> This page is for SDKs, version Legacy. > For other versions, use one of these documentation indexes: > - Current (default): https://buildwithfern.com/learn/sdks/current/llms.txt > - Legacy: https://buildwithfern.com/learn/sdks/legacy/llms.txt > For clean Markdown content of this page, append .md to this URL. For the complete documentation index, see https://buildwithfern.com/learn/llms.txt. # Migrating to Rust generator v1 > Breaking changes in version 1.0.0 of the Fern Rust SDK generator and how to update code that uses your generated Rust crate. Version 1.0.0 of the Rust SDK generator keeps most public symbols and `use` paths, but changes a few pervasive shapes. A small number of root causes can produce many `cargo build` errors. Release the regenerated SDK as a new major version, and share this page with your SDK users so they can update their code. ## Before you upgrade Generator versions 1.0.0 and later read configuration from [`sdk-config.yml`](/learn/sdks/reference/sdk-config) instead of `generators.yml`. Run `fern sdk migrate` to create `sdk-config.yml` from your existing configuration. ## `DateTime` becomes `DateTime` Date-time fields and builder setters keep the offset returned by the server instead of normalizing to UTC. Comparisons are unaffected because `chrono` compares instants, but `Display` output and re-serialized values include the original offset instead of `Z`. **`Before`** ```rust title="Before" let watered_at: DateTime = plant.watered_at; let plant = Plant::builder().watered_at(Utc::now()).build()?; ``` **`After`** ```rust title="After" let watered_at: DateTime = plant.watered_at; // .with_timezone(&Utc) to convert let plant = Plant::builder().watered_at(Utc::now().fixed_offset()).build()?; ``` **To fix:** Update type annotations to `DateTime`, and call `.fixed_offset()` when passing a `DateTime` to a setter. ## `Default` is only derived when every field is optional Model types with required fields no longer derive `Default`, so `..Default::default()` fails with `E0277`. **`Before`** ```rust title="Before" let request = CreatePlantRequest { body: plant, ..Default::default() }; ``` **`After`** ```rust title="After" let request = CreatePlantRequest::builder().body(plant).build()?; ``` **To fix:** Use the type's builder instead of struct update syntax with `Default`. ## `ApiError` variants carry the error body Error variants are tuple variants that hold the error body type, instead of struct variants with a generated `message` field. `NotFoundError` and `ConflictError` aren't generated unless the API declares them; undeclared statuses arrive as `ApiError::Http`. **`Before`** ```rust title="Before" Err(ApiError::UnauthorizedError { message, .. }) => log(&message), Err(ApiError::NotFoundError { resource_id, .. }) => retry(resource_id), ``` **`After`** ```rust title="After" Err(ApiError::UnauthorizedError(body)) => log(&body.response_text), Err(ApiError::Http { status: 404, message }) => retry(message), ``` `ApiError` also adds `DecodeError` and `WebSocket` variants. Because `ApiError` isn't `#[non_exhaustive]`, an exhaustive `match` without a wildcard arm fails to compile. **To fix:** Update `match` arms to the new variant shapes, and add arms or a wildcard for the new variants. ## Paginated list methods return a stream Paginated endpoints keep their original method name, are no longer `async`, and return a `Stream` of typed pages. The separate `*_paginated` method, `AsyncPaginator`, and `core::pagination` module are removed, and the single-page fetch isn't public. **`Before`** ```rust title="Before" let page: CursorPage = client.plants.list(&request, None).await?; let mut pager = client.plants.list_paginated(&request, None).await?; ``` **`After`** ```rust title="After" let mut pages = client.plants.list(&request, None); while let Some(page) = pages.next().await { for plant in page? { process(plant); } } ``` **To fix:** Remove `.await` from the list call and iterate over the stream. Each item is a `Result>` for one page. ## Header parameters are method arguments Declared header parameters were previously omitted from method signatures, so those headers were never sent. Each header parameter is an `Option` argument before `options`. **`Before`** ```rust title="Before" client.plants.create(&plant, None).await?; ``` **`After`** ```rust title="After" client.plants.create(&plant, Some(idempotency_key), None).await?; ``` Query parameters may stay in the request struct or move to a separate `{Operation}QueryRequest` argument, depending on the endpoint. **To fix:** Add the header arguments to affected calls, and check query parameters for each endpoint. ## Union variants hold a data struct The JSON on the wire is unchanged. Instead of inline fields and per-variant constructors, each union variant holds a single `data` struct, and the union has `is_*`, `as_*`, and `into_*` accessors. **`Before`** ```rust title="Before" let care = CarePlan::watering_with_schedule(vec![schedule], Some(true)); match &care { CarePlan::Watering { schedule, notify } => apply(schedule, *notify), _ => {} } ``` **`After`** ```rust title="After" let care = CarePlan::Watering { data: CarePlanWatering { schedule: Some(vec![schedule]), notify: Some(true) }, }; if let Some(watering) = care.as_watering() { apply(&watering.schedule, watering.notify); } ``` **To fix:** Construct variants with a `data` struct, and replace `match` destructuring with the accessor methods. ## Other changes * Raw binary bodies are taken by value: `&Vec` becomes `Vec`. * `RequestOptions` isn't in the prelude. Import it with `use ::core::RequestOptions`. * The crate name comes from the SDK configuration instead of `_api`. Check `Cargo.toml`. * The catch-all field on open models is renamed from `extra` to `additional_properties`. * The redundant inner module is removed. `resources::plants::plants::PlantsClient` no longer resolves, but `resources::plants::PlantsClient` does. ## New capabilities * **Shared configuration:** `set_base_url`, `set_access_token`, `set_api_key`, and `set_timeout` apply to clients that were already created. * **Wire formats:** Form-urlencoded bodies, multipart text parts, vendor `+json` media types, and non-default parameter styles (`deepObject`, `pipeDelimited`, `spaceDelimited`, `matrix`, `label`) are serialized according to the spec. * **Headers, cookies, and text or XML responses** are supported.