Fix app dir vanilla-extract support (#48306)

Closes #48222.

This PR introduces a new utility function that checks the resource path,
module type, and loaders to replace the existing CSS regex in the Flight
loaders and plugins. The new function is more robust and can be extended
to correctly support other libraries such as `vanilla-extract`.

cc @SuttonJack and @SukkaW (this could be related to the other issue
we've discussed too).

packages/next/src/build/webpack/loaders/utils.ts

@@ -15,6 +15,25 @@ export function isClientComponentModule(mod: {
 
 export const regexCSS = /\.(css|scss|sass)(\?.*)?$/
 
+// This function checks if a module is able to emit CSS resources. You should
+// never only rely on a single regex to do that.
+export function isCSSMod(mod: {
+  resource: string
+  type?: string
+  loaders?: { loader: string }[]
+}): boolean {
+  return !!(
+    mod.type === 'css/mini-extract' ||
+    (mod.resource && regexCSS.test(mod.resource)) ||
+    mod.loaders?.some(
+      ({ loader }) =>
+        loader.includes('next-style-loader/index.js') ||
+        loader.includes('mini-css-extract-plugin/loader.js') ||
+        loader.includes('@vanilla-extract/webpack-plugin/loader/')
+    )
+  )
+}
+
 export function getActions(mod: {
   resource: string
   buildInfo: any

packages/next/src/build/webpack/plugins/flight-client-entry-plugin.ts

@@ -27,7 +27,7 @@ import {
   generateActionId,
   getActions,
   isClientComponentModule,
-  regexCSS,
+  isCSSMod,
 } from '../loaders/utils'
 import { traverseModules, forEachEntryModule } from '../utils'
 import { normalizePathSep } from '../../../shared/lib/page-path/normalize-path-sep'
@@ -354,7 +354,7 @@ export class ClientReferenceEntryPlugin {
         const resource = mod.resource
         const modId = resource
         if (modId) {
-          if (regexCSS.test(modId)) {
+          if (isCSSMod(mod)) {
             cssImportsForChunk[entryName].add(modId)
           }
         }
@@ -506,8 +506,7 @@ export class ClientReferenceEntryPlugin {
         compilation.moduleGraph.getResolvedModule(dependencyToFilter)
       if (!mod) return
 
-      const rawRequest = mod.rawRequest
-      const isCSS = regexCSS.test(rawRequest)
+      const isCSS = isCSSMod(mod)
 
       // We have to always use the resolved request here to make sure the
       // server and client are using the same module path (required by RSC), as

packages/next/src/build/webpack/plugins/flight-manifest-plugin.ts

@@ -12,7 +12,7 @@ import {
   SYSTEM_ENTRYPOINTS,
 } from '../../../shared/lib/constants'
 import { relative, sep } from 'path'
-import { isClientComponentModule, regexCSS } from '../loaders/utils'
+import { isClientComponentModule, isCSSMod } from '../loaders/utils'
 import { getProxiedPluginState } from '../../build-context'
 
 import { traverseModules } from '../utils'
@@ -157,17 +157,7 @@ export class ClientReferenceManifestPlugin {
         mod: webpack.NormalModule,
         chunkCSS: string[]
       ) => {
-        const isCSSModule =
-          regexCSS.test(mod.resource) ||
-          mod.type === 'css/mini-extract' ||
-          (!!mod.loaders &&
-            (dev
-              ? mod.loaders.some((item) =>
-                  item.loader.includes('next-style-loader/index.js')
-                )
-              : mod.loaders.some((item) =>
-                  item.loader.includes('mini-css-extract-plugin/loader.js')
-                )))
+        const isCSSModule = isCSSMod(mod)
 
         // Skip all modules from the pages folder. CSS modules are a special case
         // as they are generated by mini-css-extract-plugin and these modules