Read JSON avoid extra fields - deny_unknown_fields

---

• What should happen if a new field is added to the JSON, but our code is not updated yet?
• Should we let it slide, or should we report an error?

examples/json/avoid-extra-fields/src/main.rs

use serde::Deserialize;

#[derive(Deserialize, Debug)]
#[serde(deny_unknown_fields)]
struct Person {
    fname: String,
    lname: String,
    married: bool,
}

fn main() {
    let filename = get_filename();

    let content = std::fs::read_to_string(filename).unwrap();
    match serde_json::from_str::<Person>(&content) {
        Ok(data) => {
            println!("{:#?}", &data);
            assert_eq!(data.fname, "Foo");
            assert_eq!(data.lname, "Bar");
            assert!(data.married);
        }
        Err(err) => {
            eprintln!("There was an error: {err}");
            //std::process::exit(1);
        }
    }
    println!("Still here");
}

fn get_filename() -> String {
    let args: Vec<String> = std::env::args().collect();
    if args.len() != 2 {
        eprintln!("Usage: {} FILENAME", args[0]);
        std::process::exit(1);
    }
    args[1].to_owned()
}

thread 'main' panicked at src/main.rs:16:57:
JSON parsing error: Error("unknown field `year`, expected one of `fname`, `lname`, `married`", line: 4, column: 10)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace