[Build System: build-script] Updated the presets module to simplify the API and improve testability.

Author: Ross Bayer

utils/build-script

@@ -968,8 +968,8 @@ def main_preset():
     preset_parser = presets.PresetParser()
 
     try:
-        preset_parser.read(args.preset_file_names)
-    except presets.Error as e:
+        preset_parser.read_files(args.preset_file_names)
+    except presets.PresetError as e:
         diagnostics.fatal(e.message)
 
     if args.show_presets:
@@ -987,12 +987,13 @@ def main_preset():
         args.preset_substitutions[name] = value
 
     try:
-        preset = preset_parser.get_preset(args.preset,
-                                          vars=args.preset_substitutions)
-    except presets.Error as e:
+        preset = preset_parser.get_preset(
+            args.preset,
+            vars=args.preset_substitutions)
+    except presets.PresetError as e:
         diagnostics.fatal(e.message)
 
-    preset_args = migration.migrate_swift_sdks(preset.format_args())
+    preset_args = migration.migrate_swift_sdks(preset.args)
 
     if args.distcc and (args.cmake_c_launcher or args.cmake_cxx_launcher):
         diagnostics.fatal(

utils/build_swift/build_swift/class_utils.py

@@ -0,0 +1,40 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+
+"""
+Class utility functions and decorators.
+"""
+
+
+from __future__ import absolute_import, unicode_literals
+
+
+__all__ = [
+    'generate_repr',
+]
+
+
+def generate_repr(*attrs):
+    """Generates a standardized __repr__ implementation for the decorated class
+    using the provided attributes and the class name.
+    """
+
+    def _repr(self):
+        args = []
+        for attr in attrs:
+            value = getattr(self, attr)
+            args.append('{}={}'.format(attr, repr(value)))
+
+        return '{}({})'.format(type(self).__name__, ', '.join(args))
+
+    def decorator(cls):
+        setattr(cls, '__repr__', _repr)
+        return cls
+
+    return decorator