the "registry" part has a direct analogue in the

--query

PR, even without pipelining (https://github.com/pantsbuild/pants/pull/7356/files#diff-25eb113080fbb829d55e428771ef22f3R424-R428):

_T = TypeVar('_T', bound=QueryParser)

@dataclass(frozen=True)
class KnownQueryExpressions:
  components: Dict[str, Type[_T]]

@rule
def known_query_expressions(union_membership: UnionMembership) -> KnownQueryExpressions:
  return KnownQueryExpressions({
    union_member.function_name: union_member
    for union_member in union_membership.union_rules[QueryParser]
  })