Extract rule: template-no-unbound

Author: NullVoxPopuli

README.md

@@ -340,6 +340,7 @@ rules in templates can be disabled with eslint directives with mustache or html
 | [template-no-attrs-in-components](docs/rules/template-no-attrs-in-components.md)                 | disallow attrs in component templates                     |    |    |    |
 | [template-no-link-to-positional-params](docs/rules/template-no-link-to-positional-params.md)     | disallow positional params in LinkTo component            |    |    |    |
 | [template-no-link-to-tagname](docs/rules/template-no-link-to-tagname.md)                         | disallow tagName attribute on LinkTo component            |    |    |    |
+| [template-no-unbound](docs/rules/template-no-unbound.md)                                         | disallow {{unbound}} helper                               |    |    |    |
 | [template-no-with](docs/rules/template-no-with.md)                                               | disallow {{with}} helper                                  |    |    |    |
 
 ### Ember Data

docs/rules/template-no-unbound.md

@@ -0,0 +1,27 @@
+# ember/template-no-unbound
+
+> **HBS Only**: This rule applies to classic `.hbs` template files only (loose mode). It is not relevant for `gjs`/`gts` files (strict mode), where these patterns cannot occur.
+
+<!-- end auto-generated rule header -->
+
+`{{unbound}}` is a legacy hold over from the days in which Ember's template engine was less performant. Its use today
+is vestigial, and it no longer offers performance benefits.
+
+It is also a poor practice to use it for rendering only the initial value of a property that may later change.
+
+## Examples
+
+This rule **forbids** the following:
+
+```hbs
+{{unbound aVar}}
+```
+
+```hbs
+{{some-component foo=(unbound aVar)}}
+```
+
+## References
+
+- [deprecations/unbound block syntax](https://deprecations.emberjs.com/v1.x/#toc_block-and-multi-argument-unbound-helper)
+- [Ember api/unbound helper](https://api.emberjs.com/ember/release/classes/Ember.Templates.helpers/methods/each?anchor=unbound)

lib/rules/template-no-unbound.js

@@ -0,0 +1,32 @@
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'disallow {{unbound}} helper',
+      category: 'Deprecations',
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/template-no-unbound.md',
+      templateMode: 'loose',
+    },
+    schema: [],
+    messages: { unexpected: 'Unexpected unbound helper usage.' },
+    originallyFrom: {
+      name: 'ember-template-lint',
+      rule: 'lib/rules/no-unbound.js',
+      docs: 'docs/rule/no-unbound.md',
+      tests: 'test/unit/rules/no-unbound-test.js',
+    },
+  },
+  create(context) {
+    function check(node) {
+      if (node.path?.type === 'GlimmerPathExpression' && node.path.original === 'unbound') {
+        context.report({ node, messageId: 'unexpected' });
+      }
+    }
+    return {
+      GlimmerMustacheStatement: check,
+      GlimmerBlockStatement: check,
+      GlimmerSubExpression: check,
+    };
+  },
+};

tests/lib/rules/template-no-unbound.js

@@ -0,0 +1,51 @@
+const rule = require('../../../lib/rules/template-no-unbound');
+const RuleTester = require('eslint').RuleTester;
+
+const ruleTester = new RuleTester({
+  parser: require.resolve('ember-eslint-parser'),
+  parserOptions: { ecmaVersion: 2022, sourceType: 'module' },
+});
+ruleTester.run('template-no-unbound', rule, {
+  valid: [
+    '<template>{{this.value}}</template>',
+    '<template>{{foo}}</template>',
+    '<template>{{button}}</template>',
+  ],
+  invalid: [
+    {
+      code: '<template>{{unbound foo}}</template>',
+      output: null,
+      errors: [{ messageId: 'unexpected' }],
+    },
+
+    {
+      code: '<template>{{my-thing foo=(unbound foo)}}</template>',
+      output: null,
+      errors: [{ messageId: 'unexpected' }],
+    },
+  ],
+});
+
+const hbsRuleTester = new RuleTester({
+  parser: require.resolve('ember-eslint-parser/hbs'),
+  parserOptions: {
+    ecmaVersion: 2022,
+    sourceType: 'module',
+  },
+});
+
+hbsRuleTester.run('template-no-unbound', rule, {
+  valid: ['{{foo}}', '{{button}}'],
+  invalid: [
+    {
+      code: '{{unbound foo}}',
+      output: null,
+      errors: [{ message: 'Unexpected unbound helper usage.' }],
+    },
+    {
+      code: '{{my-thing foo=(unbound foo)}}',
+      output: null,
+      errors: [{ message: 'Unexpected unbound helper usage.' }],
+    },
+  ],
+});