Add ESLint rule to prevent 'use client' in server files

eslint-rules/no-use-client-in-server-files.cjs

@@ -0,0 +1,67 @@
+/**
+ * @fileoverview Prevent 'use client' directive in .server.tsx files
+ * @author React on Rails Team
+ */
+
+/**
+ * ESLint rule to prevent 'use client' directives in .server.tsx files.
+ *
+ * Files ending with .server.tsx are intended for server-side rendering in
+ * React Server Components architecture. The 'use client' directive forces
+ * webpack to bundle these as client components, which causes errors when
+ * using React's react-server conditional exports with Shakapacker 9.3.0+.
+ *
+ * @type {import('eslint').Rule.RuleModule}
+ */
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: "Prevent 'use client' directive in .server.tsx files",
+      category: 'Best Practices',
+      recommended: true,
+      url: 'https://github.com/shakacode/react_on_rails/pull/1919',
+    },
+    messages: {
+      useClientInServerFile: `Files with '.server.tsx' extension should not have 'use client' directive. Server files are for React Server Components and should not use client-only APIs. If this component needs client-side features, rename it to .client.tsx or .tsx instead.`,
+    },
+    schema: [],
+    fixable: 'code',
+  },
+
+  create(context) {
+    const filename = context.filename || context.getFilename();
+
+    // Only check .server.tsx files
+    if (!filename.endsWith('.server.tsx') && !filename.endsWith('.server.ts')) {
+      return {};
+    }
+
+    return {
+      Program(node) {
+        const sourceCode = context.sourceCode || context.getSourceCode();
+        const text = sourceCode.getText();
+
+        // Check for 'use client' directive at the start of the file
+        // Uses backreference (\1) to ensure matching quotes (both single or both double)
+        // Only matches at the very beginning of the file
+        const useClientPattern = /^\s*(['"])use client\1;?\s*\n?/;
+        const match = text.match(useClientPattern);
+
+        if (match) {
+          // Find the exact position of the directive
+          const directiveIndex = text.indexOf(match[0]);
+
+          context.report({
+            node,
+            messageId: 'useClientInServerFile',
+            fix(fixer) {
+              // Remove the 'use client' directive (regex already captures trailing newline)
+              return fixer.removeRange([directiveIndex, directiveIndex + match[0].length]);
+            },
+          });
+        }
+      },
+    };
+  },
+};

eslint-rules/no-use-client-in-server-files.test.cjs

@@ -0,0 +1,159 @@
+/**
+ * @fileoverview Tests for no-use-client-in-server-files rule
+ */
+
+const { RuleTester } = require('eslint');
+const rule = require('./no-use-client-in-server-files.cjs');
+
+const ruleTester = new RuleTester({
+  languageOptions: {
+    ecmaVersion: 2022,
+    sourceType: 'module',
+    parserOptions: {
+      ecmaFeatures: {
+        jsx: true,
+      },
+    },
+  },
+});
+
+ruleTester.run('no-use-client-in-server-files', rule, {
+  valid: [
+    {
+      code: `
+import React from 'react';
+
+export function ServerComponent() {
+  return <div>Server Component</div>;
+}
+`,
+      filename: 'Component.server.tsx',
+    },
+    {
+      code: `
+import { renderToString } from 'react-dom/server';
+
+export function render() {
+  return renderToString(<div>Hello</div>);
+}
+`,
+      filename: 'ComponentRenderer.server.tsx',
+    },
+    {
+      code: `
+'use client';
+
+import React from 'react';
+
+export function ClientComponent() {
+  return <div>Client Component</div>;
+}
+`,
+      filename: 'Component.client.tsx',
+    },
+    {
+      code: `
+'use client';
+
+import React from 'react';
+
+export function ClientComponent() {
+  return <div>Client Component</div>;
+}
+`,
+      filename: 'Component.tsx',
+    },
+    {
+      code: `
+import React from 'react';
+
+// This is fine - no 'use client' directive
+export function ServerComponent() {
+  return <div>Server</div>;
+}
+`,
+      filename: 'App.server.ts',
+    },
+  ],
+
+  invalid: [
+    {
+      code: `'use client';
+
+import React from 'react';
+
+export function Component() {
+  return <div>Component</div>;
+}
+`,
+      filename: 'Component.server.tsx',
+      errors: [
+        {
+          messageId: 'useClientInServerFile',
+        },
+      ],
+      output: `import React from 'react';
+
+export function Component() {
+  return <div>Component</div>;
+}
+`,
+    },
+    {
+      code: `"use client";
+
+import React from 'react';
+`,
+      filename: 'Component.server.tsx',
+      errors: [
+        {
+          messageId: 'useClientInServerFile',
+        },
+      ],
+      output: `import React from 'react';
+`,
+    },
+    {
+      code: `'use client'
+
+import React from 'react';
+`,
+      filename: 'Component.server.tsx',
+      errors: [
+        {
+          messageId: 'useClientInServerFile',
+        },
+      ],
+      output: `import React from 'react';
+`,
+    },
+    {
+      code: `  'use client';
+
+import wrapServerComponentRenderer from 'react-on-rails-pro/wrapServerComponentRenderer/server';
+`,
+      filename: 'AsyncOnServerSyncOnClient.server.tsx',
+      errors: [
+        {
+          messageId: 'useClientInServerFile',
+        },
+      ],
+      output: `import wrapServerComponentRenderer from 'react-on-rails-pro/wrapServerComponentRenderer/server';
+`,
+    },
+    {
+      code: `'use client';
+
+import React from 'react';
+`,
+      filename: 'Component.server.ts',
+      errors: [
+        {
+          messageId: 'useClientInServerFile',
+        },
+      ],
+      output: `import React from 'react';
+`,
+    },
+  ],
+});

eslint.config.ts

@@ -8,6 +8,7 @@ import tsEslint from 'typescript-eslint';
 import { includeIgnoreFile } from '@eslint/compat';
 import js from '@eslint/js';
 import { FlatCompat } from '@eslint/eslintrc';
+import noUseClientInServerFiles from './eslint-rules/no-use-client-in-server-files.cjs';
 
 const compat = new FlatCompat({
   baseDirectory: __dirname,
@@ -165,6 +166,19 @@ const config = tsEslint.config([
       'import/named': 'off',
     },
   },
+  {
+    files: ['**/*.server.ts', '**/*.server.tsx'],
+    plugins: {
+      'react-on-rails': {
+        rules: {
+          'no-use-client-in-server-files': noUseClientInServerFiles,
+        },
+      },
+    },
+    rules: {
+      'react-on-rails/no-use-client-in-server-files': 'error',
+    },
+  },
   {
     files: ['lib/generators/react_on_rails/templates/**/*'],
     rules: {