Function parse_cfg_attr_internal 

fn parse_cfg_attr_internal<'a>(
    parser: &mut Parser<'a>,
    sess: &'a Session,
    features: Option<&Features>,
    attribute: &Attribute,
) -> PResult<'a, (CfgEntry, Vec<(AttrItem, Span)>)>

Parses cfg_attr(pred, attr_item_list) where attr_item_list is comma-delimited.