From 2f399445f88c34e7377596f83fbb45dda7b50e8f Mon Sep 17 00:00:00 2001
From: Artem Chikin <achikin@apple.com>
Date: Tue, 14 Nov 2023 13:19:02 -0800
Subject: [PATCH] Add fallback to auxiliary legacy driver executable on
 '--disallow-use-new-driver'

With the swift-driver executable becoming the default target of 'swift' and 'swiftc' sumlinks, we need to allow for the prior mechanism afforded by this flag
---
 Sources/swift-driver/main.swift               | 14 ++++++++++++++
 Tests/SwiftDriverTests/IntegrationTests.swift |  1 +
 2 files changed, 15 insertions(+)

diff --git a/Sources/swift-driver/main.swift b/Sources/swift-driver/main.swift
index 69448fed5..f16055ede 100644
--- a/Sources/swift-driver/main.swift
+++ b/Sources/swift-driver/main.swift
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 import SwiftDriverExecution
 import SwiftDriver
+import SwiftOptions
 #if os(Windows)
 import CRT
 #elseif os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
@@ -65,6 +66,19 @@ do {
   }
   interruptSignalSource.resume()
 
+  // Fallback to legacy driver if forced to
+  if CommandLine.arguments.contains(Option.disallowForwardingDriver.spelling) ||
+     ProcessEnv.vars["SWIFT_USE_OLD_DRIVER"] != nil {
+    if let legacyExecutablePath = Process.findExecutable(CommandLine.arguments[0] + "-legacy-driver"),
+       localFileSystem.exists(legacyExecutablePath) {
+      let legacyDriverCommand = [legacyExecutablePath.pathString] +
+                                  CommandLine.arguments[1...]
+      try exec(path: legacyExecutablePath.pathString, args: legacyDriverCommand)
+    } else {
+      throw Driver.Error.unknownOrMissingSubcommand(CommandLine.arguments[0] + "-legacy-driver")
+    }
+  }
+
   if ProcessEnv.vars["SWIFT_ENABLE_EXPLICIT_MODULE"] != nil {
     CommandLine.arguments.append("-explicit-module-build")
   }
diff --git a/Tests/SwiftDriverTests/IntegrationTests.swift b/Tests/SwiftDriverTests/IntegrationTests.swift
index 414d888d5..17be025c4 100644
--- a/Tests/SwiftDriverTests/IntegrationTests.swift
+++ b/Tests/SwiftDriverTests/IntegrationTests.swift
@@ -224,6 +224,7 @@ final class IntegrationTests: IntegrationTestCase {
       let extraEnv = [
         "SWIFT": swift.pathString,
         "SWIFTC": swiftc.pathString,
+        "SWIFT_FORCE_TEST_NEW_DRIVER": "1",
         "SWIFT_DRIVER_SWIFT_EXEC": swiftFile.pathString,
         "SWIFT_DRIVER_SWIFT_FRONTEND_EXEC": frontendFile.pathString,
         "LC_ALL": "en_US.UTF-8"