Bug: an empty struct marked with #nserde(default) does not accept non-empty JSON objects

[netheril96]

I'm using an empty struct as a placeholder to assert that some object with a given key exists, but nanoserde fails if the object actually has any keys.

Here is a demo code

use nanoserde::DeJson;

/*
 * BUG DEMONSTRATION: nanoserde
 *
 * An empty struct marked with #[nserde(default)] does not accept any non-empty JSON object.
 * However, if the struct has at least one field, it correctly accepts (and ignores) extra fields.
 */

#[derive(DeJson, Debug)]
#[nserde(default)]
struct EmptyStruct {}

#[derive(DeJson, Debug)]
#[nserde(default)]
struct NonEmptyStruct {
    #[allow(dead_code)]
    _dummy: Option<i32>,
}

fn main() {
    let json = r#"{"extra": 123}"#;

    println!("Attempting to deserialize '{}' into EmptyStruct...", json);
    match EmptyStruct::deserialize_json(json) {
        Ok(s) => println!("Success: {:?}", s),
        Err(e) => println!("FAILED: {}", e),
    }

    println!("\nAttempting to deserialize '{}' into NonEmptyStruct...", json);
    match NonEmptyStruct::deserialize_json(json) {
        Ok(s) => println!("Success: {:?}", s),
        Err(e) => println!("FAILED: {}", e),
    }
}

Run result

Attempting to deserialize '{"extra": 123}' into EmptyStruct...
FAILED: Json Deserialize error: Unexpected token Str expected , or } , line:1 col:10

Attempting to deserialize '{"extra": 123}' into NonEmptyStruct...
Success: NonEmptyStruct { _dummy: None }