Archive schema update?

[XavDesbordes]

Hi,
Say at time T0, Ihave the following struct:

#[derive(Archive, Serialize, Deserialize)]
struct A {
        a: u32,
        b: String,
}

Later, and at T1, I decide to add a field:

#[derive(Archive, Serialize, Deserialize)]
struct A {
        a: u32,
        b: String,
        c: u32
}

Doing like so, I get "attempt to subtract with overflow", which is kind of expected.
Currently, the only way I found is to add the struct version in the name and have a conversion function:

#[derive(Archive, Serialize, Deserialize)]
struct A0 {
        a: u32,
        b: String,
}

#[derive(Archive, Serialize, Deserialize)]
struct A1 {
        a: u32,
        b: String,
        c : u32
}

fn convert_a0_to_a1(...)->A1{
      //deserialize A0, reserialize to A1 here
}

Is there a way (attribute or anything) to avoid this ?

Is an attribute possible, like:

#[derive(Archive, Serialize, Deserialize)]
struct A {
        a: u32,
        b: String,
        #[optional]
        c : u32
}

that would not cause a hard error if the field is not present in the calculation (but I guess that would really make the code more difficult).

Thanks