v1.0.181 introduced a breaking change - was that intentional/can that happen again?

[jost-s]

I've outlined the situation as a response to the PR that introduced a breaking change from v1.0.180 to v1.0.181.

I asked for clarification there but haven't received an answer, that's why I'm creating this issue. Please respond either here or on the PR.

[dtolnay]

Sorry about the breakage. I don't know a lot about messagepack, but I don't think I anticipated that this change would affect it.

I tried reproducing the example from #2505 (comment) using this code:

// [dependencies]
// rmp-serde = "1.1.2"
// serde = { version = "1", features = ["derive"] }

#[derive(serde::Serialize, serde::Deserialize, Debug)]
#[serde(tag = "type", content = "data")]
enum Message {
    A(i32),
}

fn main() {
    let m = Message::A(100);

    let bytes = rmp_serde::to_vec_named(&m).unwrap();
    println!("{:?}", bytes);
    println!("{:?}", rmp_serde::from_slice::<Message>(&bytes).unwrap());

    let bytes = rmp_serde::to_vec(&m).unwrap();
    println!("{:?}", bytes);
    println!("{:?}", rmp_serde::from_slice::<Message>(&bytes).unwrap());
}

And with both cargo update -p serde --precise 1.0.180 and cargo update -p serde --precise 1.0.181 it produces the same output. The to_vec_named output is:

[130, 164, 116, 121, 112, 101, 161, 65, 164, 100, 97, 116, 97, 100]

I was not able to reproduce the other output ([130, 164, 116, 121, 112, 101, 129, 161, 65, 192, 164, 100, 97, 116, 97, 100]). Do I need to configure the serializer in some non-default way to get that?

Regarding whether it can happen again — I think the best way to help it not happen would be by participating in reviewing serde PRs.

[jost-s]

Thank you for the swift response and investigation. It turns out that the project I was referring to is using rmp-serde v0.15, and with that version you get different outputs with serde v1.0.180 and v1.0.181.

$ cargo update -p serde --precise 1.0.180
    Updating crates.io index
 Downgrading serde v1.0.181 -> v1.0.180
 Downgrading serde_derive v1.0.181 -> v1.0.180
$ cargo run
[130, 164, 116, 121, 112, 101, 161, 65, 164, 100, 97, 116, 97, 100]
[146, 161, 65, 100]
$ cargo update -p serde --precise 1.0.181
    Updating crates.io index
    Updating serde v1.0.180 -> v1.0.181
    Updating serde_derive v1.0.180 -> v1.0.181
$ cargo run
[130, 164, 116, 121, 112, 101, 129, 0, 192, 164, 100, 97, 116, 97, 100]
[146, 129, 0, 192, 100]

As you say with v1 it produces identical outputs. So the breakage somehow comes from the interplay of serde with that version of rmp-serde.

I've built in a serialization test as a guard in our system that'll prevent unknown serialization deviations. I'll close this issue.