ArgumentParser: Return leftover arguments from parse()

libos/include/os/ArgumentParser.h

@@ -15,7 +15,7 @@ class ArgumentParser
     Result<void> add_value_argument(StringView& out, char short_flag, StringView long_flag, bool value_required);
     Result<void> add_value_argument(StringView& out, char short_flag, StringView long_flag, StringView fallback);
 
-    void parse(int argc, char* const* argv);
+    Result<Vector<StringView>> parse(int argc, char* const* argv);
 
   private:
     struct PositionalArgument

libos/src/ArgumentParser.cpp

@@ -49,10 +49,12 @@ static bool looks_like_long_flag(StringView arg)
     return arg.length() > 2 && arg[0] == '-' && arg[1] == '-';
 }
 
-void ArgumentParser::parse(int argc, char* const* argv)
+Result<Vector<StringView>> ArgumentParser::parse(int argc, char* const* argv)
 {
     StringView program_name = argv[0];
 
+    Vector<StringView> leftovers;
+
     Option<ValueArgument> current_value_argument = {};
     bool is_parsing_value_argument = false;
 
@@ -144,7 +146,7 @@ void ArgumentParser::parse(int argc, char* const* argv)
         Option<PositionalArgument> current = m_positional_args.try_dequeue();
         if (!current.has_value())
         {
-            fprintf(stderr, "%s: unused argument '%s'\n", program_name.chars(), arg.chars());
+            TRY(leftovers.try_append(arg));
             continue;
         }
 
@@ -172,4 +174,6 @@ void ArgumentParser::parse(int argc, char* const* argv)
         }
         else { *arg.out = arg.fallback; }
     }
+
+    return leftovers;
 }