Distro-overridable default Unix linker

Package.swift

@@ -46,13 +46,18 @@ let package = Package(
     .target(name: "CSwiftScan",
             exclude: [ "CMakeLists.txt" ]),
 
+    /// Header containing distribution-configurable defaults.
+    .target(name: "DriverDefaults",
+            exclude: [ "CMakeLists.txt" ]),
+
     /// The driver library.
     .target(
       name: "SwiftDriver",
       dependencies: [
         "SwiftOptions",
         .product(name: "SwiftToolsSupport-auto", package: "swift-tools-support-core"),
         "CSwiftScan",
+        "DriverDefaults",
         .product(name: "Yams", package: "yams"),
       ],
       exclude: ["CMakeLists.txt"]),

Sources/DriverDefaults/constants.c

@@ -0,0 +1,6 @@
+#include "constants.h"
+
+#define Q(X) #X
+#define QUOTE(X) Q(X)
+
+const char * defaultLinker = QUOTE(SWIFT_DEFAULT_LINKER);

Sources/DriverDefaults/include/constants.h

@@ -0,0 +1,16 @@
+#ifndef DRIVER_DEFAULTS_CONSTANTS_H
+#define DRIVER_DEFAULTS_CONSTANTS_H
+
+/// Default Linker
+///
+/// This macro allows configuring the Swift driver build to specify a default
+/// linker, overriding the existing heuristic.
+///
+/// NOTE: Darwin and Windows use clang as the linker. This does not affect the
+///       linker that the clang linker selects.
+#ifndef SWIFT_DEFAULT_LINKER
+#define SWIFT_DEFAULT_LINKER
+#endif
+const char *defaultLinker;
+
+#endif // DRIVER_DEFAULTS_CONSTANTS_H

Sources/DriverDefaults/include/module.modulemap

@@ -0,0 +1,4 @@
+module DriverDefaults {
+  header "constants.h"
+  export *
+}

Sources/SwiftDriver/Jobs/GenericUnixToolchain+LinkerSupport.swift

@@ -11,12 +11,19 @@
 //===----------------------------------------------------------------------===//
 
 import SwiftOptions
+import DriverDefaults
 
 import func TSCBasic.lookupExecutablePath
 import struct TSCBasic.AbsolutePath
 
 extension GenericUnixToolchain {
   private func defaultLinker(for targetTriple: Triple) -> String? {
+    let constantDefaultLinker = String(cString: DriverDefaults.defaultLinker)
+    if !constantDefaultLinker.isEmpty {
+      print("Using linker: \(constantDefaultLinker)")
+      return constantDefaultLinker
+    }
+
     if targetTriple.os == .openbsd || targetTriple.os == .freeBSD ||
         targetTriple.environment == .android {
       return "lld"