Add ParsableArguments/Command.usageString (#634)

Adds a new API to ParsableArguments and ParsableCommand for getting the
usage string. This allows clients to use argument-parser in a more
piecemeal way to construct their own error screens.

Sources/ArgumentParser/Parsable Types/ParsableArguments.swift

@@ -231,6 +231,18 @@ extension ParsableArguments {
       exit(withError: error)
     }
   }
+
+  /// Returns the usage text for this type.
+  ///
+  /// - Parameters:
+  ///   - includeHidden: Include hidden help information in the generated
+  ///     message.
+  /// - Returns: The usage text for this type.
+  public static func usageString(
+    includeHidden: Bool = false
+  ) -> String {
+    HelpGenerator(self, visibility: includeHidden ? .hidden : .default).usage
+  }
 }
 
 /// Unboxes the given value if it is a `nil` value.

Sources/ArgumentParser/Parsable Types/ParsableCommand.swift

@@ -113,6 +113,22 @@ extension ParsableCommand {
         .rendered(screenWidth: columns)
   }
 
+  /// Returns the usage text for the given subcommand of this command.
+  ///
+  /// - Parameters:
+  ///   - includeHidden: Include hidden help information in the generated
+  ///     message.
+  /// - Returns: The usage text for this type.
+  public static func usageString(
+    for subcommand: ParsableCommand.Type,
+    includeHidden: Bool = false
+  ) -> String {
+    HelpGenerator(
+      commandStack: CommandParser(self).commandStack(for: subcommand),
+      visibility: includeHidden ? .hidden : .default)
+        .usage
+  }
+
   /// Executes this command, or one of its subcommands, with the given
   /// arguments.
   ///

Tests/ArgumentParserUnitTests/HelpGenerationTests.swift

@@ -680,7 +680,7 @@ extension HelpGenerationTests {
     static let configuration = CommandConfiguration(
       commandName: "parserBug",
       subcommands: [Sub.self])
-    
+
     struct CommonOptions: ParsableCommand {
       @Flag(help: "example flag")
       var example: Bool = false
@@ -689,12 +689,12 @@ extension HelpGenerationTests {
     struct Sub: ParsableCommand {
       @OptionGroup()
       var commonOptions: CommonOptions
-      
+
       @Argument(help: "Non-mandatory argument")
       var argument: String?
     }
   }
-  
+
   func testIssue278() {
     AssertHelp(.default, for: ParserBug.Sub.self, root: ParserBug.self, equals: """
       USAGE: parserBug sub [--example] [<argument>]
@@ -708,42 +708,87 @@ extension HelpGenerationTests {
 
       """)
   }
+}
+
+extension HelpGenerationTests {
+  struct NonCustomUsage: ParsableCommand {
+    struct ExampleSubcommand: ParsableCommand {
+      static let configuration = CommandConfiguration()
+      @Argument var output: String
+    }
+
+    static let configuration = CommandConfiguration(
+      subcommands: [ExampleSubcommand.self])
+
+    @Argument var file: String
+    @Flag var verboseMode = false
+  }
 
   struct CustomUsageShort: ParsableCommand {
-    static var configuration: CommandConfiguration {
-      CommandConfiguration(usage: """
+    static let configuration = CommandConfiguration(
+      usage: """
         example [--verbose] <file-name>
         """)
-    }
-
+
     @Argument var file: String
     @Flag var verboseMode = false
   }
 
   struct CustomUsageLong: ParsableCommand {
-    static var configuration: CommandConfiguration {
-      CommandConfiguration(usage: """
+    static let configuration = CommandConfiguration(
+      usage: """
         example <file-name>
         example --verbose <file-name>
         example --help
         """)
-    }
-
+
     @Argument var file: String
     @Flag var verboseMode = false
   }
 
   struct CustomUsageHidden: ParsableCommand {
-    static var configuration: CommandConfiguration {
-      CommandConfiguration(usage: "")
-    }
+    static let configuration = CommandConfiguration(usage: "")
 
     @Argument var file: String
     @Flag var verboseMode = false
   }
 
-  func testCustomUsageHelp() {
-    XCTAssertEqual(CustomUsageShort.helpMessage(columns: 80), """
+  func test_usageCustomization_helpMessage() {
+    AssertEqualStrings(
+      actual: NonCustomUsage.helpMessage(columns: 80),
+      expected: """
+      USAGE: non-custom-usage <file> [--verbose-mode] <subcommand>
+
+      ARGUMENTS:
+        <file>
+
+      OPTIONS:
+        --verbose-mode
+        -h, --help              Show help information.
+      
+      SUBCOMMANDS:
+        example-subcommand
+
+        See 'non-custom-usage help <subcommand>' for detailed help.
+      """)
+
+    AssertEqualStrings(
+      actual: NonCustomUsage.helpMessage(
+        for: NonCustomUsage.ExampleSubcommand.self, columns: 80),
+      expected: """
+      USAGE: non-custom-usage example-subcommand <output>
+
+      ARGUMENTS:
+        <output>
+
+      OPTIONS:
+        -h, --help              Show help information.
+
+      """)
+
+    AssertEqualStrings(
+      actual: CustomUsageShort.helpMessage(columns: 80),
+      expected: """
       USAGE: example [--verbose] <file-name>
 
       ARGUMENTS:
@@ -755,7 +800,9 @@ extension HelpGenerationTests {
       
       """)
 
-    XCTAssertEqual(CustomUsageLong.helpMessage(columns: 80), """
+    AssertEqualStrings(
+      actual: CustomUsageLong.helpMessage(columns: 80),
+      expected: """
       USAGE: example <file-name>
              example --verbose <file-name>
              example --help
@@ -769,7 +816,9 @@ extension HelpGenerationTests {
       
       """)
 
-    XCTAssertEqual(CustomUsageHidden.helpMessage(columns: 80), """
+    AssertEqualStrings(
+      actual: CustomUsageHidden.helpMessage(columns: 80),
+      expected: """
       ARGUMENTS:
         <file>
 
@@ -780,24 +829,74 @@ extension HelpGenerationTests {
       """)
   }
 
-  func testCustomUsageError() {
-    XCTAssertEqual(CustomUsageShort.fullMessage(for: ValidationError("Test")), """
+  func test_usageCustomization_fullMessage() {
+    AssertEqualStrings(
+      actual: NonCustomUsage.fullMessage(for: ValidationError("Test")),
+      expected: """
+      Error: Test
+      Usage: non-custom-usage <file> [--verbose-mode] <subcommand>
+        See 'non-custom-usage --help' for more information.
+      """)
+
+    AssertEqualStrings(
+      actual: CustomUsageShort.fullMessage(for: ValidationError("Test")),
+      expected: """
       Error: Test
       Usage: example [--verbose] <file-name>
         See 'custom-usage-short --help' for more information.
       """)
-    XCTAssertEqual(CustomUsageLong.fullMessage(for: ValidationError("Test")), """
+
+    AssertEqualStrings(
+      actual: CustomUsageLong.fullMessage(for: ValidationError("Test")),
+      expected: """
       Error: Test
       Usage: example <file-name>
              example --verbose <file-name>
              example --help
         See 'custom-usage-long --help' for more information.
       """)
-    XCTAssertEqual(CustomUsageHidden.fullMessage(for: ValidationError("Test")), """
+
+    AssertEqualStrings(
+      actual: CustomUsageHidden.fullMessage(for: ValidationError("Test")),
+      expected: """
       Error: Test
         See 'custom-usage-hidden --help' for more information.
       """)
   }
+
+  func test_usageCustomization_usageString() {
+    AssertEqualStrings(
+      actual: NonCustomUsage.usageString(),
+      expected: """
+      non-custom-usage <file> [--verbose-mode] <subcommand>
+      """)
+
+    AssertEqualStrings(
+      actual: NonCustomUsage.usageString(
+        for: NonCustomUsage.ExampleSubcommand.self),
+      expected: """
+      non-custom-usage example-subcommand <output>
+      """)
+
+    AssertEqualStrings(
+      actual: CustomUsageShort.usageString(),
+      expected: """
+      example [--verbose] <file-name>
+      """)
+
+    AssertEqualStrings(
+      actual: CustomUsageLong.usageString(),
+      expected: """
+      example <file-name>
+      example --verbose <file-name>
+      example --help
+      """)
+
+    AssertEqualStrings(
+      actual: CustomUsageHidden.usageString(),
+      expected: """
+      """)
+  }
 }
 
 extension HelpGenerationTests {