only log export-no-custom-routes warning if unsupported (#57298)

This doesn't need to error, we can instead warn that the functionality will not work as expected out of the box. Support can be added for outside of Next for this to behave as expected. 

These are supported when deployed via the Nextjs builder ([x-ref](https://github.com/vercel/vercel/blob/main/packages/next/src/index.ts#L851-L855)).

Author: ztanner

packages/next/src/server/config.ts

@@ -23,6 +23,7 @@ import { pathHasPrefix } from '../shared/lib/router/utils/path-has-prefix'
 
 import { ZodParsedType, util as ZodUtil } from 'next/dist/compiled/zod'
 import type { ZodError, ZodIssue } from 'next/dist/compiled/zod'
+import { hasNextSupport } from '../telemetry/ci-info'
 
 export { normalizeConfig } from './config-shared'
 export type { DomainLocale, NextConfig } from './config-shared'
@@ -257,20 +258,23 @@ function assignDefaults(
         'Specified "i18n" cannot be used with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-i18n'
       )
     }
-    if (result.rewrites) {
-      throw new Error(
-        'Specified "rewrites" cannot be used with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
-      )
-    }
-    if (result.redirects) {
-      throw new Error(
-        'Specified "redirects" cannot be used with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
-      )
-    }
-    if (result.headers) {
-      throw new Error(
-        'Specified "headers" cannot be used with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
-      )
+
+    if (!hasNextSupport) {
+      if (result.rewrites) {
+        Log.warn(
+          'Specified "rewrites" will not automatically work with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
+        )
+      }
+      if (result.redirects) {
+        Log.warn(
+          'Specified "redirects" will not automatically work with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
+        )
+      }
+      if (result.headers) {
+        Log.warn(
+          'Specified "headers" will not automatically work with "output: export". See more info here: https://nextjs.org/docs/messages/export-no-custom-routes'
+        )
+      }
     }
   }
 

test/integration/config-output-export/test/index.test.ts

@@ -64,39 +64,86 @@ describe('config-output-export', () => {
     )
   })
 
-  it('should error with "rewrites" config', async () => {
-    const { stderr } = await runDev({
-      output: 'export',
-      rewrites: [{ source: '/from', destination: '/to' }],
+  describe('when hasNextSupport = false', () => {
+    it('should error with "rewrites" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        rewrites: [{ source: '/from', destination: '/to' }],
+      })
+      expect(stderr).toContain(
+        'Specified "rewrites" will not automatically work with "output: export".'
+      )
     })
-    expect(stderr).toContain(
-      'Specified "rewrites" cannot be used with "output: export".'
-    )
-  })
 
-  it('should error with "redirects" config', async () => {
-    const { stderr } = await runDev({
-      output: 'export',
-      redirects: [{ source: '/from', destination: '/to', permanent: true }],
+    it('should error with "redirects" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        redirects: [{ source: '/from', destination: '/to', permanent: true }],
+      })
+      expect(stderr).toContain(
+        'Specified "redirects" will not automatically work with "output: export".'
+      )
+    })
+
+    it('should error with "headers" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        headers: [
+          {
+            source: '/foo',
+            headers: [{ key: 'x-foo', value: 'val' }],
+          },
+        ],
+      })
+      expect(stderr).toContain(
+        'Specified "headers" will not automatically work with "output: export".'
+      )
     })
-    expect(stderr).toContain(
-      'Specified "redirects" cannot be used with "output: export".'
-    )
   })
 
-  it('should error with "headers" config', async () => {
-    const { stderr } = await runDev({
-      output: 'export',
-      headers: [
-        {
-          source: '/foo',
-          headers: [{ key: 'x-foo', value: 'val' }],
-        },
-      ],
+  describe('when hasNextSupport = true', () => {
+    beforeAll(() => {
+      process.env.NOW_BUILDER = '1'
+    })
+
+    afterAll(() => {
+      delete process.env.NOW_BUILDER
+    })
+
+    it('should error with "rewrites" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        rewrites: [{ source: '/from', destination: '/to' }],
+      })
+      expect(stderr).not.toContain(
+        'Specified "rewrites" will not automatically work with "output: export".'
+      )
+    })
+
+    it('should error with "redirects" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        redirects: [{ source: '/from', destination: '/to', permanent: true }],
+      })
+      expect(stderr).not.toContain(
+        'Specified "redirects" will not automatically work with "output: export".'
+      )
+    })
+
+    it('should error with "headers" config', async () => {
+      const { stderr } = await runDev({
+        output: 'export',
+        headers: [
+          {
+            source: '/foo',
+            headers: [{ key: 'x-foo', value: 'val' }],
+          },
+        ],
+      })
+      expect(stderr).not.toContain(
+        'Specified "headers" will not automatically work with "output: export".'
+      )
     })
-    expect(stderr).toContain(
-      'Specified "headers" cannot be used with "output: export".'
-    )
   })
 
   it('should error with api routes function', async () => {