Two schema generation failures involving `Schema::Null` in unions

[ksato9700]

Hi, I ran into two separate cases where rsgen-avro v0.19.3 fails to generate code.
I've minimized them into small repro schemas — hope this helps!

---

Case 1: Union containing only null

Schema

{
  "type": "record",
  "name": "Example",
  "namespace": "com.example",
  "fields": [
    { "name": "only_null", "type": ["null"], "default": null }
  ]
}

Error

Templating error: Invalid Schema::Union of only Schema::Null

What I'd expect

A union of only null is an unusual pattern, but it does appear in the wild as a
placeholder field. It might be reasonable to map it to Option<()>, or at least
emit a warning and skip the field gracefully.

---

Case 2: null as a non-first member of map values

Schema

{
  "type": "record",
  "name": "Example",
  "namespace": "com.example",
  "fields": [
    { "name": "map_with_null_last", "type": {"type": "map", "values": ["string", "null"]}, "default": {} }
  ]
}

Error

Templating error: Invalid use of Schema::Null

What I'd expect

Map values of ["string", "null"] could be represented as HashMap<String, Option<String>>.

---

Notes

• These seem to be separate from the previously fixed issue Cannot generate an Optional array #8, which covered
  ["null", {"type": "array", ...}] and produced a different error message.
• For reference, ["null", {"type": "long", "logicalType": "timestamp-millis"}]
  (a null-first union with a logical type) works correctly and generates Option<i64> —
  so null-first unions in general are fine, just these two specific patterns aren't.

Environment

• rsgen-avro version: 0.19.3
• Rust edition: 2021
• OS: macOS 15

[martin-g]

It might be reasonable to map it to Option<()>

This will fail when apache_avro 0.22 is released
See apache/avro-rs#486 (comment) and apache/avro-rs#489 (comment).

[martin-g]

• so null-first unions in general are fine, just these two specific patterns aren't.

Yes, I don't remember where I learned this from but I also know that null should be first in the union. Otherwise such kind of problems appear.
Recently we had short discussion about it at
apache/avro-rs#477 (comment)
But since there is no such requirement in the Avro specification it is just a "good practice"

[Kriskras99]

In my enum rework, this could be represented as

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum UnionOfNull {
    Null
}

I'll make sure to add a test for this use case

[lerouxrgd]

As for case 1, let's see when apache-avro 0.22 is released.

As for case2, it is not supported in this library as mentioned in the limitations:

Rust Option are supported through Avro unions having "null" in their first position only (See #39)

[ksato9700]

Thank you for your response. I understand that Case 2 is not supported. I should have read the limitations carefully; my apologies.