Handle subcommands

Sources/SwiftDriver/Driver/Driver.swift

@@ -43,6 +43,7 @@ public struct Driver {
   enum Error: Swift.Error {
     case invalidDriverName(String)
     case invalidInput(String)
+    case subcommandPassedToDriver
   }
 
   /// The set of environment variables that are visible to the driver and
@@ -193,11 +194,15 @@ public struct Driver {
     diagnosticsHandler: @escaping DiagnosticsEngine.DiagnosticsHandler = Driver.stderrDiagnosticsHandler
   ) throws {
     self.env = env
-
-    // FIXME: Determine if we should run as subcommand.
 
     self.diagnosticEngine = DiagnosticsEngine(handlers: [diagnosticsHandler])
+
+    if case .subcommand = try Self.invocationRunMode(forArgs: args).mode {
+      throw Error.subcommandPassedToDriver
+    }
+
     var args = try Self.expandResponseFiles(args, diagnosticsEngine: self.diagnosticEngine)[...]
+
     self.driverKind = try Self.determineDriverKind(args: &args)
     self.optionTable = OptionTable()
     self.parsedOptions = try optionTable.parse(Array(args))
@@ -351,6 +356,54 @@ public struct Driver {
   }
 }
 
+extension Driver {
+
+  public enum InvocationRunMode: Equatable {
+    case normal(isRepl: Bool)
+    case subcommand(String)
+  }
+
+  /// Determines whether the given arguments constitute a normal invocation,
+  /// or whether they invoke a subcommand.
+  ///
+  /// Returns the invocation mode along with the arguments modified for that mode.
+  public static func invocationRunMode(
+    forArgs args: [String]
+  ) throws -> (mode: InvocationRunMode, args: [String]) {
+
+    assert(!args.isEmpty)
+
+    let execName = try VirtualPath(path: args[0]).basenameWithoutExt
+
+    // If we are not run as 'swift' or there are no program arguments, always invoke as normal.
+    guard execName == "swift", args.count > 1 else { return (.normal(isRepl: false), args) }
+
+    // Otherwise, we have a program argument.
+    let firstArg = args[1]
+
+    // If it looks like an option or a path, then invoke in interactive mode with the arguments as given.
+    if firstArg.hasPrefix("-") || firstArg.hasPrefix("/") || firstArg.contains(".") {
+        return (.normal(isRepl: false), args)
+    }
+
+    // Otherwise, we should have some sort of subcommand.
+
+    var updatedArgs = args
+
+    // If it is the "built-in" 'repl', then use the normal driver.
+    if firstArg == "repl" {
+        updatedArgs.remove(at: 1)
+        return (.normal(isRepl: true), updatedArgs)
+    }
+
+    let subcommand = "swift-\(firstArg)"
+
+    updatedArgs.replaceSubrange(0...1, with: [subcommand])
+
+    return (.subcommand(subcommand), updatedArgs)
+  }
+}
+
 // MARK: - Response files.
 extension Driver {
   /// Tokenize a single line in a response file.

Sources/SwiftDriver/Driver/OutputFileMap.swift

@@ -31,21 +31,12 @@ public struct OutputFileMap: Equatable {
       return output
     }
 
-    // Create a temporary file
-    let baseName: String
-    switch inputFile {
-    case .absolute(let path):
-      baseName = path.basenameWithoutExt
-    case .relative(let path), .temporary(let path):
-      baseName = path.basenameWithoutExt
-    case .standardInput:
-      baseName = ""
-    case .standardOutput:
+    if inputFile == .standardOutput {
       fatalError("Standard output cannot be an input file")
     }
 
     // Form the virtual path.
-    return .temporary(RelativePath(baseName.appendingFileTypeExtension(outputType)))
+    return .temporary(RelativePath(inputFile.basenameWithoutExt.appendingFileTypeExtension(outputType)))
   }
 
   public func existingOutput(inputFile: VirtualPath, outputType: FileType) -> VirtualPath? {

Sources/SwiftDriver/Utilities/VirtualPath.swift

@@ -79,6 +79,18 @@ public enum VirtualPath: Hashable {
     default: return nil
     }
   }
+
+  /// Retrieve the basename of the path without the extension.
+  public var basenameWithoutExt: String {
+    switch self {
+    case .absolute(let path):
+      return path.basenameWithoutExt
+    case .relative(let path), .temporary(let path):
+      return path.basenameWithoutExt
+    case .standardInput, .standardOutput:
+      return ""
+    }
+  }
 }
 
 extension VirtualPath: Codable {

Sources/swift-driver/main.swift

@@ -23,7 +23,23 @@ do {
     processSet.terminate()
   }
 
-  var driver = try Driver(args: CommandLine.arguments)
+  let (mode, arguments) = try Driver.invocationRunMode(forArgs: CommandLine.arguments)
+
+  if case .subcommand(let subcommand) = mode {
+    // We are running as a subcommand, try to find the subcommand adjacent to the executable we are running as.
+    // If we didn't find the tool there, let the OS search for it.
+    let subcommandPath = Process.findExecutable(arguments[0])?.parentDirectory.appending(component: subcommand)
+                         ?? Process.findExecutable(subcommand)
+
+    if subcommandPath == nil || !localFileSystem.exists(subcommandPath!) {
+      fatalError("cannot find subcommand executable '\(subcommand)'")
+    }
+
+    // Execute the subcommand.
+    try exec(path: subcommandPath?.pathString ?? "", args: Array(arguments.dropFirst()))
+  }
+
+  var driver = try Driver(args: arguments)
   let resolver = try ArgsResolver()
   try driver.run(resolver: resolver, processSet: processSet)
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -47,6 +47,43 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+  func testInvocationRunModes() throws {
+
+    let driver1 = try Driver.invocationRunMode(forArgs: ["swift"])
+    XCTAssertEqual(driver1.mode, .normal(isRepl: false))
+    XCTAssertEqual(driver1.args, ["swift"])
+
+    let driver2 = try Driver.invocationRunMode(forArgs: ["swift", "-buzz"])
+    XCTAssertEqual(driver2.mode, .normal(isRepl: false))
+    XCTAssertEqual(driver2.args, ["swift", "-buzz"])
+
+    let driver3 = try Driver.invocationRunMode(forArgs: ["swift", "/"])
+    XCTAssertEqual(driver3.mode, .normal(isRepl: false))
+    XCTAssertEqual(driver3.args, ["swift", "/"])
+
+    let driver4 = try Driver.invocationRunMode(forArgs: ["swift", "./foo"])
+    XCTAssertEqual(driver4.mode, .normal(isRepl: false))
+    XCTAssertEqual(driver4.args, ["swift", "./foo"])
+
+    let driver5 = try Driver.invocationRunMode(forArgs: ["swift", "repl"])
+    XCTAssertEqual(driver5.mode, .normal(isRepl: true))
+    XCTAssertEqual(driver5.args, ["swift"])
+
+    let driver6 = try Driver.invocationRunMode(forArgs: ["swift", "foo", "bar"])
+    XCTAssertEqual(driver6.mode, .subcommand("swift-foo"))
+    XCTAssertEqual(driver6.args, ["swift-foo", "bar"])
+  }
+
+  func testSubcommandsHandling() throws {
+
+    XCTAssertNoThrow(try Driver(args: ["swift"]))
+    XCTAssertNoThrow(try Driver(args: ["swift", "-I=foo"]))
+    XCTAssertNoThrow(try Driver(args: ["swift", ".foo"]))
+    XCTAssertNoThrow(try Driver(args: ["swift", "/foo"]))
+
+    XCTAssertThrowsError(try Driver(args: ["swift", "foo"]))
+  }
+
   func testDriverKindParsing() throws {
     func assertArgs(
       _ args: String...,