support explicitly tagged types encoding

[yaozongyou]

use rasn::{AsnType,Decode,Encode};
use std::error::Error;

#[derive(Debug, AsnType, Decode, Encode)]
struct Message {
    id: i32,
    #[rasn(choice, enumerated)]
    body: BODY,
}

#[derive(Debug, AsnType, Decode, Encode)]
#[rasn(choice)]
enum BODY {
    #[rasn(tag(context, 3000))]
    Request(Request),

    #[rasn(tag(context, 3001))]
    Response(Response),
}

#[derive(Debug, AsnType, Decode, Encode)]
struct Request {
    num: i32,
}

#[derive(Debug, AsnType, Decode, Encode)]
struct Response {
    ret: i32,
}

fn main() -> Result<(), Box<dyn Error>> {
    let p = Message {
        id: 1,
        body: BODY::Request(Request { num: 1 }),
    };
    let e = rasn::der::encode(&p).unwrap();
    println!("e: {:02x?}", e);
    Ok(())
}

the above piece of code prints: 30, 0a, 02, 01, 01, bf, 97, 38, 03, 02, 01, 01
i want it to print 30, 0c, 02, 01, 01, bf, 97, 38, 05, 30, 03, 02, 01, 01 because it need to talk to some program written using c++ and golang. This encoding requires each CHOICE of BODY explicitly tagged. See 5.2 Explicitly tagged types of http://luca.ntop.org/Teaching/Appunti/asn1.html
Is it possible to achieve this?

[XAMPPRocky]

Is it possible to achieve this?

Thank you for your issue! Yes, but not currently through the macro. (I'm adding it soon). For now, you have to use the Explicit new type wrapper, or write the codec implementation yourself and call Encoder::encode_explicit_prefix/Decoder::decode_explicit_prefix.

#[derive(rasn::Encode, rasn::Decode)]
struct Body;

struct C0;

impl AsnType for C0 {
    const TAG: rasn::Tag = rasn::Tag::new(rasn::types::Context, 0);
}

type WrappedBody = rasn::types::Explicit<C0, Body>;

let body = WrappedBody(Body);

assert_eq!(body, rasn::ber::decode(&rasn::ber::encode(&body).unwrap()).unwrap());

[yaozongyou]

Thanks for your reply, I will have a try with the Explicit new type wrapper.

update: @XAMPPRocky I have tried with the Explicit new type wrapper following your guide. And it prints:
30, 0c, 02, 01, 01, 9f, 97, 38, 05, 30, 03, 02, 01, 01
rather than
30, 0c, 02, 01, 01, bf, 97, 38, 05, 30, 03, 02, 01, 01
maybe, rasn regards rasn::types::Explicit as primitive instead of constructed. Is it a bug?

the full code as follows:

use rasn::{AsnType,Decode,Encode};

#[derive(Debug, AsnType, Decode, Encode)]
struct Message {
    id: i32,
    #[rasn(choice)]
    body: BODY,
}

#[derive(Debug, AsnType, Decode, Encode)]
#[rasn(choice)]
enum BODY {
    #[rasn(tag(context, 3000))]
    Request(WrappedRequest),
}

#[derive(Debug, AsnType, Decode, Encode)]
struct Request {
    num: i32,
}

#[derive(Debug)]
struct C0;

impl AsnType for C0 {
    const TAG: rasn::Tag = rasn::Tag::new(rasn::types::Class::Context, 0);
}

type WrappedRequest = rasn::types::Explicit<C0, Request>;

fn main() {
    let p = Message {
        id: 1,
        body: BODY::Request(rasn::types::Explicit::new(Request { num: 1 })),
    };

    let e = rasn::der::encode(&p).unwrap();
    println!("e: {:02x?}", e);
}

[XAMPPRocky]

This is now available in the macro by using explicit around your tag definition. E.g. #[rasn(tag(explicit(context, 3000)))]