WIP: New: no-unused-vars (fixes #100).

Author: aladdin-add

docs/rules/no-unused-vars.md

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+# Disallow unused variable definitions of v-for directives or scope attributes. (no-unused-vars)
+
+This rule report variable definitions of v-for directives or scope attributes if those are not used.
+
+## :book: Rule Details
+
+:-1: Examples of **incorrect** code for this rule:
+
+```html
+<template>
+    <ol v-for="i in 5"><!-- "i" is defined but never used. -->
+        <li>item</li>
+    </ol>
+</template>
+```
+
+:+1: Examples of **correct** code for this rule:
+
+```html
+<template>
+    <ol v-for="i in 5">
+        <li>{{i}}</li><!-- "i" is defined and used. -->
+    </ol>
+</template>
+```
+
+## :wrench: Options
+
+Nothing.

lib/rules/no-unused-vars.js

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+/**
+ * @fileoverview warn variable definitions of v-for directives or scope attributes if those are not used.
+ * @author 薛定谔的猫<hh_2013@foxmail.com>
+ */
+'use strict'
+
+const utils = require('../utils')
+
+/**
+ * Collect used variables recursively.
+ *
+ * @param {Node} node - The node to collect.
+ * @param {Object} used - The object to restore result.
+ * @returns {Object} used variables.
+ */
+function collectUsed (node, used) {
+  if (node.type === 'VExpressionContainer') {
+    node.references.forEach(ref => {
+      used[ref.id.name] = true
+    })
+  }
+
+  if (node.type === 'VElement') {
+    node.children.forEach(child => collectUsed(child, used))
+  }
+
+  return used
+}
+
+/**
+ * Creates AST event handlers for require-v-for-key.
+ *
+ * @param {RuleContext} context - The rule context.
+ * @returns {Object} AST event handlers.
+ */
+function create (context) {
+  utils.registerTemplateBodyVisitor(context, {
+    "VAttribute[directive=true][key.name='for']": function (node) {
+      const vars = node.value.expression.left
+      const used = collectUsed(node.parent.parent, {})
+
+      // report unused.
+      vars.filter(v => !used[v.name]).forEach(v => {
+        context.report({
+          node,
+          loc: v.loc,
+          message: `'{{name}}' is defined but never used.`,
+          data: {
+            name: v.name
+          }
+        })
+      })
+    }
+  })
+
+  return {}
+}
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  create,
+  meta: {
+    docs: {
+      description: 'warn variable definitions of v-for directives or scope attributes if those are not used',
+      category: 'Possible Errors',
+      recommended: false
+    },
+    fixable: null,
+    schema: []
+  }
+}

tests/lib/rules/no-unused-vars.js

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+/**
+ * @author 薛定谔的猫<hh_2013@foxmail.com>
+ * @copyright 2017 薛定谔的猫. All rights reserved.
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const RuleTester = require('eslint').RuleTester
+const rule = require('../../../lib/rules/no-unused-vars')
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const tester = new RuleTester({
+  parser: 'vue-eslint-parser',
+  parserOptions: { ecmaVersion: 2015 }
+})
+
+tester.run('no-unused-vars', rule, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: '<template><ol v-for="i in 5"><li>{{i}}</li></ol></template>'
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: '<template><ol v-for="i in 5"></ol></template>',
+      errors: ['\'i\' is defined but never used.']
+    }
+  ]
+})