ESLint: custom directories from next.config.js (vercel#26401)

Attempt to fix vercel#26393

~~Sorry, I wasn't able to run tests on local, but I hope at least test should be valid.~~
Edit: Tests are working correctly on my local :) 

fixes vercel#26393



## Bug

- [x] Related issues linked using `fixes vercel#26393`
- [x] Integration tests added

docs/basic-features/eslint.md

@@ -54,22 +54,24 @@ If you do not want ESLint to run as a build step, refer to the documentation for
 
 ## Linting Custom Directories
 
-By default, Next.js will run ESLint for all files in the `pages/`, `components/`, and `lib/` directories. However, you can specify which directories using the `dirs` option in the `eslint` config in `next.config.js` for production builds:
+By default, Next.js will run ESLint for all files in the `pages/`, `components/`, and `lib/` directories. However, you can specify which directories using the `dirs` option in the `eslint` config in `next.config.js`:
 
 ```js
 module.exports = {
   eslint: {
-    dirs: ['pages', 'utils'], // Only run ESLint on the 'pages' and 'utils' directories during production builds (next build)
+    dirs: ['pages', 'utils'], // Only run ESLint on the 'pages' and 'utils' directories (next build and next lint)
   },
 }
 ```
 
-Similarly, the `--dir` flag can be used for `next lint`:
+Also, the `--dir` flag can be used for `next lint`:
 
 ```bash
 yarn lint --dir pages --dir utils
 ```
 
+This will take precedence over the configuration from `next.config.js`.
+
 ## ESLint Plugin
 
 Next.js provides an ESLint plugin, [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next), making it easier to catch common issues and problems in a Next.js application. The full set of rules is as follows:

packages/next/cli/next-lint.ts

@@ -33,7 +33,7 @@ const eslintOptions = (args: arg.Spec) => ({
     : false,
 })
 
-const nextLint: cliCommand = (argv) => {
+const nextLint: cliCommand = async (argv) => {
   const validArgs: arg.Spec = {
     // Types
     '--help': Boolean,
@@ -140,7 +140,9 @@ const nextLint: cliCommand = (argv) => {
     printAndExit(`> No such directory exists as the project root: ${baseDir}`)
   }
 
-  const dirs: string[] = args['--dir']
+  const conf = await loadConfig(PHASE_PRODUCTION_BUILD, baseDir)
+
+  const dirs: string[] = args['--dir'] ?? conf.eslint?.dirs
   const lintDirs = (dirs ?? ESLINT_DEFAULT_DIRS).reduce(
     (res: string[], d: string) => {
       const currDir = join(baseDir, d)
@@ -169,7 +171,6 @@ const nextLint: cliCommand = (argv) => {
         typeof lintResults === 'string' ? lintResults : lintResults?.output
 
       if (typeof lintResults !== 'string' && lintResults?.eventInfo) {
-        const conf = await loadConfig(PHASE_PRODUCTION_BUILD, baseDir)
         const telemetry = new Telemetry({
           distDir: join(baseDir, conf.distDir),
         })

test/integration/eslint/test/index.test.js

@@ -268,6 +268,21 @@ describe('ESLint', () => {
       )
     })
 
+    test('custom directories', async () => {
+      const { stdout, stderr } = await nextLint(dirCustomDirectories, [], {
+        stdout: true,
+        stderr: true,
+      })
+
+      const output = stdout + stderr
+      expect(output).toContain(
+        'Error: Comments inside children section of tag should be placed inside braces'
+      )
+      expect(output).toContain(
+        'Warning: External synchronous scripts are forbidden'
+      )
+    })
+
     test('max warnings flag errors when warnings exceed threshold', async () => {
       const { stdout, stderr } = await nextLint(
         dirMaxWarnings,