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

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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.

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