Add vue/require-slots-as-functions rule.

docs/rules/README.md

@@ -74,6 +74,7 @@ Enforce all the rules in this category, as well as all higher priority rules, wi
 | [vue/require-component-is](./require-component-is.md) | require `v-bind:is` of `<component>` elements |  |
 | [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-slots-as-functions](./require-slots-as-functions.md) | enforce properties of `$slots` to be used as a function |  |
 | [vue/require-toggle-inside-transition](./require-toggle-inside-transition.md) | require control the display of the content inside `<transition>` |  |
 | [vue/require-v-for-key](./require-v-for-key.md) | require `v-bind:key` with `v-for` directives |  |
 | [vue/require-valid-default-prop](./require-valid-default-prop.md) | enforce props default values to be valid |  |

docs/rules/require-slots-as-functions.md

@@ -0,0 +1,48 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/require-slots-as-functions
+description: enforce properties of `$slots` to be used as a function
+---
+# vue/require-slots-as-functions
+> enforce properties of `$slots` to be used as a function
+
+- :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 enforces the properties of `$slots` to be used as a function.  
+`this.$slots.default` was an array of VNode in Vue.js 2.x, but changed to a function that returns an array of VNode in Vue.js 3.x.
+
+<eslint-code-block :rules="{'vue/require-slots-as-functions': ['error']}">
+
+```vue
+<script>
+export default {
+  render(h) {
+    /* ✓ GOOD */
+    var children = this.$slots.default()
+    var children = this.$slots.default && this.$slots.default()
+
+    /* ✗ BAD */
+    var children = [...this.$slots.default]
+    var children = this.$slots.default.filter(test)
+  }
+}
+</script>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :books: Further reading
+
+- [Vue RFCs - 0006-slots-unification](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0006-slots-unification.md)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/require-slots-as-functions.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/require-slots-as-functions.js)

lib/configs/vue3-essential.js

@@ -42,6 +42,7 @@ module.exports = {
     'vue/require-component-is': 'error',
     'vue/require-prop-type-constructor': 'error',
     'vue/require-render-return': 'error',
+    'vue/require-slots-as-functions': 'error',
     'vue/require-toggle-inside-transition': 'error',
     'vue/require-v-for-key': 'error',
     'vue/require-valid-default-prop': 'error',

lib/index.js

@@ -111,6 +111,7 @@ module.exports = {
     'require-prop-type-constructor': require('./rules/require-prop-type-constructor'),
     'require-prop-types': require('./rules/require-prop-types'),
     'require-render-return': require('./rules/require-render-return'),
+    'require-slots-as-functions': require('./rules/require-slots-as-functions'),
     'require-toggle-inside-transition': require('./rules/require-toggle-inside-transition'),
     'require-v-for-key': require('./rules/require-v-for-key'),
     'require-valid-default-prop': require('./rules/require-valid-default-prop'),

lib/rules/require-slots-as-functions.js

@@ -0,0 +1,124 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const utils = require('../utils')
+const { findVariable } = require('eslint-utils')
+
+/**
+ * @typedef {import('vue-eslint-parser').AST.ESLintMemberExpression} MemberExpression
+ * @typedef {import('vue-eslint-parser').AST.ESLintIdentifier} Identifier
+ * @typedef {import('vue-eslint-parser').AST.ESLintExpression} Expression
+ */
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'enforce properties of `$slots` to be used as a function',
+      categories: ['vue3-essential'],
+      url: 'https://eslint.vuejs.org/rules/require-slots-as-functions.html'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      unexpected: 'Property in `$slots` should be used as function.'
+    }
+  },
+
+  create(context) {
+    /**
+     * Verify the given node
+     * @param {MemberExpression | Identifier} node The node to verify
+     * @param {Expression} reportNode The node to report
+     */
+    function verify(node, reportNode) {
+      const parent = node.parent
+
+      if (
+        parent.type === 'VariableDeclarator' &&
+        parent.id.type === 'Identifier'
+      ) {
+        // const children = this.$slots.foo
+        verifyReferences(parent.id, reportNode)
+        return
+      }
+
+      if (
+        parent.type === 'AssignmentExpression' &&
+        parent.right === node &&
+        parent.left.type === 'Identifier'
+      ) {
+        // children = this.$slots.foo
+        verifyReferences(parent.left, reportNode)
+        return
+      }
+
+      if (
+        // this.$slots.foo.xxx
+        parent.type === 'MemberExpression' ||
+        // var [foo] = this.$slots.foo
+        parent.type === 'VariableDeclarator' ||
+        // [...this.$slots.foo]
+        parent.type === 'SpreadElement' ||
+        // [this.$slots.foo]
+        parent.type === 'ArrayExpression'
+      ) {
+        context.report({
+          node: reportNode,
+          messageId: 'unexpected'
+        })
+      }
+    }
+    /**
+     * Verify the references of the given node.
+     * @param {Identifier} node The node to verify
+     * @param {Expression} reportNode The node to report
+     */
+    function verifyReferences(node, reportNode) {
+      // @ts-ignore
+      const variable = findVariable(context.getScope(), node)
+      if (!variable) {
+        return
+      }
+      for (const reference of variable.references) {
+        if (!reference.isRead()) {
+          continue
+        }
+        /** @type {Identifier} */
+        const id = reference.identifier
+        verify(id, reportNode)
+      }
+    }
+
+    return utils.defineVueVisitor(context, {
+      /** @param {MemberExpression} node */
+      MemberExpression(node) {
+        const object = node.object
+        if (object.type !== 'MemberExpression') {
+          return
+        }
+        if (
+          object.property.type !== 'Identifier' ||
+          object.property.name !== '$slots'
+        ) {
+          return
+        }
+        if (!utils.isThis(object.object, context)) {
+          return
+        }
+        verify(node, node.property)
+      }
+    })
+  }
+}

tests/lib/rules/require-slots-as-functions.js

@@ -0,0 +1,115 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/require-slots-as-functions')
+
+const RuleTester = require('eslint').RuleTester
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parser: require.resolve('vue-eslint-parser'),
+  parserOptions: { ecmaVersion: 2018, sourceType: 'module' }
+})
+ruleTester.run('require-slots-as-functions', rule, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <script>
+      export default {
+        render (h) {
+          var children = this.$slots.default()
+          var children = this.$slots.default && this.$slots.default()
+
+          return h('div', this.$slots.default)
+        }
+      }
+      </script>
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <script>
+      export default {
+        render (h) {
+          var children = unknown.$slots.default
+          var children = unknown.$slots.default.filter(test)
+
+          return h('div', [...children])
+        }
+      }
+      </script>
+      `
+    }
+  ],
+
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <script>
+      export default {
+        render (h) {
+          var children = this.$slots.default
+          var children = this.$slots.default.filter(test)
+
+          return h('div', [...children])
+        }
+      }
+      </script>
+      `,
+      errors: [
+        {
+          message: 'Property in `$slots` should be used as function.',
+          line: 5,
+          column: 38,
+          endLine: 5,
+          endColumn: 45
+        },
+        {
+          message: 'Property in `$slots` should be used as function.',
+          line: 6,
+          column: 38,
+          endLine: 6,
+          endColumn: 45
+        }
+      ]
+    },
+
+    {
+      filename: 'test.vue',
+      code: `
+      <script>
+      export default {
+        render (h) {
+          let children
+
+          const [node] = this.$slots.foo
+          const bar = [this.$slots[foo]]
+
+          children = this.$slots.foo
+
+          return h('div', children.filter(test))
+        }
+      }
+      </script>
+      `,
+      errors: [
+        'Property in `$slots` should be used as function.',
+        'Property in `$slots` should be used as function.',
+        'Property in `$slots` should be used as function.'
+      ]
+    }
+  ]
+})