More examples on OneOrMany and newtypes

[benjamin-bergia]

I am stugglinh with OneOrMany on one of my types. I tried few of the things that I found in #308. But unfortunately, I can't get it to work and I also don't understand most of it :/

Here is a sample of my code:

#[derive(Clone, Debug, Deserialize)]
struct ActionSet(HashMap<String, ActionList>);

#[serde_as]
#[derive(Clone, Debug, Deserialize)]
struct ActionList(
    #[serde_as(deserialize_as = "OneOrMany<Action, PreferMany>")]
    Vec<Action>
);

#[derive(Clone, Debug, Deserialize)]
#[serde(rename_all = "lowercase")]
enum Action {
    Add(Value),
    Remove(Value),
}

In the HashMap contained in ActionSet I would like to always have Vectors of Actions. But my file might contain either an Action or a list of Actions.

At the moment I keep getting:

error[E0277]: the trait bound `Action: DeserializeAs<'_, _>` is not satisfied
   --> src/main.rs:193:1
    |
193 | #[serde_as]
    | ^^^^^^^^^^^ the trait `DeserializeAs<'_, _>` is not implemented for `Action`
    |
    = help: the following other types implement trait `DeserializeAs<'de, T>`:
              <(As0, As1) as DeserializeAs<'de, (T0, T1)>>
              <(As0, As1, As2) as DeserializeAs<'de, (T0, T1, T2)>>
              <(As0, As1, As2, As3) as DeserializeAs<'de, (T0, T1, T2, T3)>>
              <(As0, As1, As2, As3, As4) as DeserializeAs<'de, (T0, T1, T2, T3, T4)>>
              <(As0, As1, As2, As3, As4, As5) as DeserializeAs<'de, (T0, T1, T2, T3, T4, T5)>>
              <(As0, As1, As2, As3, As4, As5, As6) as DeserializeAs<'de, (T0, T1, T2, T3, T4, T5, T6)>>
              <(As0, As1, As2, As3, As4, As5, As6, As7) as DeserializeAs<'de, (T0, T1, T2, T3, T4, T5, T6, T7)>>
              <(As0, As1, As2, As3, As4, As5, As6, As7, As8) as DeserializeAs<'de, (T0, T1, T2, T3, T4, T5, T6, T7, T8)>>
            and 169 others
    = note: required for `OneOrMany<Action, PreferMany>` to implement `DeserializeAs<'_, Vec<_>>`
note: required by a bound in `serde_with::de::<impl As<T>>::deserialize`
   --> /home/***/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_with-3.3.0/src/de/mod.rs:153:12
    |
153 |         T: DeserializeAs<'de, I>,
    |            ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `serde_with::de::<impl As<T>>::deserialize`
    = note: this error originates in the attribute macro `serde_as` (in Nightly builds, run with -Z macro-backtrace for more info)

When removing the type like in #[serde_as(deserialize_as = "OneOrMany<_, PreferMany>")]
I get something totally different:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("untagged and internally tagged enums do not support enum input")', src/main.rs:265:78

[benjamin-bergia]

I tried adding an intermediary type like in:

#[derive(Clone, Debug, Deserialize)]
struct ActionSet(HashMap<String, ActionList>);

#[derive(Clone, Debug, Deserialize)]
#[serde(from = "ActionListI")]
struct ActionList(Vec<Action>);
impl From<ActionListI> for ActionList {
  fn from(item: ActionListI) -> Self {
    return ActionList(item.vec);
  }
}

#[serde_as]
#[derive(Clone, Debug, Deserialize)]
struct ActionListI {
    // With our without #[serde(flatten)] here doesn't change anything
    #[serde_as(deserialize_as = "OneOrMany<Action, PreferMany>")]
    vec: Vec<Action>
}

#[derive(Clone, Debug, Deserialize)]
#[serde(rename_all = "lowercase")]
enum Action {
    Add(Value),
    Remove(Value),
}

But it doesn't seem to help

[jonasbb]

Please check if #202 covers your issue too. Basically, if you want to use the default Serialize/Deserialize behavior of a type, you have to write _ within the serde_as attribute. In your case that would be

#[serde_as]
#[derive(Clone, Debug, Deserialize)]
struct ActionList(
    #[serde_as(deserialize_as = "OneOrMany<_, PreferMany>")]
    Vec<Action>
);

[benjamin-bergia]

Hi @jonasbb,
So after switching to _ the trait error does go away.
I am still stuck with a "untagged and internally tagged enums do not support enum input". This doesn't seem to be related to serde_with but do you have any idea what might be triggering this?

[jonasbb]

That sounds like a serde issue, maybe caused by serde-rs/serde#1183. The use of OneOrMany might contribute here, since it buffers internally. But I cannot reproduce the issue with the types you shared here. So I can't really help you.