fix: keep arbitrary keys on an unconstrained object union member (#70)

[lightsofapollo]

Fixes #70.

The bug

Every member of a type: [string, number, boolean, object, array, "null"] union shares a single SchemaDetails, so the object member can carry no object shape at all. build_typed_multi_union_variant still routed it through add_allocated_object_schema, which projected it as a closed, empty struct:

pub struct GetEventResponsePayloadObject {}

{"payload":{"key":"value"}} deserialized without error and then serialized back as {"payload":{}} — every key silently dropped. Scalars and the array member were fine.

Root cause

The single-type path already guards this: analyze_single_typed_schema's Object arm checks should_use_dynamic_json before building a struct. The typed-multi path had no equivalent check. It couldn't reuse is_dynamic_object_pattern directly either, because that starts from schema.schema_type(), which on a TypedMulti reports only the first non-null member (string, here).

So the object-shape half of is_dynamic_object_pattern is split into object_shape_is_unconstrained(&SchemaDetails), which asks the question without the type check, and both callers use it.

The fix

An unconstrained object member now gets a map carrier instead of a struct:

///Object variant in union
pub type GetEventResponsePayloadObject = std::collections::BTreeMap<String, serde_json::Value>;

#[serde(untagged)]
pub enum GetEventResponsePayload {
    String(String),
    Number(f64),
    Boolean(bool),
    GetEventResponsePayloadObject(GetEventResponsePayloadObject),
    GetEventResponsePayloadArray(GetEventResponsePayloadArray),
}

BTreeMap<String, serde_json::Value> is the shape this codebase already uses for an untyped map (UntypedShape::ValueMap, the #[serde(flatten)] additional-properties carrier). Critically it matches only a JSON object, so the untagged enum keeps routing arrays and scalars to their own members — serde_json::Value would swallow them. The generated type name is unchanged, so only its definition moves.

Members the spec does shape are untouched and keep their generated structs: properties, required, additionalProperties (including false, which stays a closed empty struct), patternProperties, propertyNames, minProperties/maxProperties, dependentRequired/dependentSchemas, if/then/else.

Verification

tests/unconstrained_object_union_member_test.rs:

• the generated shape for an unconstrained member, and that it is not a struct;
• a member with properties and a member with additionalProperties: false still generate structs;
• a scratch-crate test that compiles the generated code and round-trips {"key":"value"}, {"nested":{"a":[1,2]},"b":null}, {}, "text", 1.5, true, ["a",1], [] through the union, and asserts an object lands in the object member.

cargo test --all-features, cargo clippy --all-features -- -D warnings, and cargo fmt --check are clean.

Follow-up (not in this PR)

Found while verifying: the anyOf/oneOf branch path still gives a bare {"type": "object"} branch a serde_json::Value alias, which is greedy in an untagged enum — with anyOf: [{type: object}, {type: string}] a JSON string deserializes into the object branch. No data loss, but the wrong variant is selected. Tracked separately; the same map carrier is the fix.

🤖 Generated with Claude Code

https://claude.ai/code/session_012mBAVbYp2iUSL8VwpyDf19

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Project	Deployment	Actions	Updated
openapi-to-rust	Ready	Preview	Sep 8, 2026 1:03pm UTC

[lightsofapollo]

Folding this into #72 so everything from this investigation lands as one reviewable change — #72 now targets main and contains this commit plus the related union fixes. Closing in favor of that.