interpreterbase: Add validator keyword argument to typed_kwargs

This attribute is a callable that returns a string if the value is
invalid, otherwise None. This intended for cases like the `install_*`
function's `install_mode` paramater, which is either an int or the
string "preserve", which allows us to do nice things like:

```python

class Kwargs(TypedDict):

    install_mode: T.Union[int, T.Literal['preserve']]

@typed_kwargs(
  'foo', KwargInfo('install_mode', ...,
  validator=lambda x: None if isinstance(x, int) or x == 'preserve' else 'must be the literal "preserve"),
)
def install_data(self, node, args, kwargs: 'Kwargs'):
  ...
```

In this case mypy *knows* that the string is preserve, as do we, and we
can simply do tests like:
```python
if kwargs['install_mode'] == 'preserve':
   ...
else:
   # this is an int
```

mesonbuild/interpreterbase.py

@@ -424,12 +424,18 @@ class KwargInfo(T.Generic[_T]):
         itself contain mutable types, typed_kwargs will copy the default
     :param since: Meson version in which this argument has been added. defaults to None
     :param deprecated: Meson version in which this argument has been deprecated. defaults to None
+    :param validator: A callable that does additional validation. This is mainly
+        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
     """
 
     def __init__(self, name: str, types: T.Union[T.Type[_T], T.Tuple[T.Type[_T], ...], ContainerTypeInfo],
                  *, required: bool = False, listify: bool = False,
                  default: T.Optional[_T] = None,
-                 since: T.Optional[str] = None, deprecated: T.Optional[str] = None):
+                 since: T.Optional[str] = None,
+                 deprecated: T.Optional[str] = None,
+                 validator: T.Optional[T.Callable[[_T], T.Optional[str]]] = None):
         self.name = name
         self.types = types
         self.required = required
@@ -437,6 +443,7 @@ class KwargInfo(T.Generic[_T]):
         self.default = default
         self.since = since
         self.deprecated = deprecated
+        self.validator = validator
 
 
 def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
@@ -492,6 +499,11 @@ def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
                             else:
                                 shouldbe = f'"{info.types.__name__}"'
                             raise InvalidArguments(f'{name} keyword argument "{info.name}"" was of type "{type(value).__name__}" but should have been {shouldbe}')
+
+                    if info.validator is not None:
+                        msg = info.validator(value)
+                        if msg is not None:
+                            raise InvalidArguments(f'{name} keyword argument "{info.name}" {msg}')
                 elif info.required:
                     raise InvalidArguments(f'{name} is missing required keyword argument "{info.name}"')
                 else:

run_unittests.py

@@ -1629,6 +1629,21 @@ class InternalTests(unittest.TestCase):
             self.assertRegex(out.getvalue(), r'WARNING:.*deprecated.*input arg in testfunc')
             self.assertNotRegex(out.getvalue(), r'WARNING:.*introduced.*input arg in testfunc')
 
+    def test_typed_kwarg_validator(self) -> None:
+        @typed_kwargs(
+            'testfunc',
+            KwargInfo('input', str, validator=lambda x: 'invalid!' if x != 'foo' else None)
+        )
+        def _(obj, node, args: T.Tuple, kwargs: T.Dict[str, str]) -> None:
+            pass
+
+        # Should be valid
+        _(None, mock.Mock(), tuple(), dict(input='foo'))
+
+        with self.assertRaises(MesonException) as cm:
+            _(None, mock.Mock(), tuple(), dict(input='bar'))
+        self.assertEqual(str(cm.exception), "testfunc keyword argument \"input\" invalid!")
+
 
 @unittest.skipIf(is_tarball(), 'Skipping because this is a tarball release')
 class DataTests(unittest.TestCase):