driver: forward driver invocation to the new driver by default

include/swift/AST/DiagnosticsDriver.def

@@ -193,8 +193,5 @@ WARNING(warn_drv_darwin_sdk_invalid_settings, none,
 REMARK(remark_forwarding_to_new_driver, none,
        "new Swift driver at '%0' will be used", (StringRef))
 
-REMARK(remark_forwarding_driver_not_there, none,
-      "new Swift driver at '%0' cannot be found; C++ driver will be used", (StringRef))
-
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

test/Driver/opt-record.swift

@@ -8,7 +8,8 @@
 // RUN: %target-swiftc_driver -O -wmo -save-optimization-record -save-optimization-record-passes unknown -save-optimization-record-path %t/optrecordmod-filtered.opt.yaml %s -module-name optrecordmod -o %t/opt-record 2>&1 | %FileCheck -allow-empty %s
 // RUN: %FileCheck -allow-empty -check-prefix=YAML-FILTERED %s < %t/optrecordmod-filtered.opt.yaml
 
-// CHECK-NOT: remark
+// CHECK-NOT: warning
+// CHECK-NOT: error
 
 var a: Int = 1
 

test/Driver/tools_directory.swift

@@ -6,18 +6,18 @@
 // RUN: %swiftc_driver -### -target x86_64-linux-unknown -tools-directory /Something/obviously/fake %s 2>&1 | %FileCheck -check-prefix BINUTILS %s
 
 // CLANGSUB: swift
-// CLANGSUB-SAME: -o [[OBJECTFILE:.*]]
+// CLANGSUB: -o [[OBJECTFILE:.*]]
 // CLANGSUB: swift-autolink-extract{{(\.exe)?"?}} [[OBJECTFILE]]
-// CLANGSUB-SAME: -o {{"?}}[[AUTOLINKFILE:.*]]
+// CLANGSUB: -o {{"?}}[[AUTOLINKFILE:.*]]
 // CLANGSUB: {{[^ ]+(\\\\|/)}}Inputs{{/|\\\\}}fake-toolchain{{/|\\\\}}clang
 // CLANGSUB-DAG: [[OBJECTFILE]]
 // CLANGSUB-DAG: @[[AUTOLINKFILE]]
 // CLANGSUB: -o tools_directory
 
 // BINUTILS: swift
-// BINUTILS-SAME: -o [[OBJECTFILE:.*]]
+// BINUTILS: -o [[OBJECTFILE:.*]]
 // BINUTILS: swift-autolink-extract{{(\.exe)?"?}} [[OBJECTFILE]]
-// BINUTILS-SAME: -o {{"?}}[[AUTOLINKFILE:.*]]
+// BINUTILS: -o {{"?}}[[AUTOLINKFILE:.*]]
 // BINUTILS: clang
 // BINUTILS-DAG: [[OBJECTFILE]]
 // BINUTILS-DAG: @[[AUTOLINKFILE]]
@@ -31,6 +31,6 @@
 // RUN: %swiftc_driver -### -target x86_64-apple-macosx10.9 -tools-directory %S/Inputs/fake-toolchain %s 2>&1 | %FileCheck -check-prefix LDSUB %s
 
 // LDSUB: swift
-// LDSUB-SAME: -o [[OBJECTFILE:.*]]
+// LDSUB: -o [[OBJECTFILE:.*]]
 // LDSUB: {{[^ ]+(\\\\|/)}}Inputs{{/|\\\\}}fake-toolchain{{(\\\\|/)ld"?}} [[OBJECTFILE]]
 // LDSUB: -o tools_directory

tools/driver/driver.cpp

@@ -126,6 +126,24 @@ static bool shouldRunAsSubcommand(StringRef ExecName,
   return true;
 }
 
+static bool shouldDisallowNewDriver(StringRef ExecName,
+                                    const ArrayRef<const char *> argv) {
+  // We are not invoking the driver, so don't forward.
+  if (ExecName != "swift" && ExecName != "swiftc") {
+    return true;
+  }
+  // If user specified using the old driver, don't forward.
+  if (llvm::find_if(argv, [](const char* arg) {
+    return StringRef(arg) == "-disallow-use-new-driver";
+  }) != argv.end()) {
+    return true;
+  }
+  if (llvm::sys::Process::GetEnv("SWIFT_USE_OLD_DRIVER").hasValue()) {
+    return true;
+  }
+  return false;
+}
+
 static int run_driver(StringRef ExecName,
                        const ArrayRef<const char *> argv) {
   // Handle integrated tools.
@@ -160,23 +178,16 @@ static int run_driver(StringRef ExecName,
   DiagnosticEngine Diags(SM);
   Diags.addConsumer(PDC);
 
-  std::string newDriverName;
+  std::string newDriverName = "swift-driver-new";
   if (auto driverNameOp = llvm::sys::Process::GetEnv("SWIFT_USE_NEW_DRIVER")) {
     newDriverName = driverNameOp.getValue();
   }
-  auto disallowForwarding = llvm::find_if(argv, [](const char* arg) {
-    return StringRef(arg) == "-disallow-use-new-driver";
-  }) != argv.end();
   // 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 (!newDriverName.empty() && !disallowForwarding &&
-      (ExecName == "swift" || ExecName == "swiftc")) {
+  if (!shouldDisallowNewDriver(ExecName, argv)) {
     SmallString<256> NewDriverPath(llvm::sys::path::parent_path(Path));
     llvm::sys::path::append(NewDriverPath, newDriverName);
-    if (!llvm::sys::fs::exists(NewDriverPath)) {
-      Diags.diagnose(SourceLoc(), diag::remark_forwarding_driver_not_there,
-                     NewDriverPath);
-    } else {
+    if (llvm::sys::fs::exists(NewDriverPath)) {
       SmallVector<const char *, 256> subCommandArgs;
       // Rewrite the program argument.
       subCommandArgs.push_back(NewDriverPath.c_str());