PackageModel: partially address #5719

Handle `-use-ld=` from the configuration in the selection of the
librarian.  We do not honour the `-Xmanifest` or `-Xswiftc` flags being
passed on the command line rather than the plist as we cannot
pre-compute the flags.

Author: compnerd

Sources/PackageModel/UserToolchain.swift

@@ -126,7 +126,8 @@ public final class UserToolchain: Toolchain {
     triple: Triple, binDir: AbsolutePath,
     useXcrun: Bool,
     environment: EnvironmentVariables,
-    searchPaths: [AbsolutePath]
+    searchPaths: [AbsolutePath],
+    extraSwiftFlags: [String]
   ) throws
     -> AbsolutePath
   {
@@ -142,7 +143,12 @@ public final class UserToolchain: Toolchain {
         {
           return librarian.basename
         }
-        // TODO(5719) use `lld-link` if the build requests lld.
+        // TODO(5719) handle `-Xmanifest` vs `-Xswiftc`
+        // `-use-ld=` is always joined in Swift.
+        if let ld = extraSwiftFlags.first(where: { $0.starts(with: "-use-ld=") }) {
+          let linker = String(ld.split(separator: "=").last!)
+          return linker == "lld" ? "lld-link" : linker
+        }
         return "link"
       }
       // TODO(compnerd) consider defaulting to `llvm-ar` universally with
@@ -438,10 +444,6 @@ public final class UserToolchain: Toolchain {
     var triple =
       destination.targetTriple ?? Triple.getHostTriple(usingSwiftCompiler: swiftCompilers.compile)
 
-    self.librarianPath = try UserToolchain.determineLibrarian(
-      triple: triple, binDir: binDir, useXcrun: useXcrun, environment: environment,
-      searchPaths: envSearchPaths)
-
     // Change the triple to the specified arch if there's exactly one of them.
     // The Triple property is only looked at by the native build system currently.
     if let archs = self.architectures, archs.count == 1 {
@@ -455,6 +457,10 @@ public final class UserToolchain: Toolchain {
     self.extraFlags.swiftCompilerFlags = try Self.deriveSwiftCFlags(
       triple: triple, destination: destination, environment: environment)
 
+    self.librarianPath = try UserToolchain.determineLibrarian(
+      triple: triple, binDir: binDir, useXcrun: useXcrun, environment: environment,
+      searchPaths: envSearchPaths, extraSwiftFlags: self.extraFlags.swiftCompilerFlags)
+
     if let sdkDir = destination.sdkRootDir {
       self.extraFlags.cCompilerFlags =
         [

Tests/PackageModelTests/PackageModelTests.swift

@@ -75,4 +75,63 @@ class PackageModelTests: XCTestCase {
         "-sdk", sdkDir.pathString,
       ])
   }
+
+  func testWindowsLibrarianSelection() throws {
+    // 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,
+    ]
+
+    #if os(Windows)
+    let suffix: String = ".exe"
+    #else
+    let suffix: String = ""
+    #endif
+
+    let triple = try Triple("x86_64-unknown-windows-msvc")
+    let fs = TSCBasic.localFileSystem
+
+    try withTemporaryFile { [contents] vfsPath in
+      try withTemporaryDirectory(removeTreeOnDeinit: true) { [contents] tmp in
+        let bin = tmp.appending(component: "bin")
+        try fs.createDirectory(bin)
+
+        let lld = bin.appending(component: "lld-link\(suffix)")
+        try fs.writeFileContents(lld, bytes: ByteString(contents))
+
+        let not = bin.appending(component: "not-a-linker\(suffix)")
+        try fs.writeFileContents(not, bytes: ByteString(contents))
+
+        #if !os(Windows)
+          try fs.chmod(.executable, path: lld, options: [])
+        #endif
+
+        try XCTAssertEqual(
+          UserToolchain.determineLibrarian(
+            triple: triple, binDir: bin, useXcrun: false, environment: [:], searchPaths: [],
+            extraSwiftFlags: ["-Xswiftc", "-use-ld=lld"]),
+          lld)
+
+        try XCTAssertEqual(
+          UserToolchain.determineLibrarian(
+            triple: triple, binDir: bin, useXcrun: false, environment: [:], searchPaths: [],
+            extraSwiftFlags: ["-Xswiftc", "-use-ld=not-a-link\(suffix)"]),
+          not)
+
+        try XCTAssertEqual(
+          UserToolchain.determineLibrarian(
+            triple: triple, binDir: bin, useXcrun: false, environment: [:], searchPaths: [],
+            extraSwiftFlags: ["-Xswiftc", "-use-ld=not-a-link\(suffix)"]),
+          AbsolutePath("link"))
+      }
+    }
+  }
 }