Bug when using `prefixItems` + `items: false` + `default` in JSON Schema

[Big-Iron-Cheems]

When defining a JSON Schema that combines prefixItems, items: false, and a default value for an array, and referencing it via $ref, the library fails to process the schema.
The schema is valid according to JSON Schema 2020-12, but triggers an internal error in the library.
Removing any one of these three elements prevents the bug.

Steps to Reproduce

Original schema causing the bug: schema_cat.json
Minimal schema that still triggers the bug:

{
  "$defs": {
    "T": {
      "type": "object",
      "properties": {
        "x": {
          "type": "array",
          "prefixItems": [
            { "type": "integer" }
          ],
          "items": false,
          "default": [0]
        }
      }
    }
  },
  "$ref": "#/$defs/T"
}

This works correctly (no error) when $ref is not used:

{
  "type": "object",
  "properties": {
    "x": {
      "type": "array",
      "prefixItems": [
        { "type": "integer" }
      ],
      "items": false,
      "default": [0]
    }
  }
}

Using typify::import_types! and passing in either schema fails at comptime with error: value does not conform to the given schema.

Expected Behavior

The library should correctly parse the schema and validate default values.

Observed Behavior

The library throws an error or fails to process the schema when all three conditions (prefixItems, items: false, default) are present and used via $ref.

Environment

• typify 0.5.0
• rustc 1.92.0

---

I think that's all I could find out via my testing, let me know if anything else could be of use in debugging this.

[ahl]

prefixItems is a feature of JSON Schema 2020-12... which we don't support yet. It's simply being ignored so the schema is effectively:

{
  "type": "object",
  "properties": {
    "x": {
      "type": "array",
      "items": false,
      "default": [0]
    }
  }
}

So... yeah! [0] isn't valid against that schema.

This is the old JSON schema version:

{
  "$defs": {
    "T": {
      "type": "object",
      "properties": {
        "x": {
          "type": "array",
          "items": [
            {
              "type": "integer"
            }
          ],
          "additionalItems": false,
          "default": [0]
        }
      }
    }
  },
  "$ref": "#/$defs/T"
}

But that fails because we don't handle that kind of schema. Fortunately, the original schema has minItems and maxItems set. So if I do something a little closer to what's in that schema:

{
  "$defs": {
    "T": {
      "type": "object",
      "properties": {
        "x": {
          "type": "array",
          "minItems": 3,
          "maxItems": 3,
          "items": [
            { "type": "integer" },
            { "type": "integer" },
            { "type": "integer" }
          ],
          "additionalItems": false,
          "default": [ -1, -1, -1 ]
        }
      }
    }
  },
  "$ref": "#/$defs/T"
}

This works and we get this:

#[derive(:: serde :: Deserialize, :: serde :: Serialize, Clone, Debug)]
pub struct T {
    #[serde(default = "defaults::t_x")]
    pub x: (i64, i64, i64),
}

pub mod defaults {
    pub(super) fn t_x() -> (i64, i64, i64) {
        (-1_i64, -1_i64, -1_i64)
    }
}

... which seems decent. Feedback is welcome.

Regarding 2020-12 support. That work is ongoing... in the background... slowly. It hasn't been a high priority, but with OpenAPI 3.1+ becoming more broadly adopted, it may be a slightly higher priority for us.

[Big-Iron-Cheems]

Thanks for the in-depth response, I'll see if we can use a modified JSON without these yet unsupported features in the meantime 👍
I'll keep this open since it might be useful in tracking progress of 2020-12 support.

[dawid-nowak]

I think I just got hit by the lack of support for 2020-12 when I was trying to generate Rust types from the MCP protocol schema