Flattening and capturing extras in Rust

flatten merges a nested struct into the parent, or sweeps up unknown keys.

use serde::{Deserialize, Serialize};
use std::collections::HashMap;

#[derive(Debug, Serialize, Deserialize)]
struct Common {
    id: u32,
    created: String,
}

#[derive(Debug, Serialize, Deserialize)]
struct Record {
    #[serde(flatten)]
    common: Common,
    name: String,
    #[serde(flatten)]
    extra: HashMap<String, serde_json::Value>,
}

fn main() -> Result<(), serde_json::Error> {
    let raw = r#"{"id":7,"created":"2026-07-29","name":"ada",
                  "streak":12}"#;
    let record: Record = serde_json::from_str(raw)?;
    println!("{} {}", record.common.id, record.name);
    println!("{:?}", record.extra.get("streak"));
    Ok(())
}

How it works

1. flatten on a struct field inlines its keys.
2. flatten on a map collects everything not otherwise named.
3. This is how you model an envelope plus a payload.

Keywords and builtins used here

• Common
• HashMap
• Ok
• Record
• Result
• String
• fn
• let
• main
• struct
• u32
• use

The run, in numbers

Lines

26

Characters to type

596

Tokens

139

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 79 seconds.

Type this snippet

Step 3 of 3 in Field attributes, step 6 of 11 in Serde & JSON.

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