An "entire" Rust struct containing a single string field is just a struct with a single field. You don't have to specify the fields you don't need and it's unlikely your handrolled parser would be faster than serde (unless you are using memchr or something, then in that case you aren't parsing JSON).

You can add a catch all field that captures those in a map. You can also choose to ignore things you don’t explicitly mention. It is up to you how strict you want the parsing to be.

So you can define a struct with one status field and the right annotations and you will get exactly what you describe without having to write the code and it will still be almost as fast as doing that parsing yourself.

If you only want a single field in json, you define a struct with just that single field. Serde does have the ability to return an error about unknown fields, but it is not enabled by default.

This also informs your second question. If new optional fields in json are added, unless you tell serde to complain about them, it won't, and you can add the new Option at your leisure.

As far as I understand, unmapped fields are simply ignored, so you can write your code like

    #[derive(Deserialize)]
    struct Response { status: u64 }

> However, if you just want a very minimal subset of a JSON response (i.e. a 'status' string field out of say 20 other values in a map), is it not better to just look for one field, as opposed to have to create an entire struct in Rust to represent the full response?

You can define a struct with just the status field.

> How does Serde work with optional extra fields? I know you can use Option<>, but that implies you know they exist - what happens if fields get added in the future silently? I guess the schema changes then, which isn't good, but that might happen?

Unknown fields are skipped by default.