Fix CSS imports tree-shaking

packages/next/build/webpack/loaders/next-app-loader.ts

@@ -33,6 +33,7 @@ async function createTreeCodeFromPath({
 }) {
   const splittedPath = pagePath.split(/[\\/]/)
   const appDirPrefix = splittedPath[0]
+  const pages: string[] = []
 
   async function createSubtreePropsFromSegmentPath(
     segments: string[]
@@ -56,6 +57,8 @@ async function createTreeCodeFromPath({
       if (parallelSegment === 'page') {
         const matchedPagePath = `${appDirPrefix}${parallelSegmentPath}`
         const resolvedPagePath = await resolve(matchedPagePath)
+        if (resolvedPagePath) pages.push(resolvedPagePath)
+
         // Use '' for segment as it's the page. There can't be a segment called '' so this is the safest way to add it.
         props[parallelKey] = `['', {}, {layoutOrPagePath: ${JSON.stringify(
           resolvedPagePath
@@ -107,7 +110,7 @@ async function createTreeCodeFromPath({
   }
 
   const tree = await createSubtreePropsFromSegmentPath([])
-  return `const tree = ${tree}.children;`
+  return [`const tree = ${tree}.children;`, pages]
 }
 
 function createAbsolutePath(appDir: string, pathToTurnAbsolute: string) {
@@ -173,14 +176,15 @@ const nextAppLoader: webpack.LoaderDefinitionFunction<{
     }
   }
 
-  const treeCode = await createTreeCodeFromPath({
+  const [treeCode, pages] = await createTreeCodeFromPath({
     pagePath,
     resolve: resolver,
     resolveParallelSegments,
   })
 
   const result = `
     export ${treeCode}
+    export const pages = ${JSON.stringify(pages)}
 
     export const AppRouter = require('next/dist/client/components/app-router.js').default
     export const LayoutRouter = require('next/dist/client/components/layout-router.js').default

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

@@ -1,6 +1,5 @@
 import { stringify } from 'querystring'
 import path from 'path'
-import { relative } from 'path'
 import { webpack, sources } from 'next/dist/compiled/webpack/webpack'
 import {
   getInvalidator,
@@ -96,7 +95,7 @@ export class FlightClientEntryPlugin {
           // Note that this isn't that reliable as webpack is still possible to assign
           // additional queries to make sure there's no conflict even using the `named`
           // module ID strategy.
-          let ssrNamedModuleId = relative(compiler.context, modResource)
+          let ssrNamedModuleId = path.relative(compiler.context, modResource)
           if (!ssrNamedModuleId.startsWith('.')) {
             // TODO use getModuleId instead
             ssrNamedModuleId = `./${ssrNamedModuleId.replace(/\\/g, '/')}`
@@ -248,8 +247,56 @@ export class FlightClientEntryPlugin {
       )
     })
 
-    // After optimizing all the modules, we collect the CSS that are still used.
+    // After optimizing all the modules, we collect the CSS that are still used
+    // by the certain chunk.
     compilation.hooks.afterOptimizeModules.tap(PLUGIN_NAME, () => {
+      const cssImportsForChunk: Record<string, string[]> = {}
+
+      function collectModule(entryName: string, mod: any) {
+        const resource = mod.resource
+        if (resource) {
+          if (regexCSS.test(resource)) {
+            cssImportsForChunk[entryName].push(resource)
+          }
+        }
+      }
+
+      compilation.chunkGroups.forEach((chunkGroup: any) => {
+        chunkGroup.chunks.forEach((chunk: webpack.Chunk) => {
+          // Here we only track page chunks.
+          if (!chunk.name) return
+          if (!chunk.name.endsWith('/page')) return
+
+          const entryName = path.join(compiler.context, chunk.name)
+
+          if (!cssImportsForChunk[entryName]) {
+            cssImportsForChunk[entryName] = []
+          }
+
+          const chunkModules = compilation.chunkGraph.getChunkModulesIterable(
+            chunk
+          ) as Iterable<webpack.NormalModule>
+          for (const mod of chunkModules) {
+            collectModule(entryName, mod)
+
+            const anyModule = mod as any
+            if (anyModule.modules) {
+              anyModule.modules.forEach((concatenatedMod: any) => {
+                collectModule(entryName, concatenatedMod)
+              })
+            }
+          }
+
+          const entryCSSInfo: Record<string, string[]> =
+            flightCSSManifest.__entry_css__ || {}
+          entryCSSInfo[entryName] = cssImportsForChunk[entryName]
+
+          Object.assign(flightCSSManifest, {
+            __entry_css__: entryCSSInfo,
+          })
+        })
+      })
+
       forEachEntryModule(({ entryModule }) => {
         for (const connection of compilation.moduleGraph.getOutgoingConnections(
           entryModule

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

@@ -70,6 +70,10 @@ export type FlightManifest = {
 
 export type FlightCSSManifest = {
   [modulePath: string]: string[]
+} & {
+  __entry_css__?: {
+    [entry: string]: string[]
+  }
 }
 
 const PLUGIN_NAME = 'FlightManifestPlugin'

packages/next/server/app-render.tsx

@@ -512,7 +512,8 @@ function getSegmentParam(segment: string): {
 function getCssInlinedLinkTags(
   serverComponentManifest: FlightManifest,
   serverCSSManifest: FlightCSSManifest,
-  filePath: string
+  filePath: string,
+  serverCSSForEntries: string[]
 ): string[] {
   const layoutOrPageCss =
     serverCSSManifest[filePath] ||
@@ -525,24 +526,46 @@ function getCssInlinedLinkTags(
   const chunks = new Set<string>()
 
   for (const css of layoutOrPageCss) {
-    const mod = serverComponentManifest[css]
-    if (mod) {
-      for (const chunk of mod.default.chunks) {
-        chunks.add(chunk)
+    // We only include the CSS if it's a global CSS, or it is used by this
+    // entrypoint.
+    if (serverCSSForEntries.includes(css) || !/\.module\.css/.test(css)) {
+      const mod = serverComponentManifest[css]
+      if (mod) {
+        for (const chunk of mod.default.chunks) {
+          chunks.add(chunk)
+        }
       }
     }
   }
 
   return [...chunks]
 }
 
+function getServerCSSForEntries(
+  serverCSSManifest: FlightCSSManifest,
+  entries: string[]
+) {
+  const css = []
+  for (const entry of entries) {
+    const entryName = entry.replace(/\.[^.]+$/, '')
+    if (
+      serverCSSManifest.__entry_css__ &&
+      serverCSSManifest.__entry_css__[entryName]
+    ) {
+      css.push(...serverCSSManifest.__entry_css__[entryName])
+    }
+  }
+  return css
+}
+
 /**
  * Get inline <link rel="preload" as="font"> tags based on server CSS manifest and font loader manifest. Only used when rendering to HTML.
  */
 function getPreloadedFontFilesInlineLinkTags(
   serverComponentManifest: FlightManifest,
   serverCSSManifest: FlightCSSManifest,
   fontLoaderManifest: FontLoaderManifest | undefined,
+  serverCSSForEntries: string[],
   filePath?: string
 ): string[] {
   if (!fontLoaderManifest || !filePath) {
@@ -559,10 +582,14 @@ function getPreloadedFontFilesInlineLinkTags(
   const fontFiles = new Set<string>()
 
   for (const css of layoutOrPageCss) {
-    const preloadedFontFiles = fontLoaderManifest.app[css]
-    if (preloadedFontFiles) {
-      for (const fontFile of preloadedFontFiles) {
-        fontFiles.add(fontFile)
+    // We only include the CSS if it's a global CSS, or it is used by this
+    // entrypoint.
+    if (serverCSSForEntries.includes(css) || !/\.module\.css/.test(css)) {
+      const preloadedFontFiles = fontLoaderManifest.app[css]
+      if (preloadedFontFiles) {
+        for (const fontFile of preloadedFontFiles) {
+          fontFiles.add(fontFile)
+        }
       }
     }
   }
@@ -908,6 +935,15 @@ export async function renderToHTMLOrFlight(
 
     let defaultRevalidate: false | undefined | number = false
 
+    // Collect all server CSS imports used by this specific entry (or entries, for parallel routes).
+    // Not that we can't rely on the CSS manifest because it tracks CSS imports per module,
+    // which can be used by multiple entries and cannot be tree-shaked in the module graph.
+    // More info: https://github.com/vercel/next.js/issues/41018
+    const serverCSSForEntries = getServerCSSForEntries(
+      serverCSSManifest!,
+      ComponentMod.pages
+    )
+
     /**
      * Use the provided loader tree to create the React Component tree.
      */
@@ -941,13 +977,16 @@ export async function renderToHTMLOrFlight(
         ? getCssInlinedLinkTags(
             serverComponentManifest,
             serverCSSManifest!,
-            layoutOrPagePath
+            layoutOrPagePath,
+            serverCSSForEntries
           )
         : []
+
       const preloadedFontFiles = getPreloadedFontFilesInlineLinkTags(
         serverComponentManifest,
         serverCSSManifest!,
         fontLoaderManifest,
+        serverCSSForEntries,
         layoutOrPagePath
       )
       const Template = template

test/e2e/app-dir/index.test.ts

@@ -1269,8 +1269,7 @@ describe('app dir', () => {
             ).toBe(false)
           })
 
-          // TODO-APP: Should not include unused css modules in pages
-          it.skip('should not include unused css modules in nested pages in prod', async () => {
+          it('should not include unused css modules in nested pages in prod', async () => {
             const browser = await webdriver(
               next.url,
               '/css/css-page/unused-nested/inner'