Next.js plugin for alternate bundler (#76186)

This adds a new package allowing users to opt-into using Rspack in place of webpack in Next.js. While there is some logic to handle rspack in core, the actual third-party dependencies are not included there, only conditionally loaded. This package:
    
- Includes the actual npm dependencies on `@rspack/core` and `@rspack/plugin-react-refresh`
- Sets the `NEXT_RSPACK` environment variable, which Next.js itself uses to load the dependencies
- It exists as a kind of no-op `withRspack` decorator. @timneutkens is this the best approach to this?

package.json

@@ -16,7 +16,7 @@
     "test-types": "tsc",
     "test-unit": "jest test/unit/ packages/next/ packages/font",
     "test-dev": "cross-env NEXT_TEST_MODE=dev pnpm testheadless",
-    "test-dev-rspack": "cross-env NEXT_TEST_MODE=dev NEXT_RSPACK=1 BUILTIN_FLIGHT_CLIENT_ENTRY_PLUGIN=1 BUILTIN_APP_LOADER=1 BUILTIN_SWC_LOADER=1 pnpm testheadless",
+    "test-dev-rspack": "cross-env NEXT_TEST_MODE=dev NEXT_TEST_USE_RSPACK=1 BUILTIN_FLIGHT_CLIENT_ENTRY_PLUGIN=1 BUILTIN_APP_LOADER=1 BUILTIN_SWC_LOADER=1 pnpm testheadless",
     "test-dev-turbo": "cross-env NEXT_TEST_MODE=dev TURBOPACK=1 TURBOPACK_DEV=1 pnpm testheadless",
     "test-start": "cross-env NEXT_TEST_MODE=start pnpm testheadless",
     "test-start-rspack": "cross-env NEXT_TEST_MODE=start NEXT_RSPACK=1 BUILTIN_FLIGHT_CLIENT_ENTRY_PLUGIN=1 BUILTIN_APP_LOADER=1 BUILTIN_SWC_LOADER=1 pnpm testheadless",

packages/next-plugin-rspack/README.md

@@ -0,0 +1,44 @@
+# @next/plugin-rspack
+
+This plugin allows you to use [Rspack](https://rspack.dev) in place of webpack with Next.js.
+
+## Installation
+
+```
+npm install @next/plugin-rspack
+```
+
+or
+
+```
+yarn add @next/plugin-rspack
+```
+
+## Usage
+
+Create or update a `next.config.js`/`next.config.ts` and wrap your existing configuration:
+
+```js
+const withRspack = require('@next/plugin-rspack')
+
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  /* config options here */
+}
+
+module.exports = withRspack(nextConfig)
+```
+
+## Usage with next-compose-plugins
+
+Alternatively, you can use `next-compose-plugins` to quickly integrate `@next/plugin-rspack` with other Next.js plugins:
+
+```js
+const withPlugins = require('next-compose-plugins')
+const withRspack = require('@next/plugin-rspack')
+
+module.exports = withPlugins([
+  [withRspack],
+  // your other plugins here
+])
+```

packages/next-plugin-rspack/index.js

@@ -0,0 +1,7 @@
+module.exports = function withRspack(config) {
+  process.env.NEXT_RSPACK = 'true'
+  process.env.BUILTIN_FLIGHT_CLIENT_ENTRY_PLUGIN = 'true'
+  process.env.BUILTIN_APP_LOADER = 'true'
+  process.env.BUILTIN_SWC_LOADER = 'true'
+  return config
+}

packages/next-plugin-rspack/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@next/plugin-rspack",
+  "version": "15.2.0-canary.65",
+  "repository": {
+    "url": "vercel/next.js",
+    "directory": "packages/next-plugin-rspack"
+  },
+  "dependencies": {
+    "@rspack/core": "npm:@rspack-canary/core@1.2.0-canary-37cc738d-20250207113050",
+    "@rspack/plugin-react-refresh": "1.0.1"
+  }
+}

packages/next/errors.json

@@ -644,5 +644,7 @@
   "643": "Invalid \"devIndicator.position\" provided, expected one of %s, received %s",
   "644": "@rspack/core is not available. Please make sure the appropriate Next.js plugin is installed.",
   "645": "@rspack/plugin-react-refresh is not available. Please make sure the appropriate Next.js plugin is installed.",
-  "646": "No span found for compilation"
+  "646": "No span found for compilation",
+  "647": "@rspack/core is not available. Please make sure `@next/plugin-rspack` is correctly installed.",
+  "648": "@rspack/plugin-react-refresh is not available. Please make sure `@next/plugin-rspack` is correctly installed."
 }

packages/next/src/shared/lib/get-rspack.ts

@@ -5,7 +5,7 @@ export function getRspackCore() {
   } catch (e) {
     if (e instanceof Error && 'code' in e && e.code === 'MODULE_NOT_FOUND') {
       throw new Error(
-        '@rspack/core is not available. Please make sure the appropriate Next.js plugin is installed.'
+        '@rspack/core is not available. Please make sure `@next/plugin-rspack` is correctly installed.'
       )
     }
 
@@ -20,7 +20,7 @@ export function getRspackReactRefresh() {
   } catch (e) {
     if (e instanceof Error && 'code' in e && e.code === 'MODULE_NOT_FOUND') {
       throw new Error(
-        '@rspack/plugin-react-refresh is not available. Please make sure the appropriate Next.js plugin is installed.'
+        '@rspack/plugin-react-refresh is not available. Please make sure `@next/plugin-rspack` is correctly installed.'
       )
     }
 

test/lib/create-next-install.js

@@ -8,6 +8,7 @@ const { linkPackages } =
   require('../../.github/actions/next-stats-action/src/prepare/repo-setup')()
 
 const PREFER_OFFLINE = process.env.NEXT_TEST_PREFER_OFFLINE === '1'
+const useRspack = process.env.NEXT_TEST_USE_RSPACK === '1'
 
 async function installDependencies(cwd, tmpDir) {
   const args = [
@@ -122,6 +123,7 @@ async function createNextInstall({
             })
           )
       }
+
       const combinedDependencies = {
         next: pkgPaths.get('next'),
         ...Object.keys(dependencies).reduce((prev, pkg) => {
@@ -131,6 +133,12 @@ async function createNextInstall({
         }, {}),
       }
 
+      if (useRspack) {
+        combinedDependencies['@next/plugin-rspack'] = pkgPaths = pkgPaths.get(
+          '@next/plugin-rspack'
+        )
+      }
+
       const scripts = {
         debug: `NEXT_PRIVATE_SKIP_CANARY_CHECK=1 NEXT_TELEMETRY_DISABLED=1 NEXT_TEST_NATIVE_DIR=${process.env.NEXT_TEST_NATIVE_DIR} node --inspect --trace-deprecation --enable-source-maps node_modules/next/dist/bin/next`,
         ...packageJson.scripts,
@@ -183,6 +191,15 @@ async function createNextInstall({
           .traceAsyncFn(() => installDependencies(installDir, tmpDir))
       }
 
+      if (useRspack) {
+        // This is what the @next/plugin-rspack plugin does.
+        // TODO: Load the plugin properly during test
+        process.env.NEXT_RSPACK = 'true'
+        process.env.BUILTIN_FLIGHT_CLIENT_ENTRY_PLUGIN = 'true'
+        process.env.BUILTIN_APP_LOADER = 'true'
+        process.env.BUILTIN_SWC_LOADER = 'true'
+      }
+
       return {
         installDir,
         pkgPaths,