interpreter: add feature.disable_auto_if()

Add a method to downgrade an option to disabled if it is not used.
This is useful to avoid unnecessary search for dependencies;
for example

    dep = dependency('dep', required: get_option('feature').disable_auto_if(not foo))

can be used instead of the more verbose and complex

    if get_option('feature').auto() and not foo then
      dep = dependency('', required: false)
    else
      dep = dependency('dep', required: get_option('feature'))
    endif

or to avoid unnecessary dependency searches:

  dep1 = dependency('dep1', required: get_option('foo'))
  # dep2 is only used together with dep1
  dep2 = dependency('dep2', required: get_option('foo').disable_auto_if(not dep1.found()))
 ```

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 0bf188c..37f486a 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -75,6 +75,7 @@ class FeatureOptionHolder(InterpreterObject, ObjectHolder[coredata.UserFeatureOp
                              'allowed': self.allowed_method,
                              'auto': self.auto_method,
                              'require': self.require_method,
+                             'disable_auto_if': self.disable_auto_if_method,
                              })
 
     @property
@@ -122,6 +123,14 @@ class FeatureOptionHolder(InterpreterObject, ObjectHolder[coredata.UserFeatureOp
             raise InterpreterException(prefix + error_message)
         return self.as_disabled()
 
+    @permittedKwargs({})
+    def disable_auto_if_method(self, args, kwargs):
+        if len(args) != 1:
+            raise InvalidArguments('Expected 1 argument, got %d.' % (len(args), ))
+        if not isinstance(args[0], bool):
+            raise InvalidArguments('boolean argument expected.')
+        return self if self.value != 'auto' or not args[0] else self.as_disabled()
+
 
 class RunProcess(InterpreterObject):