eslint-plugin: add lint rule to ensure BUI CSS is not imported in plugins

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/solid-bees-agree.md

@@ -0,0 +1,5 @@
+---
+'@backstage/eslint-plugin': minor
+---
+
+Added `@backstage/no-ui-css-imports-in-non-frontend` rule, which ensures that CSS from `@backstage/ui` is not imported outside of the frontend app.

packages/dev-utils/src/devApp/BuiCss.tsx

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+// This ensures that dev apps always have the BUI CSS loaded.
+// eslint-disable-next-line @backstage/no-ui-css-imports-in-non-frontend
 import '@backstage/ui/css/styles.css';
 
 /**

packages/eslint-plugin/README.md

@@ -42,3 +42,4 @@ The following rules are provided by this plugin:
 | [@backstage/no-undeclared-imports](./docs/rules/no-undeclared-imports.md)                           | Forbid imports of external packages that have not been declared in the appropriate dependencies field in `package.json`. |
 | [@backstage/no-top-level-material-ui-4-imports](./docs/rules/no-top-level-material-ui-4-imports.md) | Forbid top level import from Material UI v4 packages.                                                                    |
 | [@backstage/no-mixed-plugin-imports](./docs/rules/no-mixed-plugin-imports.md)                       | Disallow mixed plugin imports.                                                                                           |
+| [@backstage/no-ui-css-imports-in-non-frontend](./docs/rules/no-ui-css-imports-in-non-frontend.md)   | Ensure that only packages with the `frontend` role can import CSS files from `@backstage/ui`.                            |

packages/eslint-plugin/docs/rules/no-ui-css-imports-in-non-frontend.md

@@ -0,0 +1,50 @@
+# no-ui-css-imports-in-non-frontend
+
+Ensures that only packages with `backstage.role` set to `"frontend"` can import CSS files from `@backstage/ui`.
+
+This rule prevents non-frontend packages from accidentally importing global CSS styles from `@backstage/ui`. These CSS files should only be imported by the app, never by plugins, modules or libraries.
+
+## Rule Details
+
+This rule checks imports from `@backstage/ui` that end with `.css` and verifies that the importing package has `backstage.role: "frontend"` in its `package.json`.
+
+If a package does not have a `backstage.role` field defined at all, the import is allowed (the check is skipped).
+
+Examples of **incorrect** code for this rule:
+
+```js
+// In a package with "backstage.role": "frontend-plugin"
+import '@backstage/ui/css/styles.css';
+```
+
+```js
+// In a package with "backstage.role": "web-library"
+import '@backstage/ui/css/styles.css';
+```
+
+```js
+// In a package with "backstage.role": "backend"
+require('@backstage/ui/css/styles.css');
+```
+
+Examples of **correct** code for this rule:
+
+```js
+// In a package with "backstage.role": "frontend"
+import '@backstage/ui/css/styles.css';
+```
+
+```js
+// In a package without a "backstage.role" field
+import '@backstage/ui/css/styles.css';
+```
+
+```js
+// Non-CSS imports are allowed in any package
+import { Button, Text } from '@backstage/ui';
+```
+
+```js
+// In a package with "backstage.role": "backend"
+import { Button } from '@backstage/ui';
+```

packages/eslint-plugin/index.js

@@ -23,6 +23,7 @@ module.exports = {
         '@backstage/no-relative-monorepo-imports': 'error',
         '@backstage/no-undeclared-imports': 'error',
         '@backstage/no-mixed-plugin-imports': 'warn',
+        '@backstage/no-ui-css-imports-in-non-frontend': 'error',
       },
     },
   },
@@ -32,5 +33,6 @@ module.exports = {
     'no-undeclared-imports': require('./rules/no-undeclared-imports'),
     'no-top-level-material-ui-4-imports': require('./rules/no-top-level-material-ui-4-imports'),
     'no-mixed-plugin-imports': require('./rules/no-mixed-plugin-imports'),
+    'no-ui-css-imports-in-non-frontend': require('./rules/no-ui-css-imports-in-non-frontend'),
   },
 };

packages/eslint-plugin/rules/no-ui-css-imports-in-non-frontend.js

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// @ts-check
+
+const visitImports = require('../lib/visitImports');
+const getPackages = require('../lib/getPackages');
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    messages: {
+      noCssImport:
+        'CSS imports from @backstage/ui are only allowed in packages with backstage.role set to "frontend". Current role: "{{role}}"',
+    },
+    docs: {
+      description:
+        'Ensure that only packages with backstage.role set to "frontend" can import CSS files from @backstage/ui.',
+      url: 'https://github.com/backstage/backstage/blob/master/packages/eslint-plugin/docs/rules/no-ui-css-imports-in-non-frontend.md',
+    },
+  },
+  create(context) {
+    const packages = getPackages(context.getCwd());
+    if (!packages) {
+      return {};
+    }
+
+    const currentPackage = packages.byPath(context.filename);
+    if (!currentPackage) {
+      return {};
+    }
+
+    return visitImports(context, (node, imp) => {
+      const isBuiImport =
+        (imp.type === 'external' || imp.type === 'internal') &&
+        imp.packageName === '@backstage/ui';
+      if (!isBuiImport) {
+        return;
+      }
+
+      const isCssImport = imp.path?.endsWith('.css');
+      if (!isCssImport) {
+        return;
+      }
+
+      const backstageRole = currentPackage.packageJson.backstage?.role;
+      if (!backstageRole) {
+        // Allow if no role is defined
+        return;
+      }
+
+      if (backstageRole !== 'frontend') {
+        context.report({
+          node: node,
+          messageId: 'noCssImport',
+          data: {
+            role: backstageRole,
+          },
+        });
+      }
+    });
+  },
+};

packages/eslint-plugin/src/__fixtures__/monorepo/packages/frontend-pkg/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@internal/frontend-pkg",
+  "backstage": {
+    "role": "frontend"
+  }
+}

packages/eslint-plugin/src/__fixtures__/monorepo/packages/frontend-plugin-pkg/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@internal/frontend-plugin-pkg",
+  "backstage": {
+    "role": "frontend-plugin"
+  }
+}

packages/eslint-plugin/src/__fixtures__/monorepo/packages/no-role-pkg/package.json

@@ -0,0 +1,3 @@
+{
+  "name": "@internal/no-role-pkg"
+}

packages/eslint-plugin/src/no-ui-css-imports-in-non-frontend.test.ts

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { RuleTester } from 'eslint';
+import path from 'path';
+import rule from '../rules/no-ui-css-imports-in-non-frontend';
+
+const RULE = 'no-ui-css-imports-in-non-frontend';
+const FIXTURE = path.resolve(__dirname, '__fixtures__/monorepo');
+
+const ERR = (role: string) => ({
+  message: `CSS imports from @backstage/ui are only allowed in packages with backstage.role set to "frontend". Current role: "${role}"`,
+});
+
+// cwd must be restored
+const origDir = process.cwd();
+afterAll(() => {
+  process.chdir(origDir);
+});
+process.chdir(FIXTURE);
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    sourceType: 'module',
+    ecmaVersion: 2021,
+  },
+});
+
+ruleTester.run(RULE, rule, {
+  valid: [
+    // Frontend package can import CSS from @backstage/ui
+    {
+      code: `import '@backstage/ui/css/styles.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-pkg/index.ts'),
+    },
+    {
+      code: `import '@backstage/ui/css/other.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-pkg/index.ts'),
+    },
+    {
+      code: `require('@backstage/ui/css/styles.css')`,
+      filename: path.join(FIXTURE, 'packages/frontend-pkg/index.ts'),
+    },
+    // Package without backstage.role can import CSS (skip check)
+    {
+      code: `import '@backstage/ui/css/styles.css'`,
+      filename: path.join(FIXTURE, 'packages/no-role-pkg/index.ts'),
+    },
+    // Non-CSS imports are allowed in any package
+    {
+      code: `import { Button } from '@backstage/ui'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+    },
+    {
+      code: `import { Text } from '@backstage/ui/components'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+    },
+    // Imports from other packages are allowed
+    {
+      code: `import './styles.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+    },
+    {
+      code: `import 'some-other-package/styles.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+    },
+  ],
+  invalid: [
+    // Backend package cannot import CSS from @backstage/ui
+    {
+      code: `import '@backstage/ui/css/styles.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+      errors: [ERR('frontend-plugin')],
+    },
+    {
+      code: `import '@backstage/ui/css/other.css'`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+      errors: [ERR('frontend-plugin')],
+    },
+    {
+      code: `require('@backstage/ui/css/styles.css')`,
+      filename: path.join(FIXTURE, 'packages/frontend-plugin-pkg/index.ts'),
+      errors: [ERR('frontend-plugin')],
+    },
+  ],
+});