Fix serde "missing field `email`"

What the error means

serde's derived Deserialize treats every struct field as required unless told otherwise. When the JSON object has no key for a field, deserialization fails with missing field \email` at line 1 column 9` — the position is where the object ended, because that is when serde knew the field was never coming. There is no partial struct; the whole value is rejected.

In the example, the first record contains email and the second does not. A struct generated from the first record declares email: String, which makes the key mandatory, so the second record fails.

Why it happens

• The generation sample always contained the key.
• The key is optional in the producer's contract, present for some records and absent for others.
• A field that is nullable but not optional. email: Option<String> accepts "email": null and an absent key — serde treats a missing Option field as None by default — but a field of type String does not, and neither does a custom type without #[serde(default)].
• A rename mismatch: the JSON key is user_email and the field is email without #[serde(rename = "user_email")], so the key is present but unmatched.

How to fix it

1. Paste a sample that includes a record without the key into the generator above. It merges the records and emits Option<String> for any field absent from at least one; serde then decodes an absent key as None with no further annotation.
2. In an existing struct, change the type to Option<String>, or keep the type and add #[serde(default)] to fall back to Default::default() — an empty string, zero, an empty vector — when the key is missing.
3. For a specific fallback, use #[serde(default = "path::to::fn")].
4. If the JSON key has a different name, add #[serde(rename = "...")] or a struct-level #[serde(rename_all = "camelCase")] rather than renaming the field.

The struct that accepts both records:

use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Root {
    pub id: i64,
    pub email: Option<String>,
}

If it still fails

• #[serde(deny_unknown_fields)] is the opposite concern — it rejects extra keys — and does not interact with this error.
• If the message names a field inside a nested struct, the Option or default belongs on the inner struct's field.
• Go and Kotlin apply the same required-by-default rule; JSON to Go uses pointers with omitempty and JSON to Kotlin uses T? = null for the same optional key.