intperperterbase: Add a convertor keyword argument

This is meant to allow simple type conversions to happen before the
interpreter function is called. This should simplify some cases like the
"native" keyword arugment that are booleans in the Meson DSL, but an
Enum in the implementation

mesonbuild/interpreterbase.py

@@ -428,6 +428,10 @@ class KwargInfo(T.Generic[_T]):
         intended for cases where a string is expected, but only a few specific
         values are accepted. Must return None if the input is valid, or a
         message if the input is invalid
+    :param convertor: A callable that converts the raw input value into a
+        different type. This is intended for cases such as the meson DSL using a
+        string, but the implementation using an Enum. This should not do
+        validation, just converstion.
     """
 
     def __init__(self, name: str, types: T.Union[T.Type[_T], T.Tuple[T.Type[_T], ...], ContainerTypeInfo],
@@ -435,7 +439,8 @@ class KwargInfo(T.Generic[_T]):
                  default: T.Optional[_T] = None,
                  since: T.Optional[str] = None,
                  deprecated: T.Optional[str] = None,
-                 validator: T.Optional[T.Callable[[_T], T.Optional[str]]] = None):
+                 validator: T.Optional[T.Callable[[_T], T.Optional[str]]] = None,
+                 convertor: T.Optional[T.Callable[[_T], TYPE_nvar]] = None):
         self.name = name
         self.types = types
         self.required = required
@@ -444,6 +449,7 @@ class KwargInfo(T.Generic[_T]):
         self.since = since
         self.deprecated = deprecated
         self.validator = validator
+        self.convertor = convertor
 
 
 def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
@@ -517,6 +523,9 @@ def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
                     else:
                         kwargs[info.name] = info.default
 
+                if info.convertor:
+                    kwargs[info.name] = info.convertor(kwargs[info.name])
+
             return f(*wrapped_args, **wrapped_kwargs)
         return T.cast(TV_func, wrapper)
     return inner

run_unittests.py

@@ -1644,6 +1644,16 @@ class InternalTests(unittest.TestCase):
             _(None, mock.Mock(), tuple(), dict(input='bar'))
         self.assertEqual(str(cm.exception), "testfunc keyword argument \"input\" invalid!")
 
+    def test_typed_kwarg_convertor(self) -> None:
+        @typed_kwargs(
+            'testfunc',
+            KwargInfo('native', bool, convertor=lambda n: MachineChoice.BUILD if n else MachineChoice.HOST)
+        )
+        def _(obj, node, args: T.Tuple, kwargs: T.Dict[str, MachineChoice]) -> None:
+            assert isinstance(kwargs['native'], MachineChoice)
+
+        _(None, mock.Mock(), tuple(), dict(native=True))
+
 
 @unittest.skipIf(is_tarball(), 'Skipping because this is a tarball release')
 class DataTests(unittest.TestCase):