default field value

[benluelo]

It is possible to have any functionality similar to #[serde(default)]?

[VictorKoenders]

One issue with this is I'm thinking how this is going to interact with encode/decode:

#[derive(Encode, Decode)]
struct Foo {
    a: u32,
    #[bincode(default)]
    b: u32
}

let bytes = bincode::encode_to_vec(&Foo, bincode::config::standard()).unwrap();

What would be in bytes at this point? We can't skip b, there's numerous issues with serde skipping fields without leaving some form of tombstone.

If we're going to introduce tombstones, then we've just re-invented Option<T> and we wouldn't need an attribute like that.

What's the scenario where you'd want #[bincode(default)]?

[benluelo]

our main usecase is adding new fields to a structure in a backwards-compatible way. I attempted to implement this manually by just reading more bytes and using the default if it fails due to hitting the end of the stream, but this of course doesn't work since bincode doesn't have any notion of "structs" (if the struct with this manual implementation is nested with another struct, it will read into the following field of the parent struct). I don't think there's a clean way around this, and likely we'll end up versioning our types at the top level with a large enum a la enum Type { V1(TypeV1), V2(TypeV2) }.

[VictorKoenders]

The best way we've found is to indeed do enum Type { V1(TypeV1), V2(TypeV2) } or to append an Option<()> to each struct. We know there's a desire to have backward compatibility for bincode, but we also have no way to implement this technically with how bincode is set up.