def default_missing(self) -> PerMachine[_T]:
        """Default host to build

        This allows just specifying nothing in the native case, and just host in the
        cross non-compiler case.
        """
        assert self.build is not None, 'Cannot fill in missing when all fields are empty'
        return PerMachine(self.build, self.host if self.host is not None else self.build)
Default host to build

        This allows just specifying nothing in the native case, and just host in the
        cross non-compiler case.
        
mesonbuild/utils/universal.py