Round-trip failure: Empty maps/sequences gain an extra layer of nesting

[saethlin]

This example program probably ought to exit successfully

fn main() {
    let input = "{}";
    let v: ron::Value = ron::from_str(input).unwrap();
    println!("{:?}", v);
    let ser = ron::to_string(&v).unwrap();
    println!("{:?}", ser);
    let roundtrip = ron::from_str(&ser).unwrap();
    println!("{:?}", roundtrip);
    assert_eq!(v, roundtrip);
}

But it doesn't:

Map(Map({}))
"({})"
Seq([Map(Map({}))])
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Map(Map({}))`,
 right: `Seq([Map(Map({}))])`', src/main.rs:9:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This actually continues indefinitely. "{}" becomes "({})" which becomes "[({})]" and from then on we just add square brackets every round-trip.

[juntyr]

It seems the wrapping here is to blame:

ron/src/value.rs

Lines 25 to 26 in c4a7834

	#[derive(Clone, Debug, Default, Deserialize, Serialize)]
	pub struct Map(MapInner);

Adding the #[serde(transparent)] attribute to the struct seems to fix the problem.

[torkleyy]

This is definitely a bug and should be fixed, thanks for the report!

However, please note that in general, Value cannot be round-tripped.

[saethlin]

However, please note that in general, Value cannot be round-tripped.

Can you elaborate on this? I'm aware that textual formats like RON don't round-trip some float values correctly, is there something else specific to RON?

[torkleyy]

However, please note that in general, Value cannot be round-tripped.

Can you elaborate on this? I'm aware that textual formats like RON don't round-trip some float values correctly, is there something else specific to RON?

Yes, sure. The serde data model is inspired by JSON and thus does not differentiate struct from map, lists from tuples, possibly more I don't recall right now. Since we use serde to serialize / deserialize Value, the information gets lost. This is not an issue when using regular Rust types with #[derive(Deserialize, Serialize)] because they provide the type information.