⭐️New: Add vue/valid-v-bind-sync rule (#647)

* New: Add `vue/valid-v-bind-sync` rule

* update doc

* update use messageId

* Change: null comparison to Boolean call.

* add testcase

* Update: to use reference.variable

* fixed eslint error

* update document

* revert readme

* fix eof

* fixed doc

* update docs

* fixed lint errors

* fixed

Author: ota-meshi

docs/rules/README.md

@@ -166,6 +166,7 @@ For example:
 | [vue/static-class-names-order](./static-class-names-order.md) | enforce static class names order | :wrench: |
 | [vue/v-on-function-call](./v-on-function-call.md) | enforce or forbid parentheses after method calls without arguments in `v-on` directives | :wrench: |
 | [vue/v-slot-style](./v-slot-style.md) | enforce `v-slot` directive style | :wrench: |
+| [vue/valid-v-bind-sync](./valid-v-bind-sync.md) | enforce valid `.sync` modifier on `v-bind` directives |  |
 | [vue/valid-v-slot](./valid-v-slot.md) | enforce valid `v-slot` directives |  |
 
 ## Deprecated

docs/rules/valid-v-bind-sync.md

@@ -0,0 +1,70 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/valid-v-bind-sync
+description: enforce valid `.sync` modifier on `v-bind` directives
+---
+# vue/valid-v-bind-sync
+> enforce valid `.sync` modifier on `v-bind` directives
+
+This rule checks whether every `.sync` modifier on `v-bind` directives is valid.
+
+## :book: Rule Details
+
+This rule reports `.sync` modifier on `v-bind` directives in the following cases:
+
+- The `.sync` modifier does not have the attribute value which is valid as LHS. E.g. `<MyComponent v-bind:aaa.sync="foo() + bar()" />`
+- The `.sync` modifier is on non Vue-components. E.g. `<input v-bind:aaa.sync="foo"></div>`
+- The `.sync` modifier's reference is iteration variables. E.g. `<div v-for="x in list"><MyComponent v-bind:aaa.sync="x" /></div>`
+
+<eslint-code-block :rules="{'vue/valid-v-bind-sync': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <MyComponent v-bind:aaa.sync="foo"/>
+  <MyComponent :aaa.sync="foo"/>
+
+  <div v-for="todo in todos">
+    <MyComponent v-bind:aaa.sync="todo.name"/>
+    <MyComponent :aaa.sync="todo.name"/>
+  </div>
+
+  <!-- ✗ BAD -->
+  <MyComponent v-bind:aaa.sync="foo + bar" />
+  <MyComponent :aaa.sync="foo + bar" />
+
+  <input v-bind:aaa.sync="foo">
+  <input :aaa.sync="foo">
+
+  <div v-for="todo in todos">
+    <MyComponent v-bind:aaa.sync="todo" />
+    <MyComponent :aaa.sync="todo" />
+  </div>
+</template>
+```
+
+</eslint-code-block>
+
+::: warning Note
+This rule does not check syntax errors in directives because it's checked by [no-parsing-error] rule.
+:::
+
+## :wrench: Options
+
+Nothing.
+
+## :couple: Related rules
+
+- [no-parsing-error]
+
+[no-parsing-error]: no-parsing-error.md
+
+## :books: Further reading
+
+- [Guide - `.sync` Modifier]([https://vuejs.org/v2/guide/list.html#v-for-with-a-Component](https://vuejs.org/v2/guide/components-custom-events.html#sync-Modifier))
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/valid-v-bind-sync.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/valid-v-bind-sync.js)

lib/index.js

@@ -81,6 +81,7 @@ module.exports = {
     'v-on-style': require('./rules/v-on-style'),
     'v-slot-style': require('./rules/v-slot-style'),
     'valid-template-root': require('./rules/valid-template-root'),
+    'valid-v-bind-sync': require('./rules/valid-v-bind-sync'),
     'valid-v-bind': require('./rules/valid-v-bind'),
     'valid-v-cloak': require('./rules/valid-v-cloak'),
     'valid-v-else-if': require('./rules/valid-v-else-if'),

lib/rules/valid-v-bind-sync.js

@@ -0,0 +1,113 @@
+/**
+ * @fileoverview enforce valid `.sync` modifier on `v-bind` directives
+ * @author Yosuke Ota
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Helpers
+// ------------------------------------------------------------------------------
+
+/**
+ * Check whether the given node is valid or not.
+ * @param {ASTNode} node The element node to check.
+ * @returns {boolean} `true` if the node is valid.
+ */
+function isValidElement (node) {
+  if (
+    (!utils.isHtmlElementNode(node) && !utils.isSvgElementNode(node)) ||
+    utils.isHtmlWellKnownElementName(node.rawName) ||
+    utils.isSvgWellKnownElementName(node.rawName)
+  ) {
+    // non Vue-component
+    return false
+  }
+  return true
+}
+
+/**
+ * Check whether the given node can be LHS.
+ * @param {ASTNode} node The node to check.
+ * @returns {boolean} `true` if the node can be LHS.
+ */
+function isLhs (node) {
+  return Boolean(node) && (
+    node.type === 'Identifier' ||
+    node.type === 'MemberExpression'
+  )
+}
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'enforce valid `.sync` modifier on `v-bind` directives',
+      category: undefined,
+      url: 'https://eslint.vuejs.org/rules/valid-v-bind-sync.html'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      unexpectedInvalidElement: "'.sync' modifiers aren't supported on <{{name}}> non Vue-components.",
+      unexpectedNonLhsExpression: "'.sync' modifiers require the attribute value which is valid as LHS.",
+      unexpectedUpdateIterationVariable: "'.sync' modifiers cannot update the iteration variable '{{varName}}' itself."
+    }
+  },
+
+  create (context) {
+    return utils.defineTemplateBodyVisitor(context, {
+      "VAttribute[directive=true][key.name.name='bind']" (node) {
+        if (!node.key.modifiers.map(mod => mod.name).includes('sync')) {
+          return
+        }
+        const element = node.parent.parent
+        const name = element.name
+
+        if (!isValidElement(element)) {
+          context.report({
+            node,
+            loc: node.loc,
+            messageId: 'unexpectedInvalidElement',
+            data: { name }
+          })
+        }
+
+        if (node.value) {
+          if (!isLhs(node.value.expression)) {
+            context.report({
+              node,
+              loc: node.loc,
+              messageId: 'unexpectedNonLhsExpression'
+            })
+          }
+
+          for (const reference of node.value.references) {
+            const id = reference.id
+            if (id.parent.type !== 'VExpressionContainer') {
+              continue
+            }
+            const variable = reference.variable
+            if (variable) {
+              context.report({
+                node,
+                loc: node.loc,
+                messageId: 'unexpectedUpdateIterationVariable',
+                data: { varName: id.name }
+              })
+            }
+          }
+        }
+      }
+    })
+  }
+}