Automatically find toolchain from swift-syntax-dev-utils using xcrun on macOS

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/commands/Build.swift

@@ -24,7 +24,7 @@ struct Build: ParsableCommand {
   func run() throws {
     let executor = BuildExecutor(
       swiftPMBuilder: SwiftPMBuilder(
-        toolchain: arguments.toolchain,
+        toolchain: try arguments.toolchain,
         buildDir: arguments.buildDir,
         multirootDataFile: arguments.multirootDataFile,
         release: arguments.release,

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/commands/GenerateSourceCode.swift

@@ -23,7 +23,7 @@ struct GenerateSourceCode: ParsableCommand {
 
   func run() throws {
     let executor = GenerateSourceCodeExecutor(
-      toolchain: arguments.toolchain,
+      toolchain: try arguments.toolchain,
       verbose: arguments.verbose
     )
     try executor.run(sourceDir: Paths.sourcesDir)

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/commands/Test.swift

@@ -24,7 +24,7 @@ struct Test: ParsableCommand {
   func run() throws {
     let executor = TestExecutor(
       swiftPMBuilder: SwiftPMBuilder(
-        toolchain: arguments.toolchain,
+        toolchain: try arguments.toolchain,
         buildDir: arguments.buildDir,
         multirootDataFile: arguments.multirootDataFile,
         release: arguments.release,

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/commands/VerifySourceCode.swift

@@ -33,7 +33,7 @@ struct VerifySourceCode: ParsableCommand {
     try VerifySpiYmlExecutor().run()
 
     try VerifySourceCodeExecutor(
-      toolchain: arguments.toolchain,
+      toolchain: try arguments.toolchain,
       verbose: arguments.verbose
     ).run()
   }

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/common/BuildArguments.swift

@@ -15,10 +15,20 @@ import Foundation
 
 struct BuildArguments: ParsableArguments {
   @Option(
+    name: .customLong("toolchain"),
     help: "The path to the toolchain that shall be used to build SwiftSyntax.",
     transform: URL.init(fileURLWithPath:)
   )
-  var toolchain: URL
+  var _toolchain: URL? = defaultToolchain()
+
+  var toolchain: URL {
+    get throws {
+      guard let toolchain = self._toolchain else {
+        throw ScriptExectutionError(message: "Toolchain could not be inferred. Specify toolchain using --toolchain")
+      }
+      return toolchain
+    }
+  }
 
   @Option(
     help: """

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/common/Paths.swift

@@ -78,6 +78,12 @@ enum Paths {
     }
   }
 
+  static var xcrunExec: URL {
+    get throws {
+      return try lookupExecutable(for: "xcrun")
+    }
+  }
+
   private static var envSearchPaths: [URL] {
     // Compute search paths from PATH variable.
     #if os(Windows)

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/common/SourceCodeGeneratorArguments.swift

@@ -15,10 +15,20 @@ import Foundation
 
 struct SourceCodeGeneratorArguments: ParsableArguments {
   @Option(
+    name: .customLong("toolchain"),
     help: "The path to the toolchain that shall be used to build SwiftSyntax.",
     transform: URL.init(fileURLWithPath:)
   )
-  var toolchain: URL
+  var _toolchain: URL? = defaultToolchain()
+
+  var toolchain: URL {
+    get throws {
+      guard let toolchain = self._toolchain else {
+        throw ScriptExectutionError(message: "Toolchain could not be inferred. Specify toolchain using --toolchain")
+      }
+      return toolchain
+    }
+  }
 
   @Flag(help: "Enable verbose logging.")
   var verbose: Bool = false

SwiftSyntaxDevUtils/Sources/swift-syntax-dev-utils/common/Utils.swift

@@ -22,3 +22,25 @@ func withTemporaryDirectory<T>(_ body: (URL) throws -> T) throws -> T {
   }
   return try body(tempDirURL)
 }
+
+/// Infer the default toolchain using `xcrun`.
+///
+/// Returns `nil` on all platforms except for macOS.
+func defaultToolchain() -> URL? {
+  #if os(macOS)
+  do {
+    let swiftcPath = try ProcessRunner(executableURL: try Paths.xcrunExec, arguments: ["--find", "swiftc"]).run(verbose: false).stdout.trimmingCharacters(
+      in: .whitespacesAndNewlines
+    )
+    if swiftcPath.isEmpty {
+      return nil
+    }
+    return URL(fileURLWithPath: swiftcPath).deletingLastPathComponent().deletingLastPathComponent()
+  } catch {
+    return nil
+  }
+  #else
+  // Toolchain lookup not implemented without xcrun yet
+  return nil
+  #endif
+}