Move legacy argparser

utils/build-script

@@ -245,8 +245,6 @@ class BuildScriptInvocation(object):
 
     @staticmethod
     def apply_default_arguments(toolchain, args):
-        driver_arguments.apply_default_arguments(args)
-
         # infer if ninja is required
         ninja_required = (
             args.cmake_generator == 'Ninja' or args.build_foundation)

utils/build_swift/README.md

@@ -0,0 +1,12 @@
+# build_swift
+
+The `build_swift` module contains data-structures and functions used by
+the Swift build-script.
+
+## Unit Tests
+
+You may run the unit test suite using the command:
+
+```sh
+$ python -m unittest discover -s utils/build_swift
+```

utils/build_swift/driver_arguments.py

@@ -24,13 +24,31 @@
 
 
 __all__ = [
-    'apply_default_arguments',
     'create_argument_parser',
 ]
 
 
-def apply_default_arguments(args):
-    """Preprocess argument namespace to apply default behaviors."""
+class _ApplyDefaultsArgumentParser(argparse.ArgumentParser):
+    """Wrapper class around the default ArgumentParser that allows for
+    post-processing the parsed argument namespace to apply default argument
+    transformations.
+    """
+
+    def __init__(self, apply_defaults=None, *args, **kwargs):
+        self._apply_defaults = apply_defaults
+        super(_ApplyDefaultsArgumentParser, self).__init__(*args, **kwargs)
+
+    def parse_known_args(self, args=None, namespace=None):
+        args, argv = super(_ApplyDefaultsArgumentParser, self)\
+            .parse_known_args(args, namespace)
+
+        self._apply_defaults(args)
+        return args, argv
+
+
+def _apply_default_arguments(args):
+    """Preprocess argument namespace to apply default behaviors.
+    """
 
     # Build cmark if any cmark-related options were specified.
     if (args.cmark_build_variant is not None):
@@ -245,7 +263,10 @@ def apply_default_arguments(args):
 
 
 def create_argument_parser():
-    parser = argparse.ArgumentParser(
+    """Return a configured argument parser."""
+
+    parser = _ApplyDefaultsArgumentParser(
+        apply_defaults=_apply_default_arguments,
         formatter_class=argparse.RawDescriptionHelpFormatter,
         usage=USAGE,
         description=DESCRIPTION,
@@ -1111,6 +1132,8 @@ def create_argument_parser():
     return parser
 
 
+# ----------------------------------------------------------------------------
+
 USAGE = """
   %(prog)s [-h | --help] [OPTION ...]
   %(prog)s --preset=NAME [SUBSTITUTION ...]

utils/build_swift/tests/__init__.py

@@ -0,0 +1,7 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors