rust - How can I distinguish between a deserialized field that is missing and one that is null?

Question

I'd like to use Serde to parse some JSON as part of a HTTP PATCH request. Since PATCH requests don't pass the entire object, only the relevant data to update, I need the ability to tell between a value that was not passed, a value that was explicitly set to null, and a value that is present.

I have a value object with multiple nullable fields:

struct Resource {
    a: Option<i32>,
    b: Option<i32>,
    c: Option<i32>,
}

If the client submits JSON like this:

{"a": 42, "b": null}

I'd like to change a to Some(42), b to None, and leave c unchanged.

I tried wrapping each field in one more level of Option:

#[derive(Debug, Deserialize)]
struct ResourcePatch {
    a: Option<Option<i32>>,
    b: Option<Option<i32>>,
    c: Option<Option<i32>>,
}

playground

This does not make a distinction between b and c; both are None but I'd have wanted b to be Some(None).

I'm not tied to this representation of nested Options; any solution that can distinguish the 3 cases would be fine, such as one using a custom enum.

Answer 1

Building off of E_net4's answer, you can also create an enum for the three possibilities:

#[derive(Debug)]
enum Patch<T> {
    Missing,
    Null,
    Value(T),
}

impl<T> Default for Patch<T> {
    fn default() -> Self {
        Patch::Missing
    }
}

impl<T> From<Option<T>> for Patch<T> {
    fn from(opt: Option<T>) -> Patch<T> {
        match opt {
            Some(v) => Patch::Value(v),
            None => Patch::Null,
        }
    }
}

impl<'de, T> Deserialize<'de> for Patch<T>
where
    T: Deserialize<'de>,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        Option::deserialize(deserializer).map(Into::into)
    }
}

This can then be used as:

#[derive(Debug, Deserialize)]
struct ResourcePatch {
    #[serde(default)]
    a: Patch<i32>,
}

Unfortunately, you still have to annotate each field with #[serde(default)] (or apply it to the entire struct). Ideally, the implementation of Deserialize for Patch would handle that completely, but I haven't figured out how to do that yet.