`@next/swc` consist of multiple rust packages to enable features. See below for the high level hierarchies.
```mermaid
flowchart TD
C(next-custom-transforms) --> A(napi)
C(next-custom-transforms) --> B(wasm)
D(next-core) --> A(napi)
E(next-build) --> A(napi)
F(next-api) --> A(napi)
C(next-custom-transforms) --> D
D(next-core) --> F(next-api)
D(next-core) --> E(next-build)
```
-`next-custom-transforms`: provides next-swc specific SWC transform visitors. Turbopack, and the plain next-swc bidnings (`transform`) use these transforms. Since this is a bottom package can be imported in any place (turbopack / next-swc / wasm), it is important package do not contain specific dependencies. For example, using Turbopack's VC in this package will cause build failures to wasm bindings.
-`next-core`: Implements Turbopack features for the next.js core functionality. This is also the place where Turbopack-specific transform providers (implementing `CustomTransformer`) lives, which wraps swc's transformer in the `next-custom-transforms`.
-`napi` / `wasm`: The actual binding interfaces, napi for the node.js and wasm for the wasm. Note wasm bindings cannot import packages using turbopack's feature.
#### To add new swc transforms
1. Implements a new visitor in `next-custom-transforms`. It is highly encouraged to use `VisitMut` instead of `Fold` for the performance reasons.
2. Implements a new `CustomTransformer` under `packages/next-swc/crates/next-core/src/next_shared/transforms` to make Turbopack's ecma transform plugin, then adjust corresponding rules in `packages/next-swc/crates/next-core/src/(next_client|next_server)/context.rs`.