interpreter: add FeatureOption.enable_if and .disable_if

This adds two new methods, that are conceptually related in the same way
that `enable_auto_if` and `disable_auto_if` are. They are different
however, in that they will always replace an `auto` value with an
`enabled` or `disabled` value, or error if the feature is in the
opposite state (calling `feature(disabled).enable_if(true)`, for
example). This matters when the feature will be passed to
dependency(required : …)`, which has different behavior when passed an
enabled feature than an auto one.

The `disable_if` method will be controversial, I'm sure, since it
can be expressed via `feature.require()` (`feature.require(not
condition) == feature.disable_if(condition)`). I have two defences of
this:

1) `feature.require` is difficult to reason about, I would expect
   require to be equivalent to `feature.enable_if(condition)`, not to
   `feature.disable_if(not condition)`.
2) mixing `enable_if` and `disable_if` in the same call chain is much
   clearer than mixing `require` and `enable_if`:
   ```meson
   get_option('feat') \
     .enable_if(foo) \
     .disable_if(bar) \
     .enable_if(opt)
   ```
   vs
   ```meson
   get_option('feat') \
     .enable_if(foo) \
     .require(not bar) \
     .enable_if(opt)
   ```
   In the first chain it's immediately obvious what is happening, in the
   second, not so much, especially if you're not familiar with what
   `require` means.

1 files changed, 39 insertions, 5 deletions

diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index a750da9..fa91714 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -41,6 +41,9 @@ if T.TYPE_CHECKING:
 
         separator: str
 
+_ERROR_MSG_KW: KwargInfo[T.Optional[str]] = KwargInfo('error_message', (str, NoneType))
+
+
 def extract_required_kwarg(kwargs: 'kwargs.ExtractRequired',
                            subproject: 'SubProject',
                            feature_check: T.Optional[FeatureCheckBase] = None,
@@ -97,6 +100,8 @@ class FeatureOptionHolder(ObjectHolder[coredata.UserFeatureOption]):
                              'require': self.require_method,
                              'disable_auto_if': self.disable_auto_if_method,
                              'enable_auto_if': self.enable_auto_if_method,
+                             'disable_if': self.disable_if_method,
+                             'enable_if': self.enable_if_method,
                              })
 
     @property
@@ -134,22 +139,51 @@ class FeatureOptionHolder(ObjectHolder[coredata.UserFeatureOption]):
     def auto_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> bool:
         return self.value == 'auto'
 
+    def _disable_if(self, condition: bool, message: T.Optional[str]) -> coredata.UserFeatureOption:
+        if not condition:
+            return copy.deepcopy(self.held_object)
+
+        if self.value == 'enabled':
+            err_msg = f'Feature {self.held_object.name} cannot be enabled'
+            if message:
+                err_msg += f': {message}'
+            raise InterpreterException(err_msg)
+        return self.as_disabled()
+
     @FeatureNew('feature_option.require()', '0.59.0')
     @typed_pos_args('feature_option.require', bool)
     @typed_kwargs(
         'feature_option.require',
-        KwargInfo('error_message', (str, NoneType))
+        _ERROR_MSG_KW,
     )
     def require_method(self, args: T.Tuple[bool], kwargs: 'kwargs.FeatureOptionRequire') -> coredata.UserFeatureOption:
-        if args[0]:
+        return self._disable_if(not args[0], kwargs['error_message'])
+
+    @FeatureNew('feature_option.disable_if()', '1.1.0')
+    @typed_pos_args('feature_option.disable_if', bool)
+    @typed_kwargs(
+        'feature_option.disable_if',
+        _ERROR_MSG_KW,
+    )
+    def disable_if_method(self, args: T.Tuple[bool], kwargs: 'kwargs.FeatureOptionRequire') -> coredata.UserFeatureOption:
+        return self._disable_if(args[0], kwargs['error_message'])
+
+    @FeatureNew('feature_option.enable_if()', '1.1.0')
+    @typed_pos_args('feature_option.enable_if', bool)
+    @typed_kwargs(
+        'feature_option.enable_if',
+        _ERROR_MSG_KW,
+    )
+    def enable_if_method(self, args: T.Tuple[bool], kwargs: 'kwargs.FeatureOptionRequire') -> coredata.UserFeatureOption:
+        if not args[0]:
             return copy.deepcopy(self.held_object)
 
-        if self.value == 'enabled':
-            err_msg = f'Feature {self.held_object.name} cannot be enabled'
+        if self.value == 'disabled':
+            err_msg = f'Feature {self.held_object.name} cannot be disabled'
             if kwargs['error_message']:
                 err_msg += f': {kwargs["error_message"]}'
             raise InterpreterException(err_msg)
-        return self.as_disabled()
+        return self.as_enabled()
 
     @FeatureNew('feature_option.disable_auto_if()', '0.59.0')
     @noKwargs