Warn/revert custom devtool in development mode (#14285)

Warn users and revert their `devtool` when they manually change the `devtool` in development mode. For this addition, I check to ensure the `devtool` is custom (i.e. different than what is set by Next) and has a value (`false` is fine as a custom `devtool`!).

As described in [this issue (13963)](https://github.com/vercel/next.js/issues/13963), changing the `devtool` in development mode can cause issues with performance.

Fixes #13963

errors/improper-devtool.md

+# Improper webpack `devtool` used in development mode
+
+#### Why This Error Occurred
+
+Next.js chooses the most optimal `devtool` for use with webpack. Changing the `devtool` in development mode will cause severe performance regressions with your application.
+
+#### Possible Ways to Fix It
+
+Please remove the custom `devtool` override or only apply it to production builds in your `next.config.js`.
+
+```js
+module.exports = {
+  webpack: (config, options) => {
+    if (!options.dev) {
+      config.devtool = options.isServer ? false : 'your-custom-devtool'
+    }
+    return config
+  },
+}
+```

packages/next/build/webpack-config.ts

+import { codeFrameColumns } from '@babel/code-frame'
 import ReactRefreshWebpackPlugin from '@next/react-refresh-utils/ReactRefreshWebpackPlugin'
 import crypto from 'crypto'
 import { readFileSync } from 'fs'
@@ -5,6 +6,7 @@ import chalk from 'next/dist/compiled/chalk'
 import TerserPlugin from 'next/dist/compiled/terser-webpack-plugin'
 import path from 'path'
 import webpack from 'webpack'
+import type { Configuration } from 'webpack'
 import {
   DOT_NEXT_ALIAS,
   NEXT_PROJECT_ROOT,
@@ -21,6 +23,7 @@ import {
   SERVERLESS_DIRECTORY,
   SERVER_DIRECTORY,
 } from '../next-server/lib/constants'
+import { execOnce } from '../next-server/lib/utils'
 import { findPageFile } from '../server/lib/find-page-file'
 import { WebpackEntrypoints } from './entries'
 import {
@@ -44,12 +47,11 @@ import { ReactLoadablePlugin } from './webpack/plugins/react-loadable-plugin'
 import { ServerlessPlugin } from './webpack/plugins/serverless-plugin'
 import WebpackConformancePlugin, {
   DuplicatePolyfillsConformanceCheck,
+  GranularChunksConformanceCheck,
   MinificationConformanceCheck,
   ReactSyncScriptsConformanceCheck,
-  GranularChunksConformanceCheck,
 } from './webpack/plugins/webpack-conformance-plugin'
 import { WellKnownErrorsPlugin } from './webpack/plugins/wellknown-errors-plugin'
-import { codeFrameColumns } from '@babel/code-frame'
 
 type ExcludesFalse = <T>(x: T | false) => x is T
 
@@ -61,6 +63,15 @@ const escapePathVariables = (value: any) => {
     : value
 }
 
+const devtoolRevertWarning = execOnce((devtool: Configuration['devtool']) => {
+  console.warn(
+    chalk.yellow.bold('Warning: ') +
+      chalk.bold(`Reverting webpack devtool to '${devtool}'.\n`) +
+      'Changing the webpack devtool in development mode will cause severe performance regressions.\n' +
+      'Read more: https://err.sh/next.js/improper-devtool'
+  )
+})
+
 function parseJsonFile(filePath: string) {
   const JSON5 = require('next/dist/compiled/json5')
   const contents = readFileSync(filePath, 'utf8')
@@ -1002,6 +1013,7 @@ export default async function getBaseWebpackConfig(
     productionBrowserSourceMaps,
   })
 
+  let originalDevtool = webpackConfig.devtool
   if (typeof config.webpack === 'function') {
     webpackConfig = config.webpack(webpackConfig, {
       dir,
@@ -1014,6 +1026,11 @@ export default async function getBaseWebpackConfig(
       webpack,
     })
 
+    if (dev && originalDevtool !== webpackConfig.devtool) {
+      webpackConfig.devtool = originalDevtool
+      devtoolRevertWarning(originalDevtool)
+    }
+
     if (typeof (webpackConfig as any).then === 'function') {
       console.warn(
         '> Promise returned in next config. https://err.sh/vercel/next.js/promise-in-next-config'

test/integration/config-devtool-dev/next.config.js

+module.exports = {
+  webpack: (config) => {
+    config.devtool = false
+    return config
+  },
+}

test/integration/config-devtool-dev/pages/index.js

+import { useEffect } from 'react'
+
+export default function Index(props) {
+  useEffect(() => {
+    throw new Error('this should render')
+  }, [])
+  return <div>Index Page</div>
+}

test/integration/config-devtool-dev/test/index.test.js

+/* eslint-env jest */
+
+import {
+  check,
+  findPort,
+  getRedboxSource,
+  hasRedbox,
+  killApp,
+  launchApp,
+} from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import { join } from 'path'
+
+jest.setTimeout(1000 * 30)
+
+const appDir = join(__dirname, '../')
+
+describe('devtool set in developmemt mode in next config', () => {
+  it('should warn and revert when a devtool is set in development mode', async () => {
+    let stderr = ''
+
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      env: { __NEXT_TEST_WITH_DEVTOOL: true },
+      onStderr(msg) {
+        stderr += msg || ''
+      },
+    })
+
+    const found = await check(
+      () => stderr,
+      /Reverting webpack devtool to /,
+      false
+    )
+
+    const browser = await webdriver(appPort, '/')
+    expect(await hasRedbox(browser)).toBe(true)
+    if (process.platform === 'win32') {
+      // TODO: add win32 snapshot
+    } else {
+      expect(await getRedboxSource(browser)).toMatchInlineSnapshot(`
+        "pages/index.js (5:10) @ eval
+
+          3 | export default function Index(props) {
+          4 |   useEffect(() => {
+        > 5 |     throw new Error('this should render')
+            |          ^
+          6 |   }, [])
+          7 |   return <div>Index Page</div>
+          8 | }"
+      `)
+    }
+    await browser.close()
+
+    await killApp(app)
+    expect(found).toBeTruthy()
+  })
+})