Throw an error when target: 'serverless' is used with Middleware (#…

…37819)

Serverless target is deprecated, so we don't support it in Middleware as it's a new feature,
but we need to have a good error message for that. This commit solves that.

## Related

* Fixes #37433



## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/build/entries.ts

@@ -31,6 +31,7 @@ import {
   isMiddlewareFilename,
   isServerComponentPage,
   NestedMiddlewareError,
+  MiddlewareInServerlessTargetError,
 } from './utils'
 import { getPageStaticInfo } from './analysis/get-page-static-info'
 import { normalizePathSep } from '../shared/lib/page-path/normalize-path-sep'
@@ -348,6 +349,10 @@ export async function createEntrypoints(params: CreateEntrypointsParams) {
 
       if (isMiddlewareFile(page)) {
         middlewareRegex = staticInfo.middleware?.pathMatcher?.source || '.*'
+
+        if (target === 'serverless') {
+          throw new MiddlewareInServerlessTargetError()
+        }
       }
 
       runDependingOnPageType({

packages/next/build/utils.ts

@@ -1331,6 +1331,17 @@ export function getPossibleMiddlewareFilenames(
     path.join(folder, `${MIDDLEWARE_FILENAME}.${extension}`)
   )
 }
+
+export class MiddlewareInServerlessTargetError extends Error {
+  constructor() {
+    super(
+      'Next.js Middleware is not supported in the deprecated serverless target.\n' +
+        'Please remove `target: "serverless" from your next.config.js to use Middleware.'
+    )
+    this.name = 'MiddlewareInServerlessTargetError'
+  }
+}
+
 export class NestedMiddlewareError extends Error {
   constructor(nestedFileNames: string[], mainDir: string, pagesDir: string) {
     super(

test/production/middleware-is-not-allowed-when-using-serverless-target/app/middleware.js

@@ -0,0 +1,3 @@
+export default (req) => {
+  console.log(req.url)
+}

test/production/middleware-is-not-allowed-when-using-serverless-target/app/next.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  target: 'serverless',
+}

test/production/middleware-is-not-allowed-when-using-serverless-target/app/pages/index.js

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

test/production/middleware-is-not-allowed-when-using-serverless-target/index.test.ts

@@ -0,0 +1,14 @@
+import { nextBuild } from 'next-test-utils'
+import path from 'path'
+
+describe('Middleware is not allowed when using serverless target', () => {
+  it('fails to build', async () => {
+    const { code, stderr } = await nextBuild(
+      path.resolve(__dirname, './app'),
+      undefined,
+      { ignoreFail: true, stderr: true }
+    )
+    expect(code).toEqual(1)
+    expect(stderr).toContain('MiddlewareInServerlessTargetError')
+  })
+})