Function parse_r_data_accessors_value 

fn parse_r_data_accessors_value(
    content: ParseStream<'_>,
    key: &Ident,
    system: ClassSystem,
) -> Result<RDataAccessors>

Parses an r_data_accessors key inside s3(...) / s4(...) / env(...) / r6(...) / s7(...), after the key: nothing, or = "get" (S3, S4 and env only).