Windows: support -tools-directory

Un-hardcode the explicit target triple for the test.  This allows the
test to run on Windows where there is no `ld` (nor `ld.exe`).  This then
uncovers the fact that the windows toolchain did not honour
`-tools-directory`.  Fixing that uncovers the fact that the lookup for
the tool will fail as the empty file does not get identified as a
binary.  To work around that, we now emit a 97-byte PE binary that is
correctly identified as a binary as the contents.  This will never be
executed and so should be fine to emit on all platforms.  Note that the
identified linker is different between Darwin and non-Darwin as the
behaviour of the driver is different - non-Darwin uses clang, while
Darwin uses `ld` (ld64) directly.

Thanks to @artemcm for the discussion on how to fix this properly.

Author: compnerd

Sources/SwiftDriver/Jobs/WindowsToolchain+LinkerSupport.swift

@@ -24,8 +24,9 @@ extension WindowsToolchain {
                                             sanitizers: Set<Sanitizer>,
                                             targetInfo: FrontendTargetInfo)
       throws -> AbsolutePath {
-    let targetTriple = targetInfo.target.triple
+    var clang = try getToolPath(.clang)
 
+    let targetTriple = targetInfo.target.triple
     if !targetTriple.triple.isEmpty {
       commandLine.appendFlag("-target")
       commandLine.appendFlag(targetTriple.triple)
@@ -40,6 +41,18 @@ extension WindowsToolchain {
       break
     }
 
+    if let arg = parsedOptions.getLastArgument(.toolsDirectory) {
+      let path = try AbsolutePath(validating: arg.asSingle)
+
+      if let tool = lookupExecutablePath(filename: executableName("clang"),
+                                         searchPaths: [path]) {
+        clang = tool
+      }
+
+      commandLine.appendFlag("-B")
+      commandLine.appendPath(path)
+    }
+
     // Select the linker to use.
     if let arg = parsedOptions.getLastArgument(.useLd) {
       commandLine.appendFlag("-fuse-ld=\(arg.asSingle)")
@@ -131,6 +144,6 @@ extension WindowsToolchain {
     addLinkedLibArgs(to: &commandLine, parsedOptions: &parsedOptions)
 
     // TODO(compnerd) handle static libraries
-    return try getToolPath(.clang)
+    return clang
   }
 }

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -3296,11 +3296,26 @@ final class SwiftDriverTests: XCTestCase {
 
   func testToolsDirectory() throws {
     try withTemporaryDirectory { tmpDir in
+#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
       let ld = tmpDir.appending(component: "ld")
-      try localFileSystem.writeFileContents(ld) { $0 <<< "" }
+#else
+      let ld = tmpDir.appending(component: executableName("clang"))
+#endif
+      // tiny PE binary from: https://archive.is/w01DO
+      let contents: [UInt8] = [
+          0x4d, 0x5a, 0x00, 0x00, 0x50, 0x45, 0x00, 0x00, 0x4c, 0x01, 0x01, 0x00,
+          0x6a, 0x2a, 0x58, 0xc3, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+          0x04, 0x00, 0x03, 0x01, 0x0b, 0x01, 0x08, 0x00, 0x04, 0x00, 0x00, 0x00,
+          0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x0c, 0x00, 0x00, 0x00,
+          0x04, 0x00, 0x00, 0x00, 0x0c, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00,
+          0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
+          0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+          0x68, 0x00, 0x00, 0x00, 0x64, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+          0x02
+      ]
+      try localFileSystem.writeFileContents(ld) { $0 <<< contents }
       try localFileSystem.chmod(.executable, path: AbsolutePath(ld.pathString))
       var driver = try Driver(args: ["swiftc",
-                                     "-target", "x86_64-apple-macosx10.14",
                                      "-tools-directory", tmpDir.pathString,
                                      "foo.swift"])
       let frontendJobs = try driver.planBuild()