Serde: use container #[serde(default)], but with some required fields

Question

I have a struct that uses the #[serde(default)] container attribute.

But there is one field that should be required (if this field doesn't exist in the incoming data, the deserializer should error out instead of falling back to default value).

#[serde(default)]
#[derive(Serialize, Deserialize)]
struct Example {
  important: i32, // <-- I want this field to be required.
  a: i32, //  <-- If this field isn't in the incoming data, fallback to the default value.
  b: i32, //  <-- If this field isn't in the incoming data, fallback to the default value.
  c: i32, //  <-- If this field isn't in the incoming data, fallback to the default value.
}

Edit:

The information below isn't correct. The #[serde(default)] field attribute does not take the default value of the struct type, but rather of each field's type. (i.e. impl Default for Example isn't used. impl Default for i32 is used).

End Edit.

I could use the #[serde(default)] field attribute like this:

#[derive(Serialize, Deserialize)]
struct Example {
  important: i32,
  #[serde(default)]
  a: i32,
  #[serde(default)]
  b: i32,
  #[serde(default)]
  c: i32,
}

So important would be required, while a, b, and c would have default values.

But copy-pasting #[serde(default)] for all but one field doesn't seem like a good solution (my struct has ~10 fields). Is there a better way?

Is it an either all or none situation? Because what would happen if important and b are specified e.g. the following JSON { "important": 1, "b": 2 }. Should a and c still be default. If yes, then repeating it 10 times, might be the shortest solution. Edit: I was about to suggest using a second struct and serde(default) + serde(flatten) but that doesn't seem to be supported (see issue #1626) — vallentin
My question was unclear. Yes, if there is only important and b, a and c should be default. Your suggestion to use serde(flatten) would require an additional layer when accessing the fields, like example.data.a. I've decided to just copy paste ten times. — wisha
Given that serde(default) + serde(flatten) is currently unsupported, then yeah, the most straightforward solution is just to use serde(default) x10 — vallentin

Buzzec Buzzec · Accepted Answer · 2021-01-30T22:58:34

Responding to your edit you can utilize #[serde(default = "path")] to set the default values for each field. The path is to a function that returns the default value for that field.

Reference: https://serde.rs/field-attrs.html

Example:

const A_DEFAULT: i32 = 1;
const B_DEFAULT: i32 = 2;
const C_DEFAULT: i32 = 3;

#[derive(Serialize, Deserialize)]
struct Example {
  important: i32,
  #[serde(default = "a_default")]
  a: i32,
  #[serde(default = "b_default")]
  b: i32,
  #[serde(default = "c_default")]
  c: i32,
}

fn a_default() -> i32{
  A_DEFAULT
}
fn b_default() -> i32{
  B_DEFAULT
}
fn c_default() -> i32{
  C_DEFAULT
}

Serde: use container #[serde(default)], but with some required fields

1 Answers