Add experimental manual page generation (#332)

- Adds a swift package manager command plugin called
  GenerateManualPlugin. The plugin can be invoked from the command line
  using `swift package experimental-generate-manual`. The plugin is
  prefixed for now with "experimental-" to indicate it is not mature and
  may see breaking changes to its CLI and output in the future. The
  plugin can be can be used to generate a manual in MDoc syntax for any
  swift-argument-parser tool that can be executed via
  `tool --experimental-dump-info`.
- The plugin works by converting the `ToolInfoV0` structure from the
  `ArgumentParserToolInfo` library into MDoc AST nodes using a custom
  (SwiftUI-esk) result builder DSL. The MDoc AST is then lowered to a
  string and written to disk.
- The MDoc AST included is not general purpose and doesn't represent the
  true language exactly, so it is private to the underlying
  `generate-manual` tool. In the future it would be interesting to
  finish fleshing out this MDoc library and spin it out, however this is
  not a priority.
- Next steps include:
  - Improving the command line interface for the plugin.
  - Adding support for "extended discussions" to Commands and exposing
    this information in manuals.
  - Further improve the escaping logic to properly escape MDoc macros
    that might happen to appear in user's help strings.
  - Ingesting external content a-la swift-docc so the entire tool
    documentation does not need to be included in the binary itself.
  - Bug fixes and addressing developer/user feedback.

Built with love,
@rauhul

Package.swift

@@ -21,6 +21,7 @@ var package = Package(
     ],
     dependencies: [],
     targets: [
+        // Core Library
         .target(
             name: "ArgumentParser",
             dependencies: ["ArgumentParserToolInfo"],
@@ -34,6 +35,7 @@ var package = Package(
             dependencies: [],
             exclude: ["CMakeLists.txt"]),
 
+        // Examples
         .executableTarget(
             name: "roll",
             dependencies: ["ArgumentParser"],
@@ -47,6 +49,7 @@ var package = Package(
             dependencies: ["ArgumentParser"],
             path: "Examples/repeat"),
 
+        // Tests
         .testTarget(
             name: "ArgumentParserEndToEndTests",
             dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
@@ -68,10 +71,13 @@ var package = Package(
 
 #if swift(>=5.6) && os(macOS)
 package.targets.append(contentsOf: [
+    // Examples
     .executableTarget(
         name: "count-lines",
         dependencies: ["ArgumentParser"],
         path: "Examples/count-lines"),
+
+    // Tools
     .executableTarget(
         name: "changelog-authors",
         dependencies: ["ArgumentParser"],

Package@swift-5.6.swift

@@ -0,0 +1,104 @@
+// swift-tools-version:5.6
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import PackageDescription
+
+var package = Package(
+    name: "swift-argument-parser",
+    products: [
+        .library(
+            name: "ArgumentParser",
+            targets: ["ArgumentParser"]),
+    ],
+    dependencies: [],
+    targets: [
+        // Core Library
+        .target(
+            name: "ArgumentParser",
+            dependencies: ["ArgumentParserToolInfo"],
+            exclude: ["CMakeLists.txt"]),
+        .target(
+            name: "ArgumentParserTestHelpers",
+            dependencies: ["ArgumentParser", "ArgumentParserToolInfo"],
+            exclude: ["CMakeLists.txt"]),
+        .target(
+            name: "ArgumentParserToolInfo",
+            dependencies: [ ],
+            exclude: ["CMakeLists.txt"]),
+
+        // Plugins
+        .plugin(
+            name: "GenerateManualPlugin",
+            capability: .command(
+                intent: .custom(
+                    verb: "experimental-generate-manual",
+                    description: "Generate a manual entry for a specified target.")),
+            dependencies: ["generate-manual"]),
+
+        // Examples
+        .executableTarget(
+            name: "roll",
+            dependencies: ["ArgumentParser"],
+            path: "Examples/roll"),
+        .executableTarget(
+            name: "math",
+            dependencies: ["ArgumentParser"],
+            path: "Examples/math"),
+        .executableTarget(
+            name: "repeat",
+            dependencies: ["ArgumentParser"],
+            path: "Examples/repeat"),
+
+        // Tools
+        .executableTarget(
+            name: "generate-manual",
+            dependencies: ["ArgumentParser", "ArgumentParserToolInfo"],
+            path: "Tools/generate-manual"),
+
+        // Tests
+        .testTarget(
+            name: "ArgumentParserEndToEndTests",
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
+        .testTarget(
+            name: "ArgumentParserExampleTests",
+            dependencies: ["ArgumentParserTestHelpers"],
+            resources: [.copy("CountLinesTest.txt")]),
+        .testTarget(
+            name: "ArgumentParserGenerateManualTests",
+            dependencies: ["ArgumentParserTestHelpers"]),
+        .testTarget(
+            name: "ArgumentParserPackageManagerTests",
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
+        .testTarget(
+            name: "ArgumentParserUnitTests",
+            dependencies: ["ArgumentParser", "ArgumentParserTestHelpers"],
+            exclude: ["CMakeLists.txt"]),
+    ]
+)
+
+#if os(macOS)
+package.targets.append(contentsOf: [
+    // Examples
+    .executableTarget(
+        name: "count-lines",
+        dependencies: ["ArgumentParser"],
+        path: "Examples/count-lines"),
+
+    // Tools
+    .executableTarget(
+        name: "changelog-authors",
+        dependencies: ["ArgumentParser"],
+        path: "Tools/changelog-authors"),
+    ])
+#endif

Plugins/GenerateManualPlugin/GenerateManualPlugin.swift

@@ -0,0 +1,95 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import PackagePlugin
+import Foundation
+
+@main
+struct GenerateManualPlugin: CommandPlugin {
+  func performCommand(
+    context: PluginContext,
+    arguments: [String]
+  ) async throws {
+    // Locate generation tool.
+    let generationToolFile = try context.tool(named: "generate-manual").path
+
+    // Create an extractor to extract plugin-only arguments from the `arguments`
+    // array.
+    var extractor = ArgumentExtractor(arguments)
+
+    // Run generation tool once if help is requested.
+    if extractor.helpRequest() {
+      try generationToolFile.exec(arguments: ["--help"])
+      print("""
+        ADDITIONAL OPTIONS:
+          --configuration <configuration>
+                                  Tool build configuration used to generate the
+                                  manual. (default: release)
+
+        NOTE: The "GenerateManual" plugin handles passing the "<tool>" and
+        "--output-directory <output-directory>" arguments. Manually supplying
+        these arguments will result in a runtime failure.
+        """)
+      return
+    }
+
+    // Extract configuration argument before making it to the
+    // "generate-manual" tool.
+    let configuration = try extractor.configuration()
+
+    // Build all products first.
+    print("Building package in \(configuration) mode...")
+    let buildResult = try packageManager.build(
+      .all(includingTests: false),
+      parameters: .init(configuration: configuration))
+
+    guard buildResult.succeeded else {
+      throw GenerateManualPluginError.buildFailed(buildResult.logText)
+    }
+    print("Built package in \(configuration) mode")
+
+    // Run generate-manual on all executable artifacts.
+    for builtArtifact in buildResult.builtArtifacts {
+      // Skip non-executable targets
+      guard builtArtifact.kind == .executable else { continue }
+
+      // Skip executables without a matching product.
+      guard let product = builtArtifact.matchingProduct(context: context)
+      else { continue }
+
+      // Skip products without a dependency on ArgumentParser.
+      guard product.hasDependency(named: "ArgumentParser") else { continue }
+
+      // Get the artifacts name.
+      let executableName = builtArtifact.path.lastComponent
+      print("Generating manual for \(executableName)...")
+
+      // Create output directory.
+      let outputDirectory = context
+        .pluginWorkDirectory
+        .appending(executableName)
+      try outputDirectory.createOutputDirectory()
+
+      // Create generation tool arguments.
+      var generationToolArguments = [
+        builtArtifact.path.string,
+        "--output-directory",
+        outputDirectory.string
+      ]
+      generationToolArguments.append(
+        contentsOf: extractor.unextractedOptionsOrFlags)
+
+      // Spawn generation tool.
+      try generationToolFile.exec(arguments: generationToolArguments)
+      print("Generated manual in '\(outputDirectory)'")
+    }
+  }
+}

Plugins/GenerateManualPlugin/GenerateManualPluginError.swift

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import PackagePlugin
+
+enum GenerateManualPluginError: Error {
+  case unknownBuildConfiguration(String)
+  case buildFailed(String)
+  case createOutputDirectoryFailed(Error)
+  case subprocessFailedNonZeroExit(Path, Int32)
+  case subprocessFailedError(Path, Error)
+}
+
+extension GenerateManualPluginError: CustomStringConvertible {
+  var description: String {
+    switch self {
+    case .unknownBuildConfiguration(let configuration):
+      return "Build failed: Unknown build configuration '\(configuration)'."
+    case .buildFailed(let logText):
+      return "Build failed: \(logText)."
+    case .createOutputDirectoryFailed(let error):
+      return """
+        Failed to create output directory: '\(error.localizedDescription)'
+        """
+    case .subprocessFailedNonZeroExit(let tool, let exitCode):
+      return """
+        '\(tool.lastComponent)' invocation failed with a nonzero exit code: \
+        '\(exitCode)'.
+        """
+    case .subprocessFailedError(let tool, let error):
+      return """
+        '\(tool.lastComponent)' invocation failed: \
+        '\(error.localizedDescription)'
+        """
+    }
+  }
+}
+
+extension GenerateManualPluginError: LocalizedError {
+  var localizedDescription: String { self.description }
+}

Plugins/GenerateManualPlugin/PackagePlugin+Helpers.swift

@@ -0,0 +1,93 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import PackagePlugin
+
+extension ArgumentExtractor {
+  mutating func helpRequest() -> Bool {
+      self.extractFlag(named: "help") > 0
+  }
+
+  mutating func configuration() throws -> PackageManager.BuildConfiguration {
+    switch self.extractOption(named: "configuration").first {
+    case .some(let configurationString):
+      switch configurationString {
+      case "debug":
+        return .debug
+      case "release":
+        return .release
+      default:
+        throw GenerateManualPluginError
+          .unknownBuildConfiguration(configurationString)
+      }
+    case .none:
+      return .release
+    }
+  }
+}
+
+extension Path {
+  func createOutputDirectory() throws {
+    do {
+      try FileManager.default.createDirectory(
+        atPath: self.string,
+        withIntermediateDirectories: true)
+    } catch {
+      throw GenerateManualPluginError.createOutputDirectoryFailed(error)
+    }
+  }
+
+  func exec(arguments: [String]) throws {
+    do {
+      let process = Process()
+      process.executableURL = URL(fileURLWithPath: self.string)
+      process.arguments = arguments
+      try process.run()
+      process.waitUntilExit()
+      guard
+        process.terminationReason == .exit,
+        process.terminationStatus == 0
+      else {
+        throw GenerateManualPluginError.subprocessFailedNonZeroExit(
+          self, process.terminationStatus)
+      }
+    } catch {
+      throw GenerateManualPluginError.subprocessFailedError(self, error)
+    }
+  }
+}
+
+extension PackageManager.BuildResult.BuiltArtifact {
+  func matchingProduct(context: PluginContext) -> Product? {
+    context
+      .package
+      .products
+      .first { $0.name == self.path.lastComponent }
+  }
+}
+
+extension Product {
+  func hasDependency(named name: String) -> Bool {
+    recursiveTargetDependencies
+      .contains { $0.name == name }
+  }
+
+  var recursiveTargetDependencies: [Target] {
+    var dependencies = [Target.ID: Target]()
+    for target in self.targets {
+      for dependency in target.recursiveTargetDependencies {
+        dependencies[dependency.id] = dependency
+      }
+    }
+    return Array(dependencies.values)
+  }
+}