Only show help arguments for Options/Arguments with an Arity greater than zero

Author: baronfel

src/System.CommandLine.Tests/Help/HelpBuilderTests.cs

@@ -720,7 +720,7 @@ public void Command_arguments_show_argument_name_in_first_column()
                 new Argument<bool>("boolArgument") { Description = "Some value" },
                 new Argument<int>("intArgument") { Description = "Another value" },
             };
-            
+
             var helpBuilder = GetHelpBuilder(SmallMaxWidth);
 
             helpBuilder.Write(command, _console);
@@ -777,16 +777,16 @@ public void Help_describes_default_value_for_argument()
 
             help.Should().Contain("[default: the-arg-value]");
         }
-        
+
         [Fact]
         public void Help_does_not_show_default_value_for_argument_when_default_value_is_empty()
         {
             var argument = new Argument<string>("the-arg")
-            { 
+            {
                 Description = "The argument description",
                 DefaultValueFactory = (_) => ""
             };
-            
+
             var command = new Command("the-command", "The command description")
             {
                 argument
@@ -824,6 +824,38 @@ public void Help_does_not_show_default_value_for_option_when_default_value_is_em
             help.Should().NotContain("[default");
         }
 
+        [Flags]
+        public enum Letters
+        {
+            A = 1,
+            B = 2
+        }
+
+        [Fact]
+        public void Help_does_not_show_arguments_for_enum_backed_option_when_arity_is_zero()
+        {
+            var option = new Option<Letters>("--all")
+            {
+                Description = "Passes both A and B",
+                Arity = ArgumentArity.Zero,
+                CustomParser = _ => Letters.A | Letters.B
+            };
+
+            var command = new Command("the-command", "The command description")
+            {
+                option
+            };
+
+            var helpBuilder = GetHelpBuilder(SmallMaxWidth);
+
+            helpBuilder.Write(command, _console);
+
+            var help = _console.ToString();
+
+            help.Should().NotContain("--all <A|B>");
+        }
+
+
         [Fact]
         public void Command_arguments_default_value_provided()
         {
@@ -864,7 +896,7 @@ public void Command_arguments_with_default_values_that_are_enumerable_display_pi
                 new Argument<List<int>>("filter-size")
                 {
                     DefaultValueFactory = (_) => new List<int>() { 0, 2, 4 }
-                }   
+                }
             };
 
             _helpBuilder.Write(command, _console);
@@ -905,7 +937,7 @@ public void Command_shared_arguments_with_one_or_more_arity_are_displayed_as_bei
 
             _console.ToString().Should().Contain(expected);
         }
-        
+
         #endregion Arguments
 
         #region Options
@@ -1491,7 +1523,7 @@ public void Help_describes_default_value_for_subcommand_with_arguments_and_only_
             {
                 Hidden = true
             };
-            argument.DefaultValueFactory = _  => "the-arg-value";
+            argument.DefaultValueFactory = _ => "the-arg-value";
             otherArgumentHidden.DefaultValueFactory = _ => "the-other-hidden-arg-value";
 
             var command = new Command("outer", "outer command help")

src/System.CommandLine/Help/HelpBuilder.Default.cs

@@ -50,19 +50,22 @@ public static string GetArgumentUsageLabel(Symbol parameter)
             // By default Option.Name == Argument.Name, don't repeat it
             return parameter switch
             {
-                Argument argument => GetUsageLabel(argument.HelpName, argument.ValueType, argument.CompletionSources, argument) ?? $"<{argument.Name}>",
-                Option option => GetUsageLabel(option.HelpName, option.ValueType, option.CompletionSources, option) ?? "",
+                Argument argument => GetUsageLabel(argument.HelpName, argument.ValueType, argument.CompletionSources, argument, argument.Arity) ?? $"<{argument.Name}>",
+                Option option => GetUsageLabel(option.HelpName, option.ValueType, option.CompletionSources, option, option.Arity) ?? "",
                 _ => throw new InvalidOperationException()
             };
 
-            static string? GetUsageLabel(string? helpName, Type valueType, List<Func<CompletionContext, IEnumerable<CompletionItem>>> completionSources, Symbol symbol)
+            static string? GetUsageLabel(string? helpName, Type valueType, List<Func<CompletionContext, IEnumerable<CompletionItem>>> completionSources, Symbol symbol, ArgumentArity arity)
             {
                 // Argument.HelpName is always first choice
                 if (!string.IsNullOrWhiteSpace(helpName))
                 {
                     return $"<{helpName}>";
                 }
-                else if (!(valueType == typeof(bool) || valueType == typeof(bool?)) && completionSources.Count > 0)
+                else if (
+                    !(valueType == typeof(bool) || valueType == typeof(bool?))
+                    && arity.MaximumNumberOfValues > 0 // allowing zero arguments means we don't need to show usage
+                    && completionSources.Count > 0)
                 {
                     IEnumerable<string> completions = symbol
                         .GetCompletions(CompletionContext.Empty)
@@ -94,9 +97,9 @@ public static string GetOptionUsageLabel(Option symbol)
 
         private static string GetIdentifierSymbolUsageLabel(Symbol symbol, ICollection<string>? aliasSet)
         {
-            var aliases =  aliasSet is null
-                ? new [] { symbol.Name }
-                : new [] {symbol.Name}.Concat(aliasSet)
+            var aliases = aliasSet is null
+                ? new[] { symbol.Name }
+                : new[] { symbol.Name }.Concat(aliasSet)
                                 .Select(r => r.SplitPrefix())
                                 .OrderBy(r => r.Prefix, StringComparer.OrdinalIgnoreCase)
                                 .ThenBy(r => r.Alias, StringComparer.OrdinalIgnoreCase)