Enable SWC plugin cache invalidation for external file dependencies

[soujvnunes]

Goals

1. Allow SWC plugins to declare external file dependencies that trigger cache invalidation
2. Enable true HMR for plugins that transform code based on external configuration files
3. Provide parity with Vite's plugin system which supports this via server.restart()

Non-Goals

1. Full filesystem access for WASM plugins during transform
2. Changes to the SWC plugin API itself
3. Supporting arbitrary file watching patterns

Background

I'm building tailwind-expand, a set of plugins that transform CSS component aliases into Tailwind utility classes at build time.

The @tailwind-expand/swc plugin works by:

1. TypeScript wrapper reads CSS at config load time (extracts expand at-rule blocks)
2. WASM plugin receives pre-expanded aliases and transforms JSX className attributes

Current limitation: When users modify their CSS aliases, changes aren't reflected until the dev server restarts because:

• SWC plugins run in WASM sandbox (no filesystem access during transform)
• Aliases are read at config load time, not during each transform
• Turbopack caches SWC transform results with no public invalidation API

For @tailwind-expand/vite, this was resolved by using server.restart(). No equivalent exists for Turbopack.

Current workaround: nodemon (inspiration: NextJS Tip: Hot reloading for dynamic servers) to auto-restart the server, but this loses React state and is slow.

Proposal

A configuration option to declare file dependencies for SWC plugins:

// next.config.ts
export default {
  experimental: {
    swcPlugins: [
      [pluginPath, options, { watchFiles: ['./app/globals.css'] }]
    ]
  }
}

When watched files change, Turbopack would invalidate cached transforms for modules processed by that plugin and re-run the plugin's config-time setup.

Alternative approaches:

• WASI preopened directories for specific files
• Plugin transform context with file modification timestamps

I'm happy to contribute if given guidance on the preferred approach