chore: warn for incorrect _redirects file, and update docs (#1078)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: ascorbic

docs/release-notes/v4.md

@@ -42,7 +42,8 @@ If you previously set these values, they're no longer needed and can be removed:
 
 If you currently use redirects or rewrites on your site, see
 [the Rewrites and Redirects guide](https://github.com/netlify/netlify-plugin-nextjs/blob/main/docs/redirects-rewrites.md)
-for information on changes to how they are handled in this version.
+for information on changes to how they are handled in this version. If you currently have `_redirects` or `_headers`
+files in the root of your site, these will need to be moved into "public".
 
 If you want to use Next 12's beta Middleware feature, this will mostly work as expected but please
 [read the docs on some caveats and workarounds](https://github.com/netlify/netlify-plugin-nextjs/blob/main/docs/middleware.md)

src/helpers/verification.ts

@@ -1,6 +1,6 @@
 /* eslint-disable max-lines */
 import { existsSync, promises } from 'fs'
-import path, { relative } from 'path'
+import path, { relative, join } from 'path'
 
 import { NetlifyConfig, NetlifyPluginUtils } from '@netlify/build'
 import { yellowBright, greenBright, blueBright, redBright, reset } from 'chalk'
@@ -187,4 +187,14 @@ export const warnForProblematicUserRewrites = ({
     `),
   )
 }
+
+export const warnForRootRedirects = ({ appDir }: { appDir: string }) => {
+  if (existsSync(join(appDir, '_redirects'))) {
+    console.log(
+      yellowBright(
+        `You have a "_redirects" file in your root directory, which is not deployed and will be ignored. If you want it to be used, please move it into "public".`,
+      ),
+    )
+  }
+}
 /* eslint-enable max-lines */

src/index.ts

@@ -17,6 +17,7 @@ import {
   checkZipSize,
   checkForOldFunctions,
   warnForProblematicUserRewrites,
+  warnForRootRedirects,
 } from './helpers/verification'
 
 const plugin: NetlifyPlugin = {
@@ -130,8 +131,9 @@ const plugin: NetlifyPlugin = {
 
     await checkForOldFunctions({ functions })
     await checkZipSize(join(FUNCTIONS_DIST, `${ODB_FUNCTION_NAME}.zip`))
-    const { basePath } = await getNextConfig({ publish, failBuild })
+    const { basePath, appDir } = await getNextConfig({ publish, failBuild })
     warnForProblematicUserRewrites({ basePath, redirects })
+    warnForRootRedirects({ appDir })
     await unpatchNextFiles(basePath)
   },
 }