Reject duplicate fields when deserializing structs by AlixANNERAUD · Pull Request #65 · dtolnay/miniserde

AlixANNERAUD · 2026-10-19T21:22:52Z

Currently a struct deserialized with the derive accepts a repeated key and keeps the last value:

#[derive(Deserialize)]
struct Account {
    role: String,
}

json::from_str::<Account>(r#"{"role":"user","role":"admin"}"#) // Ok(Account { role: "admin" })

serde_json rejects this input with a duplicate field error. Silently picking the last value can be a problem when the JSON is assembled from several sources, since an injected key can override an earlier one.

This changes the generated Map::key to return Err(Error) when the field has already been set. To also catch duplicates of Option fields (including a repeated null), the state now starts with every field set to None, and Deserialize::default() is applied in finish for fields that never appeared, so a missing Option field still gives None.

Unknown keys are still ignored, and maps (BTreeMap, HashMap, json::Object) keep their current last-wins behavior, the same as serde_json.

A repeated key used to silently overwrite the earlier value, so {"role":"user","role":"admin"} deserialized as admin. serde_json rejects this with a duplicate field error; do the same.

Reject duplicate fields when deserializing structs

813e0f0

A repeated key used to silently overwrite the earlier value, so {"role":"user","role":"admin"} deserialized as admin. serde_json rejects this with a duplicate field error; do the same.

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Reject duplicate fields when deserializing structs - #65

Reject duplicate fields when deserializing structs#65
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AlixANNERAUD:deny-duplicate-fields

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