Serialisation regression in #1183

[aborgna-q]

#1183 included a breaking change to the serialisation format, removing the input_extensions field from the operations.

This should have either been a new v2 serialisation with backwards compatibility, or implemented so that the field gets silently ignored (if possible).

This json was generated by guppylang 0.5.2 (which uses hugr-py v0.2.0a1). Since it includes the input extensions fields it produces errors when decoded with the latest hugr crate, but works correctly once the fields are removed.
https://gist.github.com/aborgna-q/f4d6caa1fd7f9b5288935f39df4d4918

[ss2165]

Not sure this is the regression as described.
Serde should by default just ignore extra fields.
This is confirmed by noting that just removing the input_extensions key from the first node in the example is enough for deserialization to succeed (every other node still has the key).

The issue in reality appears to be hinted at by the error message

invalid type: map, expected unit struct Module

Since OpType is flattened into NodeSer it seems that for unit struct Module, the input_extensions key is being interpreted as a field of the flattened inner struct. This may be a purely serde bug. A minimal example:

    #[derive(Serialize, Deserialize)]
    struct Unit;

    #[derive(Serialize, Deserialize)]
    #[serde(tag = "u")]
    enum UnitOrNot {
        Unit(Unit),
        NotUnit { field: String },
    }
    #[derive(Serialize, Deserialize)]
    struct Nested {
        #[serde(flatten)]
        inner: UnitOrNot,
    }

    let data = r#"
        [
            {
                "u": "NotUnit",
                "field": "value",
                "ignored": "bar"
            },
            {
                "u": "Unit",
                "ignored": "foo"
            }
        
        ]"#;

    let _: Vec<Nested> = serde_json::from_str(data).unwrap();

The first element of the array deserializes correctly but the second throws:

Error: Error("invalid type: map, expected unit struct Unit", line: 11, column: 13)

Worth reporting as a serde bug? It may be a known edge case interaction of unit struct/flatten/internal tagging.

In the mean-time, believing Module is the only op that is a unit struct, going to try a heinous hack fix...

[ss2165]

Serde issue: serde-rs/serde#2775