Add vue/no-lone-template rule (#1238)

Author: ota-meshi

Diff for: docs/rules/README.md

@@ -148,6 +148,7 @@ Enforce all the rules in this category, as well as all higher priority rules, wi
 |:--------|:------------|:---|
 | [vue/attributes-order](./attributes-order.md) | enforce order of attributes | :wrench: |
 | [vue/component-tags-order](./component-tags-order.md) | enforce order of component top-level elements |  |
+| [vue/no-lone-template](./no-lone-template.md) | disallow unnecessary `<template>` |  |
 | [vue/no-multiple-slot-args](./no-multiple-slot-args.md) | disallow to pass multiple arguments to scoped slots |  |
 | [vue/no-v-html](./no-v-html.md) | disallow use of v-html to prevent XSS attack |  |
 | [vue/order-in-components](./order-in-components.md) | enforce order of properties in components | :wrench: |
@@ -256,6 +257,7 @@ Enforce all the rules in this category, as well as all higher priority rules, wi
 |:--------|:------------|:---|
 | [vue/attributes-order](./attributes-order.md) | enforce order of attributes | :wrench: |
 | [vue/component-tags-order](./component-tags-order.md) | enforce order of component top-level elements |  |
+| [vue/no-lone-template](./no-lone-template.md) | disallow unnecessary `<template>` |  |
 | [vue/no-multiple-slot-args](./no-multiple-slot-args.md) | disallow to pass multiple arguments to scoped slots |  |
 | [vue/no-v-html](./no-v-html.md) | disallow use of v-html to prevent XSS attack |  |
 | [vue/order-in-components](./order-in-components.md) | enforce order of properties in components | :wrench: |

Diff for: docs/rules/no-lone-template.md

@@ -0,0 +1,82 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/no-lone-template
+description: disallow unnecessary `<template>`
+---
+# vue/no-lone-template
+> disallow unnecessary `<template>`
+
+- :gear: This rule is included in `"plugin:vue/vue3-recommended"` and `"plugin:vue/recommended"`.
+
+## :book: Rule Details
+
+This rule aims to eliminate unnecessary and potentially confusing `<template>`.  
+In Vue.js 2.x, the `<template>` elements that have no specific directives have no effect.  
+In Vue.js 3.x, the `<template>` elements that have no specific directives render the `<template>` elements as is, but in most cases this may not be what you intended.
+
+<eslint-code-block :rules="{'vue/no-lone-template': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <template v-if="foo">...</template>
+  <template v-else-if="bar">...</template>
+  <template v-else>...</template>
+  <template v-for="e in list">...</template>
+  <template v-slot>...</template>
+
+  <!-- ✗ BAD -->
+  <template>...</template>
+  <template/>
+</template>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+```json
+{
+  "vue/no-lone-template": ["error", {
+    "ignoreAccessible": false
+  }]
+}
+```
+
+- `ignoreAccessible` ... If `true`, ignore accessible `<template>` elements. default `false`.  
+  Note: this option is useless if you are using Vue.js 2.x.
+
+### `"ignoreAccessible": true`
+
+<eslint-code-block :rules="{'vue/no-lone-template': ['error', { 'ignoreAccessible': true }]}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <template ref="foo">...</template>
+  <template id="bar">...</template>
+
+  <!-- ✗ BAD -->
+  <template class="baz">...</template>
+</template>
+```
+
+</eslint-code-block>
+
+## :mute: When Not To Use It
+
+If you are using Vue.js 3.x and want to define the `<template>` element intentionally, you will have to turn this rule off or use `"ignoreAccessible"` option.
+
+## :couple: Related rules
+
+- [vue/no-template-key]
+- [no-lone-blocks]
+
+[no-lone-blocks]: https://eslint.org/docs/rules/no-lone-blocks
+[vue/no-template-key]: ./no-template-key.md
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/no-lone-template.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/no-lone-template.js)

Diff for: lib/configs/recommended.js

@@ -8,6 +8,7 @@ module.exports = {
   rules: {
     'vue/attributes-order': 'warn',
     'vue/component-tags-order': 'warn',
+    'vue/no-lone-template': 'warn',
     'vue/no-multiple-slot-args': 'warn',
     'vue/no-v-html': 'warn',
     'vue/order-in-components': 'warn',

Diff for: lib/configs/vue3-recommended.js

@@ -8,6 +8,7 @@ module.exports = {
   rules: {
     'vue/attributes-order': 'warn',
     'vue/component-tags-order': 'warn',
+    'vue/no-lone-template': 'warn',
     'vue/no-multiple-slot-args': 'warn',
     'vue/no-v-html': 'warn',
     'vue/order-in-components': 'warn',

Diff for: lib/index.js

@@ -74,6 +74,7 @@ module.exports = {
     'no-extra-parens': require('./rules/no-extra-parens'),
     'no-irregular-whitespace': require('./rules/no-irregular-whitespace'),
     'no-lifecycle-after-await': require('./rules/no-lifecycle-after-await'),
+    'no-lone-template': require('./rules/no-lone-template'),
     'no-multi-spaces': require('./rules/no-multi-spaces'),
     'no-multiple-objects-in-class': require('./rules/no-multiple-objects-in-class'),
     'no-multiple-slot-args': require('./rules/no-multiple-slot-args'),

Diff for: lib/rules/no-lone-template.js

@@ -0,0 +1,129 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Helpers
+// ------------------------------------------------------------------------------
+
+// https://github.com/vuejs/vue-next/blob/64e2f4643602c5980361e66674141e61ba60ef70/packages/compiler-core/src/parse.ts#L405
+const SPECIAL_TEMPLATE_DIRECTIVES = new Set([
+  'if',
+  'else',
+  'else-if',
+  'for',
+  'slot'
+])
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow unnecessary `<template>`',
+      categories: ['vue3-recommended', 'recommended'],
+      url: 'https://eslint.vuejs.org/rules/no-lone-template.html'
+    },
+    fixable: null,
+    schema: [
+      {
+        type: 'object',
+        properties: {
+          ignoreAccessible: {
+            type: 'boolean'
+          }
+        },
+        additionalProperties: false
+      }
+    ],
+    messages: {
+      requireDirective: '`<template>` require directive.'
+    }
+  },
+  /** @param {RuleContext} context */
+  create(context) {
+    const options = context.options[0] || {}
+    const ignoreAccessible = options.ignoreAccessible === true
+
+    /**
+     * @param {VAttribute | VDirective} attr
+     */
+    function getKeyName(attr) {
+      if (attr.directive) {
+        if (attr.key.name.name !== 'bind') {
+          // no v-bind
+          return null
+        }
+        if (
+          !attr.key.argument ||
+          attr.key.argument.type === 'VExpressionContainer'
+        ) {
+          // unknown
+          return null
+        }
+        return attr.key.argument.name
+      }
+      return attr.key.name
+    }
+
+    return utils.defineTemplateBodyVisitor(context, {
+      /** @param {VStartTag} node */
+      "VElement[name='template'][parent.type='VElement'] > VStartTag"(node) {
+        if (
+          node.attributes.some((attr) => {
+            if (attr.directive) {
+              const directiveName = attr.key.name.name
+              if (SPECIAL_TEMPLATE_DIRECTIVES.has(directiveName)) {
+                return true
+              }
+              if (directiveName === 'slot-scope') {
+                // `slot-scope` is deprecated in Vue.js 2.6
+                return true
+              }
+              if (directiveName === 'scope') {
+                // `scope` is deprecated in Vue.js 2.5
+                return true
+              }
+            }
+
+            const keyName = getKeyName(attr)
+            if (keyName === 'slot') {
+              // `slot` is deprecated in Vue.js 2.6
+              return true
+            }
+
+            return false
+          })
+        ) {
+          return
+        }
+
+        if (
+          ignoreAccessible &&
+          node.attributes.some((attr) => {
+            const keyName = getKeyName(attr)
+            return keyName === 'id' || keyName === 'ref'
+          })
+        ) {
+          return
+        }
+
+        context.report({
+          node,
+          messageId: 'requireDirective'
+        })
+      }
+    })
+  }
+}