Add vue/block-lang rule.

docs/rules/README.md

@@ -279,13 +279,14 @@ For example:
 ```json
 {
   "rules": {
-    "vue/block-tag-newline": "error"
+    "vue/block-lang": "error"
   }
 }
 ```
 
 | Rule ID | Description |    |
 |:--------|:------------|:---|
+| [vue/block-lang](./block-lang.md) | disallow use other than available `lang` |  |
 | [vue/block-tag-newline](./block-tag-newline.md) | enforce line breaks after opening and before closing block-level tags | :wrench: |
 | [vue/component-name-in-template-casing](./component-name-in-template-casing.md) | enforce specific casing for the component naming style in template | :wrench: |
 | [vue/custom-event-name-casing](./custom-event-name-casing.md) | enforce specific casing for custom event name |  |

docs/rules/block-lang.md

@@ -0,0 +1,91 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/block-lang
+description: disallow use other than available `lang`
+---
+# vue/block-lang
+
+> disallow use other than available `lang`
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> ***This rule has not been released yet.*** </badge>
+
+## :book: Rule Details
+
+This rule disallows the use of languages other than those available in the your application for the lang attribute of block elements.
+
+## :wrench: Options
+
+```json
+{
+  "vue/block-lang": ["error",
+    {
+      "script": {
+        "lang": "ts"
+      }
+    }
+  ]
+}
+```
+
+<eslint-code-block :rules="{'vue/block-lang': ['error', { script: { lang: 'ts' } }]}">
+
+```vue
+<!-- ✓ GOOD -->
+<script lang="ts">
+</script>
+```
+
+</eslint-code-block>
+
+<eslint-code-block :rules="{'vue/block-lang': ['error', { script: { lang: 'ts' } }]}">
+
+```vue
+<!-- ✗ BAD -->
+<script>
+</script>
+```
+
+</eslint-code-block>
+
+Specify the block name for the key of the option object.  
+You can use the object as a value and use the following properties:
+
+- `lang` ... Specifies the available value for the `lang` attribute of the block. If multiple languages are available, specify them as an array. If you do not specify it, will disallow any language.
+- `allowNoLang` ... If `true`, allows the `lang` attribute not to be specified (allows the use of the default language of block).
+
+::: warning Note
+If the default language is specified for `lang` option of `<template>`, `<style>` and `<script>`, it will be enforced to not specify `lang` attribute.  
+This is to prevent unintended problems with [Vetur](https://vuejs.github.io/vetur/).
+
+See also [Vetur - Syntax Highlighting](https://vuejs.github.io/vetur/guide/highlighting.html).
+:::
+
+### `{ script: { lang: 'js' } }`
+
+Same as `{ script: { allowNoLang: true } }`.
+
+<eslint-code-block :rules="{'vue/block-lang': ['error', { script: { lang: 'js' } }]}">
+
+```vue
+<!-- ✓ GOOD -->
+<script>
+</script>
+```
+
+</eslint-code-block>
+
+<eslint-code-block :rules="{'vue/block-lang': ['error', { script: { lang: 'js' } }]}">
+
+```vue
+<!-- ✗ BAD -->
+<script lang="js">
+</script>
+```
+
+</eslint-code-block>
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/block-lang.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/block-lang.js)

lib/index.js

@@ -12,6 +12,7 @@ module.exports = {
     'arrow-spacing': require('./rules/arrow-spacing'),
     'attribute-hyphenation': require('./rules/attribute-hyphenation'),
     'attributes-order': require('./rules/attributes-order'),
+    'block-lang': require('./rules/block-lang'),
     'block-spacing': require('./rules/block-spacing'),
     'block-tag-newline': require('./rules/block-tag-newline'),
     'brace-style': require('./rules/brace-style'),

lib/rules/block-lang.js

@@ -0,0 +1,224 @@
+/**
+ * @fileoverview Disallow use other than available `lang`
+ * @author Yosuke Ota
+ */
+'use strict'
+const utils = require('../utils')
+
+/**
+ * @typedef {object} BlockOptions
+ * @property {Set<string>} lang
+ * @property {boolean} allowNoLang
+ */
+/**
+ * @typedef { { [element: string]: BlockOptions | undefined } } Options
+ */
+/**
+ * @typedef {object} UserBlockOptions
+ * @property {string[] | string} [lang]
+ * @property {boolean} [allowNoLang]
+ */
+/**
+ * @typedef { { [element: string]: UserBlockOptions | undefined } } UserOptions
+ */
+
+/**
+ * https://vuejs.github.io/vetur/guide/highlighting.html
+ * <template lang="html"></template>
+ * <style lang="css"></style>
+ * <script lang="js"></script>
+ * <script lang="javascript"></script>
+ * @type {Record<string, string[] | undefined>}
+ */
+const DEFAULT_LANGUAGES = {
+  template: ['html'],
+  style: ['css'],
+  script: ['js', 'javascript']
+}
+
+/**
+ * @param {NonNullable<BlockOptions['lang']>} lang
+ */
+function getAllowsLangPhrase(lang) {
+  const langs = [...lang].map((s) => `"${s}"`)
+  switch (langs.length) {
+    case 1:
+      return langs[0]
+    default:
+      return `${langs.slice(0, -1).join(', ')}, and ${langs[langs.length - 1]}`
+  }
+}
+
+/**
+ * Normalizes a given option.
+ * @param {string} blockName The block name.
+ * @param { UserBlockOptions } option An option to parse.
+ * @returns {BlockOptions} Normalized option.
+ */
+function normalizeOption(blockName, option) {
+  const lang = new Set(
+    Array.isArray(option.lang) ? option.lang : option.lang ? [option.lang] : []
+  )
+  let hasDefault = false
+  for (const def of DEFAULT_LANGUAGES[blockName] || []) {
+    if (lang.has(def)) {
+      lang.delete(def)
+      hasDefault = true
+    }
+  }
+  if (lang.size === 0) {
+    return {
+      lang,
+      allowNoLang: true
+    }
+  }
+  return {
+    lang,
+    allowNoLang: hasDefault || Boolean(option.allowNoLang)
+  }
+}
+/**
+ * Normalizes a given options.
+ * @param { UserOptions } options An option to parse.
+ * @returns {Options} Normalized option.
+ */
+function normalizeOptions(options) {
+  if (!options) {
+    return {}
+  }
+
+  /** @type {Options} */
+  const normalized = {}
+
+  for (const blockName of Object.keys(options)) {
+    const value = options[blockName]
+    if (value) {
+      normalized[blockName] = normalizeOption(blockName, value)
+    }
+  }
+
+  return normalized
+}
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'disallow use other than available `lang`',
+      categories: undefined,
+      url: 'https://eslint.vuejs.org/rules/block-lang.html'
+    },
+    schema: [
+      {
+        type: 'object',
+        patternProperties: {
+          '^(?:\\S+)$': {
+            oneOf: [
+              {
+                type: 'object',
+                properties: {
+                  lang: {
+                    anyOf: [
+                      { type: 'string' },
+                      {
+                        type: 'array',
+                        items: {
+                          type: 'string'
+                        },
+                        uniqueItems: true,
+                        additionalItems: false
+                      }
+                    ]
+                  },
+                  allowNoLang: { type: 'boolean' }
+                },
+                additionalProperties: false
+              }
+            ]
+          }
+        },
+        minProperties: 1,
+        additionalProperties: false
+      }
+    ],
+    messages: {
+      expected:
+        "Only {{allows}} can be used for the 'lang' attribute of '<{{tag}}>'.",
+      missing: "The 'lang' attribute of '<{{tag}}>' is missing.",
+      unexpected: "Do not specify the 'lang' attribute of '<{{tag}}>'.",
+      useOrNot:
+        "Only {{allows}} can be used for the 'lang' attribute of '<{{tag}}>'. Or, not specifying the `lang` attribute is allowed.",
+      unexpectedDefault:
+        "Do not explicitly specify the default language for the 'lang' attribute of '<{{tag}}>'."
+    }
+  },
+  /** @param {RuleContext} context */
+  create(context) {
+    const options = normalizeOptions(
+      context.options[0] || {
+        script: { allowNoLang: true },
+        template: { allowNoLang: true },
+        style: { allowNoLang: true }
+      }
+    )
+    if (!Object.keys(options).length) {
+      // empty
+      return {}
+    }
+
+    /**
+     * @param {VElement} element
+     * @returns {void}
+     */
+    function verify(element) {
+      const tag = element.name
+      const option = options[tag]
+      if (!option) {
+        return
+      }
+      const lang = utils.getAttribute(element, 'lang')
+      if (lang == null || lang.value == null) {
+        if (!option.allowNoLang) {
+          context.report({
+            node: element.startTag,
+            messageId: 'missing',
+            data: {
+              tag
+            }
+          })
+        }
+        return
+      }
+      if (!option.lang.has(lang.value.value)) {
+        let messageId
+        if (!option.allowNoLang) {
+          messageId = 'expected'
+        } else if (option.lang.size === 0) {
+          if ((DEFAULT_LANGUAGES[tag] || []).includes(lang.value.value)) {
+            messageId = 'unexpectedDefault'
+          } else {
+            messageId = 'unexpected'
+          }
+        } else {
+          messageId = 'useOrNot'
+        }
+        context.report({
+          node: lang,
+          messageId,
+          data: {
+            tag,
+            allows: getAllowsLangPhrase(option.lang)
+          }
+        })
+      }
+    }
+
+    return utils.defineDocumentVisitor(context, {
+      'VDocumentFragment > VElement': verify
+    })
+  }
+}

lib/utils/index.js

@@ -239,6 +239,18 @@ module.exports = {
    */
   defineTemplateBodyVisitor,
 
+  /**
+   * Register the given visitor to parser services.
+   * If the parser service of `vue-eslint-parser` was not found,
+   * this generates a warning.
+   *
+   * @param {RuleContext} context The rule context to use parser services.
+   * @param {TemplateListener} documentVisitor The visitor to traverse the document.
+   * @param { { triggerSelector: "Program" | "Program:exit" } } [options] The options.
+   * @returns {RuleListener} The merged visitor.
+   */
+  defineDocumentVisitor,
+
   /**
    * Wrap a given core rule to apply it to Vue.js template.
    * @param {string} coreRuleName The name of the core rule implementation to wrap.