New: Add vue/no-ref-as-operand rule

docs/rules/README.md

@@ -160,6 +160,7 @@ For example:
 | [vue/no-deprecated-slot-scope-attribute](./no-deprecated-slot-scope-attribute.md) | disallow deprecated `slot-scope` attribute (in Vue.js 2.6.0+) | :wrench: |
 | [vue/no-empty-pattern](./no-empty-pattern.md) | disallow empty destructuring patterns |  |
 | [vue/no-irregular-whitespace](./no-irregular-whitespace.md) | disallow irregular whitespace |  |
+| [vue/no-ref-as-operand](./no-ref-as-operand.md) | disallow use of value wrapped by `ref()` (Composition API) as an operand |  |
 | [vue/no-reserved-component-names](./no-reserved-component-names.md) | disallow the use of reserved names in component definitions |  |
 | [vue/no-restricted-syntax](./no-restricted-syntax.md) | disallow specified syntax |  |
 | [vue/no-static-inline-styles](./no-static-inline-styles.md) | disallow static inline `style` attributes |  |

docs/rules/no-ref-as-operand.md

@@ -0,0 +1,58 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/no-ref-as-operand
+description: disallow use of value wrapped by `ref()` (Composition API) as an operand
+---
+# vue/no-ref-as-operand
+> disallow use of value wrapped by `ref()` (Composition API) as an operand
+
+## :book: Rule Details
+
+This rule reports cases where a ref is used incorrectly as an operand.
+
+<eslint-code-block :rules="{'vue/no-ref-as-operand': ['error']}">
+
+```vue
+<script>
+import { ref } from 'vue'
+
+export default {
+  setup () {
+    const count = ref(0)
+    const ok = ref(true)
+
+    /* ✓ GOOD */
+    count.value++
+    count.value + 1
+    1 + count.value
+    var msg = ok.value ? 'yes' : 'no'
+
+    /* ✗ BAD */
+    count++
+    count + 1
+    1 + count
+    var msg = ok ? 'yes' : 'no'
+
+    return {
+      count
+    }
+  }
+}
+</script>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :books: Further reading
+
+- [Vue RFCs - 0013-composition-api](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0013-composition-api.md)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/no-ref-as-operand.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/no-ref-as-operand.js)

lib/index.js

@@ -48,6 +48,7 @@ module.exports = {
     'no-irregular-whitespace': require('./rules/no-irregular-whitespace'),
     'no-multi-spaces': require('./rules/no-multi-spaces'),
     'no-parsing-error': require('./rules/no-parsing-error'),
+    'no-ref-as-operand': require('./rules/no-ref-as-operand'),
     'no-reserved-component-names': require('./rules/no-reserved-component-names'),
     'no-reserved-keys': require('./rules/no-reserved-keys'),
     'no-restricted-syntax': require('./rules/no-restricted-syntax'),

lib/rules/no-ref-as-operand.js

@@ -0,0 +1,124 @@
+/**
+ * @author Yosuke Ota
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+const { ReferenceTracker, findVariable } = require('eslint-utils')
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'disallow use of value wrapped by `ref()` (Composition API) as an operand',
+      category: undefined,
+      url: 'https://eslint.vuejs.org/rules/no-ref-as-operand.html'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      requireDotValue: 'Must use `.value` to read or write the value wrapped by `ref()`.'
+    }
+  },
+  create (context) {
+    const refReferenceIds = new Map()
+
+    function reportIfRefWrapped (node) {
+      if (!refReferenceIds.has(node)) {
+        return
+      }
+      context.report({
+        node,
+        messageId: 'requireDotValue'
+      })
+    }
+    return {
+      'Program' () {
+        const tracker = new ReferenceTracker(context.getScope())
+        const traceMap = {
+          vue: {
+            [ReferenceTracker.ESM]: true,
+            ref: {
+              [ReferenceTracker.CALL]: true
+            }
+          }
+        }
+
+        for (const { node } of tracker.iterateEsmReferences(traceMap)) {
+          const variableDeclarator = node.parent
+          if (
+            !variableDeclarator ||
+            variableDeclarator.type !== 'VariableDeclarator' ||
+            variableDeclarator.id.type !== 'Identifier'
+          ) {
+            continue
+          }
+          const variable = findVariable(context.getScope(), variableDeclarator.id)
+          if (!variable) {
+            continue
+          }
+          const variableDeclaration = (
+            variableDeclarator.parent &&
+            variableDeclarator.parent.type === 'VariableDeclaration' &&
+            variableDeclarator.parent
+          ) || null
+          for (const reference of variable.references) {
+            if (!reference.isRead()) {
+              continue
+            }
+
+            refReferenceIds.set(reference.identifier, {
+              variableDeclarator,
+              variableDeclaration
+            })
+          }
+        }
+      },
+      // if (refValue)
+      'IfStatement>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // switch (refValue)
+      'SwitchStatement>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // -refValue, +refValue, !refValue, ~refValue, typeof refValue
+      'UnaryExpression>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // refValue++, refValue--
+      'UpdateExpression>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // refValue+1, refValue-1
+      'BinaryExpression>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // refValue+=1, refValue-=1, foo+=refValue, foo-=refValue
+      'AssignmentExpression>Identifier' (node) {
+        reportIfRefWrapped(node)
+      },
+      // refValue || other, refValue && other. ignore: other || refValue
+      'LogicalExpression>Identifier' (node) {
+        if (node.parent.left !== node) {
+          return
+        }
+        // Report only constants.
+        const info = refReferenceIds.get(node)
+        if (!info) {
+          return
+        }
+        if (!info.variableDeclaration || info.variableDeclaration.kind !== 'const') {
+          return
+        }
+        reportIfRefWrapped(node)
+      },
+      // refValue ? x : y
+      'ConditionalExpression>Identifier' (node) {
+        if (node.parent.test !== node) {
+          return
+        }
+        reportIfRefWrapped(node)
+      }
+    }
+  }
+}

package.json

@@ -47,9 +47,10 @@
     "eslint": "^5.0.0 || ^6.0.0"
   },
   "dependencies": {
+    "eslint-utils": "^2.0.0",
     "natural-compare": "^1.4.0",
-    "vue-eslint-parser": "^7.0.0",
-    "semver": "^5.6.0"
+    "semver": "^5.6.0",
+    "vue-eslint-parser": "^7.0.0"
   },
   "devDependencies": {
     "@types/node": "^4.2.16",