Add ESLint rule to disallow barrel/entrypoint imports

eslint.config.mjs

@@ -8,6 +8,10 @@ import nodePlugin from 'eslint-plugin-n';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import pluginJs from '@eslint/js';
+import { createRequire } from 'node:module';
+
+const require = createRequire(import.meta.url);
+const noBarrelImports = require('./eslint/rules/no-barrel-imports.js');
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);
@@ -41,6 +45,11 @@ export default [
     plugins: {
       'ember-internal': emberInternal,
       'disable-features': disableFeatures,
+      local: {
+        rules: {
+          'no-barrel-imports': noBarrelImports,
+        },
+      },
     },
 
     linterOptions: {
@@ -78,6 +87,11 @@ export default [
       'disable-features/disable-generator-functions': 'error',
       // Doesn't work with package.json#exports
       'import/no-unresolved': 'off',
+
+      // Prevent importing from barrel/entrypoint files in internal packages.
+      // Source files should import directly from the specific lib/ file to
+      // enable proper tree-shaking.
+      'local/no-barrel-imports': 'error',
     },
   },
   ...tseslint.configs.recommended.map((config) => ({

eslint/rules/no-barrel-imports.js

@@ -0,0 +1,79 @@
+/**
+ * ESLint rule: no-barrel-imports
+ *
+ * Disallows importing from barrel/entrypoint files of internal packages.
+ * Internal source files should import directly from the specific lib/ file
+ * to enable proper tree-shaking by bundlers.
+ *
+ * For example:
+ *   Bad:  import { Renderer } from '@ember/-internals/glimmer';
+ *   Good: import { Renderer } from '@ember/-internals/glimmer/lib/renderer';
+ */
+
+'use strict';
+
+// Barrel packages that should not be imported directly from source files.
+// Each entry maps a bare package specifier to a human-readable hint.
+const BARREL_PACKAGES = new Map([
+  [
+    '@ember/-internals/glimmer',
+    "Import from '@ember/-internals/glimmer/lib/...' instead of the barrel '@ember/-internals/glimmer'.",
+  ],
+  [
+    '@ember/-internals/environment',
+    "Import from '@ember/-internals/environment/lib/env' or '@ember/-internals/environment/lib/context' instead of the barrel '@ember/-internals/environment'.",
+  ],
+]);
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description:
+        'Disallow imports from barrel/entrypoint files; require direct file imports instead',
+    },
+    messages: {
+      noBarrelImport:
+        "Do not import from the barrel '{{source}}'. {{hint}}",
+    },
+    schema: [], // no options
+  },
+
+  create(context) {
+    // Only apply to files inside packages/ that are NOT test or type-test files.
+    const filename = context.filename || context.getFilename();
+
+    // Skip files outside packages/
+    if (!filename.includes('/packages/')) {
+      return {};
+    }
+
+    // Skip test and type-test files
+    if (/\/(tests|type-tests)\//.test(filename)) {
+      return {};
+    }
+
+    function check(node) {
+      const source = node.source && node.source.value;
+      if (typeof source !== 'string') return;
+
+      const hint = BARREL_PACKAGES.get(source);
+      if (hint) {
+        context.report({
+          node: node.source,
+          messageId: 'noBarrelImport',
+          data: { source, hint },
+        });
+      }
+    }
+
+    return {
+      ImportDeclaration: check,
+      // Also catch: export { foo } from '@ember/-internals/glimmer';
+      ExportNamedDeclaration: check,
+      // Also catch: export * from '@ember/-internals/glimmer';
+      ExportAllDeclaration: check,
+    };
+  },
+};