Clean up help generation (#385)

- Adds check to ensure that `wrapped(to:wrappingIndent:)` doesn't
  attempt to retrieve a negative prefix.
- The Usage struct was composed of an array of strings which always
  contained exact one string at runtime. This struct has been removed
  and replaced with a single usage string.
- Removes HelpGenerator._screenWidthOverride in favor of explicitly
  setting the screen width in generateHelp calls.

Sources/ArgumentParser/Usage/HelpCommand.swift

@@ -33,9 +33,10 @@ struct HelpCommand: ParsableCommand {
   mutating func buildCommandStack(with parser: CommandParser) throws {
     commandStack = parser.commandStack(for: subcommands)
   }
-  
-  func generateHelp() -> String {
-    return HelpGenerator(commandStack: commandStack).rendered()
+
+  /// Used for testing.
+  func generateHelp(screenWidth: Int) -> String {
+    HelpGenerator(commandStack: commandStack).rendered(screenWidth: screenWidth)
   }
   
   enum CodingKeys: CodingKey {

Sources/ArgumentParser/Usage/HelpGenerator.swift

@@ -12,21 +12,8 @@
 internal struct HelpGenerator {
   static var helpIndent = 2
   static var labelColumnWidth = 26
-  static var systemScreenWidth: Int {
-    _screenWidthOverride ?? _terminalSize().width
-  }
-  
-  internal static var _screenWidthOverride: Int? = nil
-  
-  struct Usage {
-    var components: [String]
-    
-    func rendered(screenWidth: Int) -> String {
-      components
-        .joined(separator: "\n")
-    }
-  }
-  
+  static var systemScreenWidth: Int { _terminalSize().width }
+
   struct Section {
     struct Element: Hashable {
       var label: String
@@ -98,7 +85,7 @@ internal struct HelpGenerator {
   
   var commandStack: [ParsableCommand.Type]
   var abstract: String
-  var usage: Usage
+  var usage: String
   var sections: [Section]
   var discussionSections: [DiscussionSection]
   
@@ -116,10 +103,10 @@ internal struct HelpGenerator {
       toolName = "\(superName) \(toolName)"
     }
 
-    var usageString = UsageGenerator(toolName: toolName, definition: [currentArgSet]).synopsis
+    var usage = UsageGenerator(toolName: toolName, definition: [currentArgSet]).synopsis
     if !currentCommand.configuration.subcommands.isEmpty {
-      if usageString.last != " " { usageString += " " }
-      usageString += "<subcommand>"
+      if usage.last != " " { usage += " " }
+      usage += "<subcommand>"
     }
     
     self.abstract = currentCommand.configuration.abstract
@@ -130,7 +117,7 @@ internal struct HelpGenerator {
       self.abstract += "\n\(currentCommand.configuration.discussion)"
     }
     
-    self.usage = Usage(components: [usageString])
+    self.usage = usage
     self.sections = HelpGenerator.generateSections(commandStack: commandStack)
     self.discussionSections = []
   }
@@ -222,9 +209,8 @@ internal struct HelpGenerator {
     ]
   }
   
-  func usageMessage(screenWidth: Int? = nil) -> String {
-    let screenWidth = screenWidth ?? HelpGenerator.systemScreenWidth
-    return "Usage: \(usage.rendered(screenWidth: screenWidth))"
+  func usageMessage() -> String {
+    return "Usage: \(usage)"
   }
   
   var includesSubcommands: Bool {
@@ -243,7 +229,7 @@ internal struct HelpGenerator {
       ? ""
       : "OVERVIEW: \(abstract)".wrapped(to: screenWidth) + "\n\n"
     
-    var helpSubcommandMessage: String = ""
+    var helpSubcommandMessage = ""
     if includesSubcommands {
       var names = commandStack.map { $0._commandName }
       if let superName = commandStack.first!.configuration._superCommandName {
@@ -259,7 +245,7 @@ internal struct HelpGenerator {
     
     return """
     \(renderedAbstract)\
-    USAGE: \(usage.rendered(screenWidth: screenWidth))
+    USAGE: \(usage)
     
     \(renderedSections)\(helpSubcommandMessage)
     """

Sources/ArgumentParser/Utilities/StringExtensions.swift

@@ -12,6 +12,13 @@
 extension String {
   func wrapped(to columns: Int, wrappingIndent: Int = 0) -> String {
     let columns = columns - wrappingIndent
+    guard columns > 0 else {
+      // Skip wrapping logic if the number of columns is less than 1 in release
+      // builds and assert in debug builds.
+      assertionFailure("`columns - wrappingIndent` should be always be greater than 0.")
+      return ""
+    }
+
     var result: [Substring] = []
     
     var currentIndex = startIndex

Tests/ArgumentParserPackageManagerTests/HelpTests.swift

@@ -26,11 +26,11 @@ func getErrorText<T: ParsableArguments>(_: T.Type, _ arguments: [String]) -> Str
   }
 }
 
-func getErrorText<T: ParsableCommand>(_: T.Type, _ arguments: [String]) -> String {
+func getErrorText<T: ParsableCommand>(_: T.Type, _ arguments: [String], screenWidth: Int) -> String {
   do {
     let command = try T.parseAsRoot(arguments)
     if let helpCommand = command as? HelpCommand {
-      return helpCommand.generateHelp()
+      return helpCommand.generateHelp(screenWidth: screenWidth)
     } else {
       XCTFail("Didn't generate a help error")
       return ""
@@ -90,7 +90,7 @@ extension HelpTests {
 
   func testConfigHelp() throws {
     XCTAssertEqual(
-      getErrorText(Package.self, ["help", "config"]).trimmingLines(),
+      getErrorText(Package.self, ["help", "config"], screenWidth: 80).trimmingLines(),
       """
                 USAGE: package config <subcommand>
 
@@ -107,11 +107,8 @@ extension HelpTests {
   }
 
   func testGetMirrorHelp() throws {
-    HelpGenerator._screenWidthOverride = 80
-    defer { HelpGenerator._screenWidthOverride = nil }
-
     XCTAssertEqual(
-      getErrorText(Package.self, ["help", "config",  "get-mirror"]).trimmingLines(),
+      getErrorText(Package.self, ["help", "config",  "get-mirror"], screenWidth: 80).trimmingLines(),
       """
                 USAGE: package config get-mirror [<options>] --package-url <package-url>