feat(cli): Show deprecation message for legacy eslint configs

Author: Tobbe

docs/docs/app-configuration-redwood-toml.md

@@ -194,3 +194,19 @@ To run a Cedar app in a container or VM, you'll want to set both the web and api
 
 You can also configure these values via `REDWOOD_WEB_HOST` and `REDWOOD_API_HOST`.
 And if you set `NODE_ENV` to production, these will be the defaults anyway.
+
+## ESLint Configuration
+
+| Key                         | Description                                                                   | Default |
+| :-------------------------- | :---------------------------------------------------------------------------- | :------ |
+| `eslintLegacyConfigWarning` | Show deprecation warnings when legacy ESLint configuration files are detected | `true`  |
+
+The `eslintLegacyConfigWarning` option controls whether Cedar shows deprecation warnings when it detects legacy ESLint configuration files (like `.eslintrc.js`, `.eslintrc.json`, or `eslint` in `package.json`).
+
+To disable these warnings, set it to `false`:
+
+```toml title="redwood.toml"
+eslintLegacyConfigWarning = false
+```
+
+This can be useful if you need to temporarily maintain legacy ESLint configurations while transitioning to the new flat config format.

packages/cli/src/commands/lint.js

@@ -1,11 +1,83 @@
 import fs from 'node:fs'
+import path from 'node:path'
 
 import execa from 'execa'
 import { terminalLink } from 'termi-link'
 
 import { recordTelemetryAttributes } from '@cedarjs/cli-helpers'
 
-import { getPaths } from '../lib/index.js'
+import { getPaths, getConfig } from '../lib/index.js'
+
+/**
+ * Checks for legacy ESLint configuration files in the project root
+ * @returns {string[]} Array of legacy config file names found
+ */
+function detectLegacyEslintConfig() {
+  const projectRoot = getPaths().base
+  const legacyConfigFiles = [
+    '.eslintrc.js',
+    '.eslintrc.cjs',
+    '.eslintrc.json',
+    '.eslintrc.yaml',
+    '.eslintrc.yml',
+  ]
+
+  const foundLegacyFiles = []
+
+  // Check for .eslintrc.* files
+  for (const configFile of legacyConfigFiles) {
+    if (fs.existsSync(path.join(projectRoot, configFile))) {
+      foundLegacyFiles.push(configFile)
+    }
+  }
+
+  // Check for eslint or eslintConfig fields in package.json
+  const packageJsonPath = path.join(projectRoot, 'package.json')
+  if (fs.existsSync(packageJsonPath)) {
+    try {
+      const packageJson = JSON.parse(fs.readFileSync(packageJsonPath, 'utf8'))
+      if (packageJson.eslintConfig) {
+        foundLegacyFiles.push('package.json (eslintConfig field)')
+      }
+      if (packageJson.eslint) {
+        foundLegacyFiles.push('package.json (eslint field)')
+      }
+    } catch (error) {
+      // Ignore JSON parse errors
+    }
+  }
+
+  return foundLegacyFiles
+}
+
+/**
+ * Shows a deprecation warning for legacy ESLint configuration
+ * @param {string[]} legacyFiles Array of legacy config file names
+ */
+function showLegacyEslintDeprecationWarning(legacyFiles) {
+  console.warn('')
+  console.warn('⚠️  DEPRECATION WARNING: Legacy ESLint Configuration Detected')
+  console.warn('')
+  console.warn('   The following legacy ESLint configuration files were found:')
+  legacyFiles.forEach((file) => {
+    console.warn(`   - ${file}`)
+  })
+  console.warn('')
+  console.warn(
+    '   Cedar has migrated to ESLint flat config format. Legacy configurations',
+  )
+  console.warn(
+    '   still work but are deprecated and will be removed in a future version.',
+  )
+  console.warn('')
+  console.warn('   To migrate to the new format:')
+  console.warn('   1. Remove the legacy config file(s) listed above')
+  console.warn('   2. Create an eslint.config.mjs')
+  console.warn('   3. Use the flat config format with @cedarjs/eslint-config')
+  console.warn('')
+  console.warn('   See more here: https://github.com/cedarjs/cedar/pull/629')
+  console.warn('')
+}
 
 export const command = 'lint [path..]'
 export const description = 'Lint your files'
@@ -37,6 +109,13 @@ export const builder = (yargs) => {
 export const handler = async ({ path, fix, format }) => {
   recordTelemetryAttributes({ command: 'lint', fix, format })
 
+  // Check for legacy ESLint configuration and show deprecation warning
+  const config = getConfig()
+  const legacyConfigFiles = detectLegacyEslintConfig()
+  if (legacyConfigFiles.length > 0 && config.eslintLegacyConfigWarning) {
+    showLegacyEslintDeprecationWarning(legacyConfigFiles)
+  }
+
   try {
     const pathString = path?.join(' ')
     const sbPath = getPaths().web.storybook

packages/project-config/src/config.ts

@@ -120,6 +120,7 @@ export interface Config {
       lintOnly: boolean
     }
   }
+  eslintLegacyConfigWarning: boolean
 }
 
 export interface CLIPlugin {
@@ -206,6 +207,7 @@ const DEFAULT_CONFIG: Config = {
       lintOnly: false,
     },
   },
+  eslintLegacyConfigWarning: true,
 }
 
 /**