Add warning when exporting with custom routes (#17538)

This adds a warning when `next export` and custom routes are defined  outside of a platform that supports them since they won't be applied anymore. 

errors/export-no-custom-routes.md

+# `next export` No Custom Routes
+
+#### Why This Error Occurred
+
+In your `next.config.js` `rewrites`, `redirects`, or `headers` were defined while `next export` was being run outside of a platform that supports them.
+
+These configs do not apply when exporting your Next.js application manually.
+
+#### Possible Ways to Fix It
+
+Disable the `rewrites`, `redirects`, and `headers` from your `next.config.js` when using `next export` to deploy your application or deploy your application using [a method](https://nextjs.org/docs/deployment#vercel-recommended) that supports these configs.
+
+### Useful Links
+
+- [Deployment Documentation](https://nextjs.org/docs/deployment#vercel-recommended)
+- [`next export` Documentation](https://nextjs.org/docs/advanced-features/static-html-export)
+- [Rewrites Documentation](https://nextjs.org/docs/api-reference/next.config.js/rewrites)
+- [Redirects Documentation](https://nextjs.org/docs/api-reference/next.config.js/redirects)
+- [Headers Documentation](https://nextjs.org/docs/api-reference/next.config.js/headers)

packages/next/export/index.ts

@@ -165,6 +165,22 @@ export default async function exportApp(
     )
   }
 
+  const customRoutesDetected = ['rewrites', 'redirects', 'headers'].filter(
+    (config) => typeof nextConfig[config] === 'function'
+  )
+
+  if (
+    !process.env.NOW_BUILDER &&
+    !options.buildExport &&
+    customRoutesDetected.length > 0
+  ) {
+    Log.warn(
+      `rewrites, redirects, and headers are not applied when exporting your application, detected (${customRoutesDetected.join(
+        ', '
+      )}). See more info here: https://err.sh/next.js/export-no-custom-routes`
+    )
+  }
+
   const buildId = readFileSync(join(distDir, BUILD_ID_FILE), 'utf8')
   const pagesManifest =
     !options.pages &&

test/integration/custom-routes/test/index.test.js

@@ -19,6 +19,7 @@ import {
   waitFor,
   normalizeRegEx,
   initNextServerScript,
+  nextExport,
 } from 'next-test-utils'
 
 jest.setTimeout(1000 * 60 * 2)
@@ -31,6 +32,7 @@ let nextConfigContent
 let externalServerPort
 let externalServer
 let stdout = ''
+let stderr = ''
 let buildId
 let appPort
 let app
@@ -1119,16 +1121,82 @@ describe('Custom routes', () => {
 
   describe('server mode', () => {
     beforeAll(async () => {
-      const { stdout: buildStdout } = await nextBuild(appDir, ['-d'], {
-        stdout: true,
-      })
+      const { stdout: buildStdout, stderr: buildStderr } = await nextBuild(
+        appDir,
+        ['-d'],
+        {
+          stdout: true,
+          stderr: true,
+        }
+      )
       stdout = buildStdout
+      stderr = buildStderr
       appPort = await findPort()
       app = await nextStart(appDir, appPort)
       buildId = await fs.readFile(join(appDir, '.next/BUILD_ID'), 'utf8')
     })
     afterAll(() => killApp(app))
     runTests()
+
+    it('should not show warning for custom routes when not next export', async () => {
+      expect(stderr).not.toContain(
+        `rewrites, redirects, and headers are not applied when exporting your application detected`
+      )
+    })
+  })
+
+  describe('export', () => {
+    let exportStderr = ''
+    let exportVercelStderr = ''
+
+    beforeAll(async () => {
+      const { stdout: buildStdout, stderr: buildStderr } = await nextBuild(
+        appDir,
+        ['-d'],
+        {
+          stdout: true,
+          stderr: true,
+        }
+      )
+      const exportResult = await nextExport(
+        appDir,
+        { outdir: join(appDir, 'out') },
+        { stderr: true }
+      )
+      const exportVercelResult = await nextExport(
+        appDir,
+        { outdir: join(appDir, 'out') },
+        {
+          stderr: true,
+          env: {
+            NOW_BUILDER: '1',
+          },
+        }
+      )
+
+      stdout = buildStdout
+      stderr = buildStderr
+      exportStderr = exportResult.stderr
+      exportVercelStderr = exportVercelResult.stderr
+    })
+
+    it('should not show warning for custom routes when not next export', async () => {
+      expect(stderr).not.toContain(
+        `rewrites, redirects, and headers are not applied when exporting your application detected`
+      )
+    })
+
+    it('should not show warning for custom routes when next export on Vercel', async () => {
+      expect(exportVercelStderr).not.toContain(
+        `rewrites, redirects, and headers are not applied when exporting your application detected`
+      )
+    })
+
+    it('should show warning for custom routes with next export', async () => {
+      expect(exportStderr).toContain(
+        `rewrites, redirects, and headers are not applied when exporting your application, detected (rewrites, redirects, headers)`
+      )
+    })
   })
 
   describe('serverless mode', () => {