Fix filesystempublicroutes test for Turbopack (#61132)

## What?

`exportPathMap` didn't work when Turbopack was enabled because the
`serializeNextConfig` function mutates the original values, overriding
`exportPathMap`.

This PR changes the serialization to copy the object and mutate only the
copied object.

Also refactored the test that was checking `_next`, the better way to
test that is to have the page render something dynamically, which is
what is added in this PR.

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Closes NEXT-2225

packages/next/src/build/swc/index.ts

@@ -1056,7 +1056,8 @@ function bindingToApi(binding: any, _wasm: boolean) {
     nextConfig: NextConfigComplete,
     projectPath: string
   ): Promise<string> {
-    let nextConfigSerializable = nextConfig as any
+    // Avoid mutating the existing `nextConfig` object.
+    let nextConfigSerializable = { ...(nextConfig as any) }
 
     nextConfigSerializable.generateBuildId =
       await nextConfig.generateBuildId?.()
@@ -1091,9 +1092,12 @@ function bindingToApi(binding: any, _wasm: boolean) {
         : undefined
 
     // loaderFile is an absolute path, we need it to be relative for turbopack.
-    if (nextConfig.images.loaderFile) {
-      nextConfig.images.loaderFile =
-        './' + path.relative(projectPath, nextConfig.images.loaderFile)
+    if (nextConfigSerializable.images.loaderFile) {
+      nextConfigSerializable.images = {
+        ...nextConfig.images,
+        loaderFile:
+          './' + path.relative(projectPath, nextConfig.images.loaderFile),
+      }
     }
 
     return JSON.stringify(nextConfigSerializable, null, 2)

test/integration/filesystempublicroutes/pages/exportpathmap-route.js

@@ -1 +1,14 @@
-export default () => <div>exportpathmap was here</div>
+import { useEffect } from 'react'
+import { useState } from 'react'
+export default function ExportPathMapRoute() {
+  const [mounted, setMounted] = useState(false)
+  useEffect(() => {
+    setMounted(true)
+  }, [])
+  return (
+    <div>
+      <h1>exportpathmap was here</h1>
+      {mounted ? <div id="page-was-loaded">Hello World</div> : null}
+    </div>
+  )
+}

test/integration/filesystempublicroutes/test/index.test.js

@@ -2,12 +2,8 @@
 
 import { join } from 'path'
 import getPort from 'get-port'
-import {
-  fetchViaHTTP,
-  initNextServerScript,
-  killApp,
-  getPageFileFromBuildManifest,
-} from 'next-test-utils'
+import { fetchViaHTTP, initNextServerScript, killApp } from 'next-test-utils'
+import webdriver from 'next-webdriver'
 
 const appDir = join(__dirname, '../')
 let appPort
@@ -40,23 +36,14 @@ describe('FileSystemPublicRoutes', () => {
     const res = await fetch('/exportpathmap-route')
     expect(res.status).toBe(200)
     const body = await res.text()
-    expect(body).toMatch(
-      process.env.TURBOPACK ? /turbopack/ : /exportpathmap was here/
-    )
+    expect(body).toMatch(/exportpathmap was here/)
   })
 
-  it('should still handle /_next routes', async () => {
-    await fetch('/exportpathmap-route') // make sure it's built
-    const pageFile = getPageFileFromBuildManifest(
-      appDir,
-      '/exportpathmap-route'
-    )
-    const res = await fetch(join('/_next', pageFile))
-    expect(res.status).toBe(200)
-    const body = await res.text()
-    expect(body).toMatch(
-      process.env.TURBOPACK ? /turbopack/ : /exportpathmap was here/
-    )
+  it('should serve JavaScript files correctly', async () => {
+    const browser = await webdriver(context.appPort, '/exportpathmap-route')
+
+    const text = await browser.waitForElementByCss('#page-was-loaded').text()
+    expect(text).toBe('Hello World')
   })
 
   it('should route to public folder files', async () => {

test/turbopack-tests-manifest.json

@@ -10749,13 +10749,12 @@
   },
   "test/integration/filesystempublicroutes/test/index.test.js": {
     "passed": [
-      "FileSystemPublicRoutes should route to public folder files",
-      "FileSystemPublicRoutes should still handle /_next routes"
-    ],
-    "failed": [
       "FileSystemPublicRoutes should not route to the index page",
-      "FileSystemPublicRoutes should route to exportPathMap defined routes in development"
+      "FileSystemPublicRoutes should route to exportPathMap defined routes in development",
+      "FileSystemPublicRoutes should route to public folder files",
+      "FileSystemPublicRoutes should serve JavaScript files correctly"
     ],
+    "failed": [],
     "pending": [],
     "flakey": [],
     "runtimeError": false