Return a const object if the value is representable as
        constant value in the generated code, otherwise it will raise
        an `Impossible` exception.

def from_untrusted(cls, value, lineno=None, environment=None):
        """Return a const object if the value is representable as
        constant value in the generated code, otherwise it will raise
        an `Impossible` exception.
        """
        from .compiler import has_safe_repr
        if not has_safe_repr(value):
            raise Impossible()
        return cls(value, lineno=lineno, environment=environment)