Fix duplicate options showing up in command help output (#68)

When using option groups to bring options into subcommands that are also
used in parent commands, the help system duplicates the options in the
generated output.

For instance, in the `SubcommandEndToEndTest` example, the `--name`
argument is listed twice for both the subcommands `CommandA` and
`CommandB`, and expected by the test assertion.

It is annoying in a simple case like this but causes a lot of unhelpful
noise in help output when you share more and more options across a
command heirarchy.

To solve this we need to keep track of what
`HelpGenerator.Section.Element` values have already been processed from
parent commands in the command stack. I acheived this by making
`Element` conform to `Hashable` to track in a `Set`.

This assumes that we don't need to support re-using command names up and
down a command heirarchy. It doesn't work at all today (you get an error
when trying to use the exact same option in a parent and child
command). If we choose to support this eventually then we'll need to
augment this solution to keep track of where the `Element` was generated
in the heirarchy.

This also updates the test to properly assert that the `--name` option
is only output once for the subcommands in `SubcommandEndToEndTest`.

Author: jonathanpenn

Sources/ArgumentParser/Usage/HelpGenerator.swift

@@ -28,7 +28,7 @@ internal struct HelpGenerator {
   }
 
   struct Section {
-    struct Element {
+    struct Element: Hashable {
       var label: String
       var abstract: String = ""
       var discussion: String = ""
@@ -125,7 +125,9 @@ internal struct HelpGenerator {
   static func generateSections(commandStack: [ParsableCommand.Type]) -> [Section] {
     var positionalElements: [Section.Element] = []
     var optionElements: [Section.Element] = []
-    
+    /// Used to keep track of elements already seen from parent commands.
+    var alreadySeenElements = Set<Section.Element>()
+
     for commandType in commandStack {
       let args = Array(ArgumentSet(commandType))
 
@@ -174,10 +176,13 @@ internal struct HelpGenerator {
         }
 
         let element = Section.Element(label: synopsis, abstract: description, discussion: arg.help.help?.discussion ?? "")
-        if case .positional = arg.kind {
-          positionalElements.append(element)
-        } else {
-          optionElements.append(element)
+        if !alreadySeenElements.contains(element) {
+          alreadySeenElements.insert(element)
+          if case .positional = arg.kind {
+            positionalElements.append(element)
+          } else {
+            optionElements.append(element)
+          }
         }
       }
     }

Tests/EndToEndTests/SubcommandEndToEndTests.swift

@@ -86,7 +86,6 @@ extension SubcommandEndToEndTests {
             USAGE: foo a --name <name> --bar <bar>
 
             OPTIONS:
-              --name <name>
               --name <name>
               --bar <bar>
               -h, --help              Show help information.
@@ -96,7 +95,6 @@ extension SubcommandEndToEndTests {
             USAGE: foo b --name <name> --baz <baz>
 
             OPTIONS:
-              --name <name>
               --name <name>
               --baz <baz>
               -h, --help              Show help information.