allOf of if/then rules on an object schema turns it into serde_json::Value and untags its discriminated union #111

Summary

An object schema with type: object and properties that also has an allOf of
validation-only if/then rules generates as serde_json::Value. A discriminated oneOf
that has this schema as a branch turns from tagged to #[serde(untagged)].

The rules do not change the shape of the object. They only constrain values, for example
"if zone is utc, name must end with Z".

Version: openapi-to-rust 0.19.0. Command:
openapi-to-rust generate spec.json --types-only --module-name generated --output-dir out.

Reproduction

{
  "openapi": "3.1.0",
  "info": {"title": "repro", "version": "1"},
  "paths": {},
  "components": {"schemas": {
    "Pet": {
      "oneOf": [{"$ref": "#/components/schemas/Cat"}, {"$ref": "#/components/schemas/Dog"}],
      "discriminator": {"propertyName": "kind"}
    },
    "Cat": {
      "type": "object",
      "required": ["kind", "name"],
      "properties": {"kind": {"type": "string", "const": "Cat"}, "name": {"type": "string"}, "zone": {"type": "string"}},
      "allOf": [{"if": {"properties": {"zone": {"const": "utc"}}, "required": ["zone"]}, "then": {"properties": {"name": {"pattern": "Z$"}}}}]
    },
    "Dog": {
      "type": "object",
      "required": ["kind", "name"],
      "properties": {"kind": {"type": "string", "const": "Dog"}, "name": {"type": "string"}}
    }
  }}
}

Actual output (excerpt):

#[derive(Debug, Clone, Deserialize, Serialize)]
#[serde(untagged)]
pub enum Pet {
    Cat(Cat),
    Dog(Dog),
}
pub type Cat = serde_json::Value;

Expected output: the same as without the allOf. Cat is a struct with kind, name
and zone, and Pet keeps the tagged serialization on kind. If you remove the allOf
from Cat, the generator emits exactly that.

Cause

1. Schema is an untagged serde enum (src/openapi.rs). The AllOf case comes before
   the object case, so Cat deserializes as Schema::AllOf. Its type and properties
   go into details.
2. branch_resolves_to_object_inner (src/analysis.rs, about line 2365) decides from the
   allOf members only, and returns before it reads the schema's own type: "Other allOf
   shapes are object-like only when at least one meaningful member is." An if/then
   member has no type, so the result is false.
3. The oneOf analysis (src/analysis.rs, about lines 4797-4813) falls back to an
   untagged union when any branch does not resolve to an object.

Suggested fix

• In branch_resolves_to_object_inner, treat a Schema::AllOf as object-like when its own
  schema_type is object or its details have properties.
• Treat an allOf member that holds only if, then and else (and annotations) as
  validation-only. It should not change the generated type.

Related: #65 (a base object combined with a variant union), a different shape with the same
serde_json::Value fallback.

Context

SiennaSchemas adds such rules
to the five branches of its TimeSeriesAssociation union. Until a fix is released, our
bundler removes rule-only allOf members before generation.