New: Add vue/require-v-if-inside-transition rule

docs/rules/README.md

@@ -62,6 +62,7 @@ Enforce all the rules in this category, as well as all higher priority rules, wi
 | [vue/require-prop-type-constructor](./require-prop-type-constructor.md) | require prop type to be a constructor | :wrench: |
 | [vue/require-render-return](./require-render-return.md) | enforce render function to always return value |  |
 | [vue/require-v-for-key](./require-v-for-key.md) | require `v-bind:key` with `v-for` directives |  |
+| [vue/require-v-if-inside-transition](./require-v-if-inside-transition.md) | require control the display of the content inside `<transition>` |  |
 | [vue/require-valid-default-prop](./require-valid-default-prop.md) | enforce props default values to be valid |  |
 | [vue/return-in-computed-property](./return-in-computed-property.md) | enforce that a return statement is present in computed property |  |
 | [vue/use-v-on-exact](./use-v-on-exact.md) | enforce usage of `exact` modifier on `v-on` |  |

docs/rules/require-v-if-inside-transition.md

@@ -0,0 +1,42 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/require-v-if-inside-transition
+description: require control the display of the content inside `<transition>`
+---
+# vue/require-v-if-inside-transition
+> require control the display of the content inside `<transition>`
+
+- :gear: This rule is included in all of `"plugin:vue/vue3-essential"`, `"plugin:vue/vue3-strongly-recommended"` and `"plugin:vue/vue3-recommended"`.
+
+## :book: Rule Details
+
+This rule reports elements inside `<transition>` that do not control the display.
+
+<eslint-code-block :rules="{'vue/require-v-if-inside-transition': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <transition><div v-if="show" /></transition>
+  <transition><div v-show="show" /></transition>
+
+  <!-- ✗ BAD -->
+  <transition><div /></transition>
+</template>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :books: Further reading
+
+- [Vue RFCs - 0017-transition-as-root](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0017-transition-as-root.md)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/require-v-if-inside-transition.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/require-v-if-inside-transition.js)

lib/configs/vue3-essential.js

@@ -30,6 +30,7 @@ module.exports = {
     'vue/require-prop-type-constructor': 'error',
     'vue/require-render-return': 'error',
     'vue/require-v-for-key': 'error',
+    'vue/require-v-if-inside-transition': 'error',
     'vue/require-valid-default-prop': 'error',
     'vue/return-in-computed-property': 'error',
     'vue/use-v-on-exact': 'error',

lib/index.js

@@ -83,6 +83,7 @@ module.exports = {
     'require-prop-types': require('./rules/require-prop-types'),
     'require-render-return': require('./rules/require-render-return'),
     'require-v-for-key': require('./rules/require-v-for-key'),
+    'require-v-if-inside-transition': require('./rules/require-v-if-inside-transition'),
     'require-valid-default-prop': require('./rules/require-valid-default-prop'),
     'return-in-computed-property': require('./rules/return-in-computed-property'),
     'script-indent': require('./rules/script-indent'),

lib/rules/require-v-if-inside-transition.js

@@ -0,0 +1,82 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Helpers
+// ------------------------------------------------------------------------------
+
+/**
+ * Check whether the given node is an well-known element or not.
+ * @param {ASTNode} node The element node to check.
+ * @returns {boolean} `true` if the name is an well-known element name.
+ */
+function isWellKnownElement (node) {
+  if (
+    (!utils.isHtmlElementNode(node) && !utils.isSvgElementNode(node)) ||
+    utils.isHtmlWellKnownElementName(node.rawName) ||
+    utils.isSvgWellKnownElementName(node.rawName)
+  ) {
+    return true
+  }
+  return false
+}
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'require control the display of the content inside `<transition>`',
+      categories: ['vue3-essential'],
+      url: 'https://eslint.vuejs.org/rules/require-v-if-inside-transition.html'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      expected: 'The element inside `<transition>` is expected to have a `v-if` or `v-show` directive.'
+    }
+  },
+
+  create (context) {
+    /**
+     * Check if the given element has display control.
+     * @param {VElement} element The element node to check.
+     */
+    function verifyInsideElement (element) {
+      if (utils.isCustomComponent(element)) {
+        return
+      }
+      if (!isWellKnownElement(element)) {
+        return
+      }
+      if (!utils.hasDirective(element, 'if') && !utils.hasDirective(element, 'show')) {
+        context.report({
+          node: element.startTag,
+          loc: element.startTag.loc,
+          messageId: 'expected'
+        })
+      }
+    }
+
+    return utils.defineTemplateBodyVisitor(context, {
+      "VElement[name='transition'] > VElement" (node) {
+        if (node.parent.children[0] !== node) {
+          return
+        }
+        verifyInsideElement(node)
+      }
+    })
+  }
+}

tests/lib/rules/require-v-if-inside-transition.js

@@ -0,0 +1,110 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const RuleTester = require('eslint').RuleTester
+const rule = require('../../../lib/rules/require-v-if-inside-transition')
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const tester = new RuleTester({
+  parser: require.resolve('vue-eslint-parser'),
+  parserOptions: { ecmaVersion: 2015 }
+})
+
+tester.run('require-v-if-inside-transition', rule, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: ''
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><div v-if="show" /></transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><div v-show="show" /></transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><div v-if="show" /></Transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><div v-show="show" /></Transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><MyComp /></Transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><component :is="component" /></Transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><div :is="component" /></Transition></template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><svg height="100" width="100"><transition><circle v-if="show" /></transition></svg> </template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><svg height="100" width="100"><transition><MyComponent /></transition></svg> </template>'
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><template v-if="show"><div /></template></transition></template>'
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: '<template><transition><div /></transition></template>',
+      errors: [
+        {
+          line: 1,
+          column: 23,
+          messageId: 'expected',
+          endLine: 1,
+          endColumn: 30
+        }
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><Transition><div /></Transition></template>',
+      errors: [{ messageId: 'expected' }]
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><div /><div /></transition></template>',
+      errors: [{ messageId: 'expected' }]
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><div v-for="e in list" /></transition></template>',
+      errors: [{ messageId: 'expected' }]
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><svg height="100" width="100"><transition><circle /></transition></svg> </template>',
+      errors: [{ messageId: 'expected' }]
+    },
+    {
+      filename: 'test.vue',
+      code: '<template><transition><template v-for="e in list"><div /></template></transition></template>',
+      errors: [{ messageId: 'expected' }]
+    }
+  ]
+})