On poky master (6204a3f5f4), either of these lines:

DISTRO_FEATURES_append_machine = " ${@bb.utils.contains('DISTRO_FEATURES', 'feature', ' another_feature', '', d)}"

DISTRO_FEATURES_machine += "${@bb.utils.contains('DISTRO_FEATURES', 'feature', 'another_feature', '', d)}"

Results in a parse error where it infinitely parses the conditional code:

$ bitbake feature-image-minimal
ERROR: Unable to parse <expansion>
Traceback (most recent call last):
  File "/home/<user>/project/poky/bitbake/lib/bb/data_smart.py", line 416, in DataSmart.expandWithRefs(s="  feature ${@bb.utils.contains('DISTRO_FEATURES', 'feature', ' another_feature', '', d)}", varname='DISTRO_FEATURES'):
                     tb = sys.exc_info()[2]
    >                raise ExpansionError(varname, s, exc).with_traceback(tb) from exc
     
  File "/home/<user>/project/poky/bitbake/lib/bb/data_smart.py", line 403, in DataSmart.expandWithRefs(s="  feature ${@bb.utils.contains('DISTRO_FEATURES', 'feature', ' another_feature', '', d)}", varname='DISTRO_FEATURES'):
                     try:
    >                    s = __expand_python_regexp__.sub(varparse.python_sub, s)
                     except SyntaxError as e:
  File "/home/<user>/project/poky/bitbake/lib/bb/data_smart.py", line 116, in VariableParse.python_sub(match=<re.Match object; span=(20, 96), match="${@bb.utils.contains('DISTRO_FEATURES', 'feature>):
     
    >            parser = bb.codeparser.PythonParser(self.varname, logger)
                 parser.parse_python(code)
  File "/home/<user>/project/poky/bitbake/lib/bb/codeparser.py", line 279, in PythonParser.__init__(name='DISTRO_FEATURES', log=<BBLogger BitBake.Data (Level 8)>):
             self.references = set()
    >        self.log = BufferedLogger('BitBake.Data.PythonParser', logging.DEBUG, log)
     
  File "/home/<user>/project/poky/bitbake/lib/bb/codeparser.py", line 184, in BufferedLogger.__init__(name='BitBake.Data.PythonParser', level=10, target=<BBLogger BitBake.Data (Level 8)>):
         def __init__(self, name, level=0, target=None):
    >        Logger.__init__(self, name)
             self.setLevel(level)
bb.data_smart.ExpansionError: Failure expanding variable DISTRO_FEATURES, expression was   feature ${@bb.utils.contains('DISTRO_FEATURES', 'feature', ' another_feature', '', d)} which triggered exception RecursionError: maximum recursion depth exceeded

This also occurs if I use a temporary variable to store the result of the conditional, and append the value of that temporary variable to DISTRO_FEATURES for the machine.

My rationale for doing this is that we have some dependent layers, and we need 'another_feature' known by the parser (for a particular machine) even if the layer implementing 'feature' (which has a dependency on 'another_feature') is optionally later in bblayers.conf. 

Regardless of the rationale, provided the inclusion isn't breaking any rules then I would expect it to either succeed or fail gracefully?