chore(lint): ban index.* filenames outside the src/index.ts barrel

Author: Mearman

eslint.config.ts

@@ -1,6 +1,31 @@
 import js from '@eslint/js';
 import tseslint from 'typescript-eslint';
 import globals from 'globals';
+import type { Rule } from 'eslint';
+
+// Forward guard: only src/index.ts may use an index.* basename. Any future module named index.ts/cts/mts/js/cjs/mjs outside the public convenience barrel is a mistake (a hidden entry point the exports map does not advertise). ESLint 9+ guarantees context.filename; the optional getFilename fallback covers an older runtime only.
+const noNonBarrelIndex: Rule.RuleModule = {
+  meta: {
+    type: 'problem',
+    schema: [],
+    messages: {
+      barrel:
+        'Only src/index.ts may be named index.* (the public convenience barrel); give any other module a descriptive filename.',
+    },
+  },
+  create(context: Rule.RuleContext & { getFilename?: () => string }) {
+    const filename = context.filename ?? context.getFilename?.() ?? '';
+    const lastSlash = filename.lastIndexOf('/');
+    const basename = lastSlash === -1 ? filename : filename.slice(lastSlash + 1);
+    if (!/^index\.[cm]?[tj]s$/.test(basename)) return {};
+    if (filename.endsWith('/src/index.ts')) return {};
+    return {
+      Program(node) {
+        context.report({ node, messageId: 'barrel' });
+      },
+    };
+  },
+};
 
 export default tseslint.config(
   js.configs.recommended,
@@ -13,6 +38,10 @@ export default tseslint.config(
       '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
     },
   },
+  {
+    plugins: { local: { rules: { 'no-non-barrel-index': noNonBarrelIndex } } },
+    rules: { 'local/no-non-barrel-index': 'error' },
+  },
   {
     ignores: ['dist/**', 'node_modules/**'],
   },