New: v-slot-style rule (fixes #801)

docs/rules/v-slot-style.md

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+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/v-slot-style
+description: enforce `v-slot` directive style
+---
+# vue/v-slot-style
+> enforce `v-slot` directive style
+
+- :wrench: The `--fix` option on the [command line](https://eslint.org/docs/user-guide/command-line-interface#fixing-problems) can automatically fix some of the problems reported by this rule.
+
+## :book: Rule Details
+
+This rule enforces `v-slot` directive style which you should use shorthand or long form.
+
+<eslint-code-block fix :rules="{'vue/v-slot-style': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <my-component v-slot="data">
+    {{data}}
+  </my-component>
+  <my-component>
+    <template #default>content</template>
+    <template #one>content</template>
+    <template #two>content</template>
+  </my-component>
+
+  <!-- ✗ BAD -->
+  <my-component #default="data">
+    {{data}}
+  </my-component>
+  <my-component>
+    <template v-slot>content</template>
+    <template v-slot:one>content</template>
+    <template v-slot:two>content</template>
+  </my-component>
+</template>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+```json
+{
+  "vue/v-slot-style": ["error", {
+    "atComponent": "shorthand" | "longform" | "v-slot",
+    "default": "shorthand" | "longform" | "v-slot",
+    "named": "shorthand" | "longform",
+  }]
+}
+```
+
+| Name | Type | Default Value | Description
+|:-----|:-----|:--------------|:------------
+| `atComponent` | `"shorthand"` \| `"longform"` \| `"v-slot"` | `"v-slot"` | The style for the default slot at custom components directly (E.g. `<my-component v-slot="">`).
+| `default` | `"shorthand"` \| `"longform"` \| `"v-slot"` | `"shorthand"` | The style for the default slot at template wrappers (E.g. `<template #default="">`).
+| `named` | `"shorthand"` \| `"longform"` | `"shorthand"` | The style for named slots (E.g. `<template #named="">`).
+
+Each value means:
+
+- `"shorthand"` ... use `#` shorthand. E.g. `#default`, `#named`, ...
+- `"longform"` ... use `v-slot:` directive notation. E.g. `v-slot:default`, `v-slot:named`, ...
+- `"v-slot"` ... use `v-slot` without that argument. This is shorter than `#default` shorthand.
+
+And a string option is supported to be consistent to similar `vue/v-bind-style` and `vue/v-on-style`.
+
+- `["error", "longform"]` is same as `["error", { atComponent: "longform", default: "longform", named: "longform" }]`.
+- `["error", "shorthand"]` is same as `["error", { atComponent: "shorthand", default: "shorthand", named: "shorthand" }]`.
+
+### `"longform"`
+
+<eslint-code-block fix :rules="{'vue/v-slot-style': ['error', 'longform']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <my-component v-slot:default="data">
+    {{data}}
+  </my-component>
+  <my-component>
+    <template v-slot:default>content</template>
+    <template v-slot:one>content</template>
+    <template v-slot:two>content</template>
+  </my-component>
+
+  <!-- ✗ BAD -->
+  <my-component v-slot="data">
+    {{data}}
+  </my-component>
+  <my-component>
+    <template #default>content</template>
+    <template #one>content</template>
+    <template #two>content</template>
+  </my-component>
+</template>
+```
+
+</eslint-code-block>
+
+## :books: Further reading
+
+- [Style guide - Directive shorthands](https://vuejs.org/v2/style-guide/#Directive-shorthands-strongly-recommended)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/v-slot-style.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/v-slot-style.js)

lib/rules/v-slot-style.js

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+/**
+ * @author Toru Nagashima
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+const { pascalCase } = require('../utils/casing')
+const utils = require('../utils')
+
+/**
+ * @typedef {Object} Options
+ * @property {"shorthand" | "longform" | "v-slot"} atComponent The style for the default slot at a custom component directly.
+ * @property {"shorthand" | "longform" | "v-slot"} default The style for the default slot at a template wrapper.
+ * @property {"shorthand" | "longform"} named The style for named slots at a template wrapper.
+ */
+
+/**
+ * Normalize options.
+ * @param {any} options The raw options to normalize.
+ * @returns {Options} The normalized options.
+ */
+function normalizeOptions (options) {
+  const normalized = {
+    atComponent: 'v-slot',
+    default: 'shorthand',
+    named: 'shorthand'
+  }
+
+  if (typeof options === 'string') {
+    normalized.atComponent = normalized.default = normalized.named = options
+  } else if (options != null) {
+    for (const key of ['atComponent', 'default', 'named']) {
+      if (options[key] != null) {
+        normalized[key] = options[key]
+      }
+    }
+  }
+
+  return normalized
+}
+
+/**
+ * Get the expected style.
+ * @param {Options} options The options that defined expected types.
+ * @param {VAttribute} node The `v-slot` node to check.
+ * @returns {"shorthand" | "longform" | "v-slot"} The expected style.
+ */
+function getExpectedStyle (options, node) {
+  const { argument } = node.key
+
+  if (argument == null || (argument.type === 'VIdentifier' && argument.name === 'default')) {
+    const element = node.parent.parent
+    return element.name === 'template' ? options.default : options.atComponent
+  }
+  return options.named
+}
+
+/**
+ * Get the expected style.
+ * @param {VAttribute} node The `v-slot` node to check.
+ * @returns {"shorthand" | "longform" | "v-slot"} The expected style.
+ */
+function getActualStyle (node) {
+  const { name, argument } = node.key
+
+  if (name.rawName === '#') {
+    return 'shorthand'
+  }
+  if (argument != null) {
+    return 'longform'
+  }
+  return 'v-slot'
+}
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'enforce `v-slot` directive style',
+      category: undefined, // strongly-recommended
+      url: 'https://eslint.vuejs.org/rules/v-slot-style.html'
+    },
+    fixable: 'code',
+    schema: [
+      {
+        anyOf: [
+          { enum: ['shorthand', 'longform'] },
+          {
+            type: 'object',
+            properties: {
+              atComponent: { enum: ['shorthand', 'longform', 'v-slot'] },
+              default: { enum: ['shorthand', 'longform', 'v-slot'] },
+              named: { enum: ['shorthand', 'longform'] }
+            },
+            additionalProperties: false
+          }
+        ]
+      }
+    ],
+    messages: {
+      expectedShorthand: "Expected '#{{argument}}' instead of '{{actual}}'.",
+      expectedLongform: "Expected 'v-slot:{{argument}}' instead of '{{actual}}'.",
+      expectedVSlot: "Expected 'v-slot' instead of '{{actual}}'."
+    }
+  },
+
+  create (context) {
+    const sourceCode = context.getSourceCode()
+    const options = normalizeOptions(context.options[0])
+
+    return utils.defineTemplateBodyVisitor(context, {
+      "VAttribute[directive=true][key.name.name='slot']" (node) {
+        const expected = getExpectedStyle(options, node)
+        const actual = getActualStyle(node)
+        if (actual === expected) {
+          return
+        }
+
+        const { name, argument } = node.key
+        const range = [name.range[0], (argument || name).range[1]]
+        const argumentText = argument ? sourceCode.getText(argument) : 'default'
+        context.report({
+          node,
+          messageId: `expected${pascalCase(expected)}`,
+          data: {
+            actual: sourceCode.text.slice(range[0], range[1]),
+            argument: argumentText
+          },
+
+          fix (fixer) {
+            switch (expected) {
+              case 'shorthand':
+                return fixer.replaceTextRange(range, `#${argumentText}`)
+              case 'longform':
+                return fixer.replaceTextRange(range, `v-slot:${argumentText}`)
+              case 'v-slot':
+                return fixer.replaceTextRange(range, 'v-slot')
+              default:
+                return null
+            }
+          }
+        })
+      }
+    })
+  }
+}