Fix SWC loader options for files outside of RSC layers (#50341)

The React Server Component transform of SWC shouldn't be enabled for
files that are not part of "app layers", for example middleware.

packages/next/src/build/webpack-config.ts

@@ -847,8 +847,8 @@ export default async function getBaseWebpackConfig(
         rootDir: dir,
         pagesDir,
         appDir,
-        hasServerComponents,
         hasReactRefresh: dev && isClient,
+        hasServerComponents: true,
         fileReading: config.experimental.swcFileReading,
         nextConfig: config,
         jsConfig,
@@ -874,13 +874,16 @@ export default async function getBaseWebpackConfig(
     : []
   const swcLoaderForClientLayer = hasServerComponents
     ? useSWCLoader
-      ? [getSwcLoader({ isServerLayer: false })]
+      ? [getSwcLoader({ hasServerComponents, isServerLayer: false })]
       : // When using Babel, we will have to add the SWC loader
         // as an additional pass to handle RSC correctly.
         // This will cause some performance overhead but
         // acceptable as Babel will not be recommended.
         [getSwcLoader({ isServerLayer: false }), getBabelLoader()]
     : []
+  const swcLoaderForMiddlewareLayer = useSWCLoader
+    ? getSwcLoader({ hasServerComponents: false })
+    : getBabelLoader()
 
   // Loader for API routes needs to be differently configured as it shouldn't
   // have RSC transpiler enabled, so syntax checks such as invalid imports won't
@@ -2022,7 +2025,7 @@ export default async function getBaseWebpackConfig(
             {
               test: codeCondition.test,
               issuerLayer: WEBPACK_LAYERS.middleware,
-              use: defaultLoaders.babel,
+              use: swcLoaderForMiddlewareLayer,
             },
             ...(hasServerComponents
               ? [

test/e2e/app-dir/app-middleware/middleware.js

@@ -1,5 +1,9 @@
 import { NextResponse } from 'next/server'
 
+// It should be able to import `headers` inside middleware
+import { headers } from 'next/headers'
+console.log(!!headers)
+
 /**
  * @param {import('next/server').NextRequest} request
  */