feat(css): add strictPostcssConfiguration flag

packages/next/build/webpack-config.ts

@@ -1210,6 +1210,7 @@ export default async function getBaseWebpackConfig(
     assetPrefix: config.assetPrefix || '',
     sassOptions: config.sassOptions,
     productionBrowserSourceMaps: config.productionBrowserSourceMaps,
+    future: config.future,
   })
 
   let originalDevtool = webpackConfig.devtool
@@ -1332,7 +1333,7 @@ export default async function getBaseWebpackConfig(
         (e) => (e as any).__next_css_remove !== true
       )
     }
-  } else {
+  } else if (!config.future.strictPostcssConfiguration) {
     await __overrideCssConfiguration(dir, !dev, webpackConfig)
   }
 

packages/next/build/webpack/config/blocks/css/index.ts

@@ -78,10 +78,7 @@ export const css = curry(async function css(
   const postCssPlugins = await getPostCssPlugins(
     ctx.rootDirectory,
     ctx.isProduction,
-    // TODO: In the future, we should stop supporting old CSS setups and
-    // unconditionally inject ours. When that happens, we should remove this
-    // function argument.
-    true
+    !ctx.future.strictPostcssConfiguration
   )
 
   // CSS cannot be imported in _document. This comes before everything because

packages/next/build/webpack/config/index.ts

@@ -1,4 +1,5 @@
 import webpack from 'webpack'
+import { NextConfig } from '../../../next-server/server/config'
 import { base } from './blocks/base'
 import { css } from './blocks/css'
 import { ConfigurationContext, pipe } from './utils'
@@ -13,6 +14,7 @@ export async function build(
     assetPrefix,
     sassOptions,
     productionBrowserSourceMaps,
+    future,
   }: {
     rootDirectory: string
     customAppFile: string | null
@@ -21,6 +23,7 @@ export async function build(
     assetPrefix: string
     sassOptions: any
     productionBrowserSourceMaps: boolean
+    future: NextConfig['future']
   }
 ): Promise<webpack.Configuration> {
   const ctx: ConfigurationContext = {
@@ -37,6 +40,7 @@ export async function build(
       : '',
     sassOptions,
     productionBrowserSourceMaps,
+    future,
   }
 
   const fn = pipe(base(ctx), css(ctx))

packages/next/build/webpack/config/utils.ts

@@ -1,4 +1,5 @@
 import webpack from 'webpack'
+import { NextConfig } from '../../../next-server/server/config'
 
 export type ConfigurationContext = {
   rootDirectory: string
@@ -14,6 +15,8 @@ export type ConfigurationContext = {
 
   sassOptions: any
   productionBrowserSourceMaps: boolean
+
+  future: NextConfig['future']
 }
 
 export type ConfigurationFn = (

packages/next/next-server/server/config.ts

@@ -31,6 +31,11 @@ export type NextConfig = { [key: string]: any } & {
   redirects?: () => Promise<Redirect[]>
 
   trailingSlash?: boolean
+
+  future: {
+    strictPostcssConfiguration: boolean
+    excludeDefaultMomentLocales: boolean
+  }
 }
 
 const defaultConfig: NextConfig = {
@@ -83,6 +88,7 @@ const defaultConfig: NextConfig = {
     scriptLoader: false,
   },
   future: {
+    strictPostcssConfiguration: false,
     excludeDefaultMomentLocales: false,
   },
   serverRuntimeConfig: {},

test/integration/css-customization/test/index.test.js

@@ -108,6 +108,39 @@ describe('Legacy Next-CSS Customization', () => {
   })
 })
 
+describe('Custom CSS Customization via Webpack', () => {
+  const appDir = join(fixturesDir, 'custom-configuration-webpack')
+
+  beforeAll(async () => {
+    await remove(join(appDir, '.next'))
+  })
+
+  it('should compile successfully', async () => {
+    const { code, stdout, stderr } = await nextBuild(appDir, [], {
+      stdout: true,
+      stderr: true,
+    })
+    expect(code).toBe(0)
+    expect(stdout).toMatch(/Compiled successfully/)
+    expect(stderr).not.toMatch(
+      /Built-in CSS support is being disabled due to custom CSS configuration being detected/
+    )
+  })
+
+  it(`should've compiled and prefixed`, async () => {
+    const cssFolder = join(appDir, '.next/static/css')
+
+    const files = await readdir(cssFolder)
+    const cssFiles = files.filter((f) => /\.css$/.test(f))
+
+    expect(cssFiles.length).toBe(1)
+    const cssContent = await readFile(join(cssFolder, cssFiles[0]), 'utf8')
+    expect(cssContent.replace(/\/\*.*?\*\//g, '').trim()).toMatchInlineSnapshot(
+      `"@media (480px <= width < 768px){::placeholder{color:green}}.video{max-width:400px;max-height:300px}"`
+    )
+  })
+})
+
 describe('CSS Customization Array', () => {
   const appDir = join(fixturesDir, 'custom-configuration-arr')
 

test/integration/css-fixtures/custom-configuration-webpack/next.config.js

@@ -0,0 +1,36 @@
+module.exports = {
+  onDemandEntries: {
+    maxInactiveAge: 1000 * 60 * 60,
+  },
+  webpack(config) {
+    modifyLoaderConfig(
+      config.module.rules,
+      [/(?<!\.module)\.css$/, /\.module\.css$/],
+      (rule) => {
+        if (!Array.isArray(rule.use)) return
+        rule.use.forEach((u) => {
+          if (u.options.postcssOptions) {
+            u.options.postcssOptions.plugins = [
+              require('postcss-short-size')({
+                // Add a prefix to test that configuration is passed
+                prefix: 'xyz',
+              }),
+            ]
+          }
+        })
+      }
+    )
+
+    return config
+  },
+  future: { strictPostcssConfiguration: true },
+}
+
+function modifyLoaderConfig(rules, regexes, cb) {
+  rules.forEach((rule) => {
+    if (rule.oneOf) return modifyLoaderConfig(rule.oneOf, regexes, cb)
+    regexes.forEach((regex) => {
+      if (rule.test && rule.test.toString() === regex.toString()) cb(rule)
+    })
+  })
+}

test/integration/css-fixtures/custom-configuration-webpack/pages/_app.js

@@ -0,0 +1,12 @@
+import React from 'react'
+import App from 'next/app'
+import '../styles/global.css'
+
+class MyApp extends App {
+  render() {
+    const { Component, pageProps } = this.props
+    return <Component {...pageProps} />
+  }
+}
+
+export default MyApp

test/integration/css-fixtures/custom-configuration-webpack/pages/index.js

@@ -0,0 +1,3 @@
+export default function Home() {
+  return <div />
+}

test/integration/css-fixtures/custom-configuration-webpack/styles/global.css

@@ -0,0 +1,11 @@
+/* this should pass through untransformed */
+@media (480px <= width < 768px) {
+  ::placeholder {
+    color: green;
+  }
+}
+
+/* this should be transformed to width/height */
+.video {
+  -xyz-max-size: 400px 300px;
+}