Account for only having template tag in no-empty-glimmer-component-classes rule (#1866)

Author: chrisrng

docs/rules/no-empty-glimmer-component-classes.md

@@ -8,7 +8,7 @@ This rule will catch and prevent the use of empty backing classes for Glimmer co
 
 ## Rule Details
 
-This rule aims to disallow the use of empty backing classes for Glimmer components when possible. Template-only Glimmer components where there is no backing class are much faster and lighter-weight than Glimmer components with backing classes, which are much lighter-weight than Ember components. Therefore, you should only have a backing class for a Glimmer component when absolutely necessary.
+This rule aims to disallow the use of empty backing classes for Glimmer components when possible including only using ember template tags in your Glimmer component. Template-only Glimmer components where there is no backing class are much faster and lighter-weight than Glimmer components with backing classes, which are much lighter-weight than Ember components. Therefore, you should only have a backing class for a Glimmer component when absolutely necessary.
 
 To fix violations of this rule:
 
@@ -26,6 +26,15 @@ import Component from '@glimmer/component';
 class MyComponent extends Component {}
 ```
 
+<!-- markdownlint-disable-next-line MD040 -->
+```
+import Component from '@glimmer/component';
+
+export default class MyComponent extends Component {
+  <template>Hello World!</template>
+}
+```
+
 Examples of **correct** code for this rule:
 
 ```js
@@ -54,7 +63,21 @@ import MyDecorator from 'my-decorator';
 class MyComponent extends Component {}
 ```
 
+<!-- markdownlint-disable-next-line MD040 -->
+```
+import Component from '@glimmer/component';
+
+export default class MyComponent extends Component {
+  foo() {
+    return this.args.bar + this.args.baz;
+  }
+
+  <template>Hello World!</template>
+}
+```
+
 ## References
 
 - [Glimmer Components - Octane Upgrade Guide](https://guides.emberjs.com/release/upgrading/current-edition/glimmer-components/)
 - [Glimmer Components RFC](https://emberjs.github.io/rfcs/0416-glimmer-components.html)
+- [First-Class Component Templates RFC](https://rfcs.emberjs.com/id/0779-first-class-component-templates/)

lib/rules/no-empty-glimmer-component-classes.js

@@ -1,8 +1,11 @@
 'use strict';
 
 const { isGlimmerComponent } = require('../utils/ember');
+const { isClassPropertyOrPropertyDefinition } = require('../utils/types');
 
 const ERROR_MESSAGE = 'Do not create empty backing classes for Glimmer components.';
+const ERROR_MESSAGE_TEMPLATE_TAG =
+  'Do not create empty backing classes for Glimmer template tag only components.';
 
 //------------------------------------------------------------------------------
 // Rule Definition
@@ -23,12 +26,21 @@ module.exports = {
   },
 
   ERROR_MESSAGE,
+  ERROR_MESSAGE_TEMPLATE_TAG,
 
   create(context) {
     return {
       ClassDeclaration(node) {
-        if (isGlimmerComponent(context, node) && !node.decorators && node.body.body.length === 0) {
+        const nodeIsGlimmerComponent = isGlimmerComponent(context, node);
+        if (nodeIsGlimmerComponent && !node.decorators && node.body.body.length === 0) {
           context.report({ node, message: ERROR_MESSAGE });
+        } else if (
+          nodeIsGlimmerComponent &&
+          node.body.body.length === 1 &&
+          isClassPropertyOrPropertyDefinition(node.body.body[0]) &&
+          node.body.body[0].key?.callee?.name === '__GLIMMER_TEMPLATE'
+        ) {
+          context.report({ node, message: ERROR_MESSAGE_TEMPLATE_TAG });
         }
       },
     };

tests/lib/rules-preprocessor/gjs-gts-processor-test.js

@@ -86,6 +86,19 @@ const valid = [
       </template>
     `,
   },
+  {
+    filename: 'my-component.gjs',
+    code: `
+      import Component from '@glimmer/component';
+      export default class MyComponent extends Component {
+        foo() {
+          return this.args.bar + this.args.baz;
+        }
+
+        <template>Hello World!</template>
+      }
+    `,
+  },
   /**
    * TODO: SKIP this scenario. Tracked in https://github.com/ember-cli/eslint-plugin-ember/issues/1685
   {
@@ -102,6 +115,21 @@ const valid = [
 ];
 
 const invalid = [
+  {
+    filename: 'my-component.gjs',
+    code: `import Component from '@glimmer/component';
+    export default class Chris extends Component {
+      <template>Hello!</template>
+    }`,
+    errors: [
+      {
+        message: 'Do not create empty backing classes for Glimmer template tag only components.',
+        line: 2,
+        column: 20,
+        endColumn: 6,
+      },
+    ],
+  },
   {
     filename: 'my-component.gjs',
     code: `