Alternative implementation of DNS headers

dns_header_2.rs

pub trait BuildFlags: Sized {
    type Error;

    fn try_build_flags(payload: [u8; 2]) -> Result<Self, Self::Error>;
}

pub struct Headers<F> {
    pub id: u16,
    pub flags: F,
    pub qdcount: u16,
    pub ancount: u16,
    pub nscount: u16,
    pub arcount: u16,
}

pub fn parse_headers<'i, F, E>(input: &'i [u8]) -> IResult<&'i [u8], Headers<F>, E>
where
    F: BuildFlags,
    E: ParseError<&'i [u8]> + ContextError<&'i [u8]> + FromExternalError<&'i [u8], F::Error>,
{
    tuple((
        be_u16.context("id"),
        take(2usize)
            // Unwrap is safe here because we know (from take(2)) that this
            // is a byte slice of length 2
            .map_res(|flags: &[u8]| F::try_build_flags(flags.try_into().unwrap()))
            .context("flags"),
        be_u16.context("qdcount"),
        be_u16.context("ancount"),
        be_u16.context("nscount"),
        be_u16.context("arcount"),
    ))
    .map(|(id, flags, qdcount, ancount, nscount, arcount)| Headers {
        id,
        flags,
        qdcount,
        ancount,
        nscount,
        arcount,
    })
    .context("headers")
    .parse(input)
}