) {
+    SmallString<256> NewDriverPath(llvm::sys::path::parent_path(Path));
+    llvm::sys::path::append(NewDriverPath, "swift-driver");
+    SmallVector<const char *, 256> subCommandArgs;
+    // Rewrite the program argument.
+    subCommandArgs.push_back(NewDriverPath.c_str());
+    if (ExecName == "swiftc") {
+      subCommandArgs.push_back("--driver-mode=swiftc");
+    } else {
+      assert(ExecName == "swift");
+      subCommandArgs.push_back("--driver-mode=swift");
+    }
+    subCommandArgs.insert(subCommandArgs.end(), argv.begin() + 1, argv.end());
+
+    // Execute the subcommand.
+    subCommandArgs.push_back(nullptr);
+    Diags.diagnose(SourceLoc(), diag::remark_forwarding_to_new_driver);
+    ExecuteInPlace(NewDriverPath.c_str(), subCommandArgs.data());
+
+    // If we reach here then an error occurred (typically a missing path).
+    std::string ErrorString = llvm::sys::StrError();
+    llvm::errs() << "error: unable to invoke subcommand: " << subCommandArgs[0]
+                 << " (" << ErrorString << ")\n";
+    return 2;
+  }
+
   Driver TheDriver(Path, ExecName, argv, Diags);
   switch (TheDriver.getDriverKind()) {
   case Driver::DriverKind::AutolinkExtract: