Add prefer-sfc-lang-attr rule

docs/rules/README.md

@@ -28,6 +28,7 @@
 | [@intlify/vue-i18n/<wbr>no-dynamic-keys](./no-dynamic-keys.html) | disallow localization dynamic keys at localization methods |  |
 | [@intlify/vue-i18n/<wbr>no-missing-keys-in-other-locales](./no-missing-keys-in-other-locales.html) | disallow missing locale message keys in other locales |  |
 | [@intlify/vue-i18n/<wbr>no-unused-keys](./no-unused-keys.html) | disallow unused localization keys | :black_nib: |
+| [@intlify/vue-i18n/<wbr>prefer-sfc-lang-attr](./prefer-sfc-lang-attr.html) | require lang attribute on `<i18n>` block | :black_nib: |
 
 ## Stylistic Issues
 

docs/rules/prefer-sfc-lang-attr.md

@@ -0,0 +1,65 @@
+---
+title: '@intlify/vue-i18n/prefer-sfc-lang-attr'
+description: require lang attribute on `<i18n>` block
+---
+
+# @intlify/vue-i18n/prefer-sfc-lang-attr
+
+> require lang attribute on `<i18n>` block
+
+- :black_nib:️ The `--fix` option on the [command line](http://eslint.org/docs/user-guide/command-line-interface#fix) can automatically fix some of the problems reported by this rule.
+
+## :book: Rule Details
+
+This rule enforce `lang` attribute to be specified to `<i18n>` custom block.
+
+:-1: Examples of **incorrect** code for this rule:
+
+locale messages:
+
+<eslint-code-block fix>
+
+<!-- eslint-skip -->
+
+```vue
+<i18n>
+{
+  "en": {
+    "message": "hello!"
+  }
+}
+</i18n>
+<script>
+/* eslint @intlify/vue-i18n/prefer-sfc-lang-attr: 'error' */
+</script>
+```
+
+</eslint-code-block>
+
+:+1: Examples of **correct** code for this rule:
+
+locale messages:
+
+<eslint-code-block fix>
+
+<!-- eslint-skip -->
+
+```vue
+<i18n lang="json">
+{
+  "en": {
+    "message": "hello!"
+  }
+}
+</i18n>
+<script>
+/* eslint @intlify/vue-i18n/prefer-sfc-lang-attr: 'error' */
+</script>
+```
+
+</eslint-code-block>
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/intlify/eslint-plugin-vue-i18n/blob/master/lib/rules/prefer-sfc-lang-attr.ts)
+- [Test source](https://github.com/intlify/eslint-plugin-vue-i18n/tree/master/tests/lib/rules/prefer-sfc-lang-attr.ts)

lib/rules.ts

@@ -13,6 +13,7 @@ import noRawText from './rules/no-raw-text'
 import noUnusedKeys from './rules/no-unused-keys'
 import noVHtml from './rules/no-v-html'
 import preferLinkedKeyWithParen from './rules/prefer-linked-key-with-paren'
+import preferSfcLangAttr from './rules/prefer-sfc-lang-attr'
 import validMessageSyntax from './rules/valid-message-syntax'
 
 export = {
@@ -30,5 +31,6 @@ export = {
   'no-unused-keys': noUnusedKeys,
   'no-v-html': noVHtml,
   'prefer-linked-key-with-paren': preferLinkedKeyWithParen,
+  'prefer-sfc-lang-attr': preferSfcLangAttr,
   'valid-message-syntax': validMessageSyntax
 }

lib/rules/prefer-sfc-lang-attr.ts

@@ -0,0 +1,61 @@
+import { getAttribute, isI18nBlock } from '../utils/index'
+import type { RuleContext, RuleListener } from '../types'
+
+function create(context: RuleContext): RuleListener {
+  const df = context.parserServices.getDocumentFragment?.()
+  if (!df) {
+    return {}
+  }
+
+  return {
+    Program() {
+      for (const i18n of df.children.filter(isI18nBlock)) {
+        const srcAttrs = getAttribute(i18n, 'src')
+        if (srcAttrs != null) {
+          continue
+        }
+        const langAttrs = getAttribute(i18n, 'lang')
+        if (
+          langAttrs == null ||
+          langAttrs.value == null ||
+          !langAttrs.value.value
+        ) {
+          context.report({
+            loc: (langAttrs?.value ?? langAttrs ?? i18n.startTag).loc,
+            messageId: 'required',
+            fix(fixer) {
+              if (langAttrs) {
+                return fixer.replaceTextRange(langAttrs.range, 'lang="json"')
+              }
+              const tokenStore = context.parserServices.getTemplateBodyTokenStore()
+              const closeToken = tokenStore.getLastToken(i18n.startTag)
+              const beforeToken = tokenStore.getTokenBefore(closeToken)
+              return fixer.insertTextBeforeRange(
+                closeToken.range,
+                (beforeToken.range[1] < closeToken.range[0] ? '' : ' ') +
+                  'lang="json" '
+              )
+            }
+          })
+        }
+      }
+    }
+  }
+}
+
+export = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'require lang attribute on `<i18n>` block',
+      category: 'Best Practices',
+      recommended: false
+    },
+    fixable: 'code',
+    schema: [],
+    messages: {
+      required: '`lang` attribute is required.'
+    }
+  },
+  create
+}

lib/utils.ts

@@ -5,6 +5,7 @@ import * as casing from './utils/casing'
 import * as collectKeys from './utils/collect-keys'
 import * as collectLinkedKeys from './utils/collect-linked-keys'
 import * as defaultTimeouts from './utils/default-timeouts'
+import * as getCwd from './utils/get-cwd'
 import * as globSync from './utils/glob-sync'
 import * as globUtils from './utils/glob-utils'
 import * as ignoredPaths from './utils/ignored-paths'
@@ -22,6 +23,7 @@ export = {
   'collect-keys': collectKeys,
   'collect-linked-keys': collectLinkedKeys,
   'default-timeouts': defaultTimeouts,
+  'get-cwd': getCwd,
   'glob-sync': globSync,
   'glob-utils': globUtils,
   'ignored-paths': ignoredPaths,

tests/lib/rules/prefer-sfc-lang-attr.ts

@@ -0,0 +1,127 @@
+import { RuleTester } from 'eslint'
+import rule = require('../../../lib/rules/prefer-sfc-lang-attr')
+
+new RuleTester({
+  parser: require.resolve('vue-eslint-parser'),
+  parserOptions: { ecmaVersion: 2020, sourceType: 'module' }
+}).run('prefer-sfc-lang-attr', rule as never, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n lang="json">{}</i18n>
+      <template></template>
+      <script></script>`
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n lang="json5">{}</i18n>
+      <template></template>
+      <script></script>`
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n lang="yaml">{}</i18n>
+      <template></template>
+      <script></script>`
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n>{}</i18n>
+      <template></template>
+      <script></script>`,
+      output: `
+      <i18n lang="json" >{}</i18n>
+      <template></template>
+      <script></script>`,
+      errors: [
+        {
+          message: '`lang` attribute is required.',
+          line: 2,
+          column: 7,
+          endLine: 2,
+          endColumn: 13
+        }
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n>{}</i18n>
+      <template></template>`,
+      output: `
+      <i18n lang="json" >{}</i18n>
+      <template></template>`,
+      errors: [
+        {
+          message: '`lang` attribute is required.',
+          line: 2,
+          column: 7,
+          endLine: 2,
+          endColumn: 13
+        }
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n locale="en" >{}</i18n>
+      <template></template>
+      <script></script>`,
+      output: `
+      <i18n locale="en" lang="json" >{}</i18n>
+      <template></template>
+      <script></script>`,
+      errors: [
+        {
+          message: '`lang` attribute is required.',
+          line: 2,
+          column: 7,
+          endLine: 2,
+          endColumn: 26
+        }
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n lang>{}</i18n>
+      <template></template>`,
+      output: `
+      <i18n lang="json">{}</i18n>
+      <template></template>`,
+      errors: [
+        {
+          message: '`lang` attribute is required.',
+          line: 2,
+          column: 13,
+          endLine: 2,
+          endColumn: 17
+        }
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <i18n lang="">{}</i18n>
+      <template></template>`,
+      output: `
+      <i18n lang="json">{}</i18n>
+      <template></template>`,
+      errors: [
+        {
+          message: '`lang` attribute is required.',
+          line: 2,
+          column: 18,
+          endLine: 2,
+          endColumn: 20
+        }
+      ]
+    }
+  ]
+})