Attributes Reference

ReflectAPI provides #[reflectapi(...)] attributes that control how Rust types are reflected into the schema and generated clients.

Struct / Enum Level

Field Level

Type Override

Transform

The transform callback receives &mut Field and can modify any field metadata — type, required, hidden, etc. It runs after type references are resolved.

ReflectAPI ships with built-in transforms in the reflectapi::transforms module:

• reflectapi::transforms::fallback_recursively — unwraps transparent wrappers (e.g. Arc<T> → T)
• reflectapi::transforms::make_required — makes a field required: Option<T> → required T, reflectapi::Option<T> → required Option<T> (nullable but not omittable). For non-Option types, marks the field as required without changing the type.
• reflectapi::transforms::make_nonnullable — makes a field non-nullable: Option<T> → T, reflectapi::Option<T> → T (keeps optionality, removes nullability). No-op for non-Option types.
• reflectapi::transforms::make_required_and_nonnullable — makes a field both required and non-nullable: Option<T> → required T, reflectapi::Option<T> → required T. For non-Option types, marks the field as required without changing the type.

Note: In reflectapi, "required" and "non-nullable" are distinct concepts. A field can be:

• Required + non-nullable (T, required) — must be present, cannot be null
• Required + nullable (Option<T>, required) — must be present, can be null
• Optional + nullable (reflectapi::Option<T>) — can be omitted or null
• Optional + non-nullable (T, not required) — can be omitted, but if present cannot be null

make_required removes the "can be omitted" (undefined) state. make_nonnullable removes the "can be null" state. make_required_and_nonnullable removes both.

Example: Controlling requiredness and nullability in the schema

A field may use an Option wrapper at the Rust level (e.g., because a middleware populates it before your handler runs) but you want generated clients to see a different contract.

#[derive(serde::Deserialize, reflectapi::Input)]
struct MyRequest {
    /// In Rust this is `Option<String>` (populated by middleware),
    /// but clients see it as a required `String` field.
    #[serde(default)]
    #[reflectapi(input_transform = "reflectapi::transforms::make_required")]
    pub tenant_id: Option<String>,
}

#[derive(serde::Deserialize, reflectapi::Input)]
struct PatchRequest {
    /// In Rust this is `reflectapi::Option<String>` (populated by middleware),
    /// but clients see it as a required `Option<String>` (nullable, not omittable).
    #[serde(default)]
    #[reflectapi(input_transform = "reflectapi::transforms::make_required")]
    pub correlation_id: reflectapi::Option<String>,

    /// In Rust this is `reflectapi::Option<String>`,
    /// but clients see it as an optional `String` (can be omitted, but not null).
    #[serde(default)]
    #[reflectapi(input_transform = "reflectapi::transforms::make_nonnullable")]
    pub display_name: reflectapi::Option<String>,

    /// In Rust this is `reflectapi::Option<String>`,
    /// but clients see it as a required `String` (must be present, cannot be null).
    #[serde(default)]
    #[reflectapi(input_transform = "reflectapi::transforms::make_required_and_nonnullable")]
    pub user_id: reflectapi::Option<String>,
}

Visibility

skip vs hidden

Both attributes remove a field from generated clients. The key difference:

• skip removes the field from the schema entirely. The field's type is never reflected, so it does not need to implement Input or Output. Use this for internal bookkeeping fields whose types are not part of your API.

• hidden keeps the field in the schema JSON (marked with "hidden": true) but excludes it from generated clients, documentation, and OpenAPI specs. The type must still implement the relevant trait. Use this for fields that are functionally required by server-side infrastructure — for example, a middleware or a proxy layer that populates the field / header before deserialization — but should not appear in client interfaces.

When to use hidden over skip: The field stays in the schema JSON so that server-side tooling (the axum adapter, middleware, or custom infrastructure) can inspect the full type structure at runtime. If nothing on the server needs the field's schema metadata, prefer skip.

Neither skip nor hidden affects serde serialization. For output types, serde will still serialize a hidden field onto the wire — hidden only controls what generated code and documentation show. If a field must never appear in responses, use #[serde(skip_serializing)] instead.

Please not that neither skip nor hidden prevent a malicious client from sending the fields in a request. A middleware may overwrite it or reject or validate as needed. It is up to the specific implementation of your server.

Example: hidden header field

#[derive(serde::Deserialize, reflectapi::Input)]
pub struct MyHeaders {
    /// Visible to clients — they must provide this
    pub authorization: String,

    /// Not visible to the generated clients and documentation
    /// Expected to be populated by a proxy or server-side middleware.
    #[reflectapi(hidden)]
    #[serde(default)]
    pub x_internal_request_id: String,
}

#[serde(default)] on skipped and hidden fields

When a field is excluded from generated clients (via skip, input_skip, or hidden), clients will not send it. Whether you add #[serde(default)] is your choice and depends on your deployment:

• With #[serde(default)]: If the field is absent, serde fills the default value. The request succeeds even if no proxy or middleware populates the field. Use this for optional metadata (trace IDs, correlation IDs) where absence is acceptable.

• Without #[serde(default)]: If the field is absent, deserialization fails with a protocol error. Use this for fields that a proxy or middleware is expected to inject — a missing value means the infrastructure is misconfigured, and you want to reject the request loudly rather than proceed silently with a zero-value.

Restrictions

#[reflectapi(hidden)] cannot be used on unnamed (tuple) struct or enum variant fields. Hiding a positional element would shift indices in generated clients, breaking wire compatibility. Use hidden only on named fields.

Enum Variant Level