fix: prepare to drop 'allowComposition' option (#1496)

Author: kazupon

docs/guide/migration/vue3.md

@@ -14,6 +14,12 @@ Developing a Vue application with a mix of Options API styles and Compostion API
 
 From Vue I18n v9.2, the Legacy API mode can also be used with Composition API mode.
 
+### About supporting
+
+:::warning NOTICE
+This migration support will be removed in the next major version v10. If your Vue 3 application project has not yet been migrated, please migrate it to v9 and then upgrade to v10.
+:::
+
 ### Limitations
 
 :::warning NOTICE

packages/vue-i18n-core/src/i18n.ts

@@ -552,6 +552,12 @@ export function createI18n(options: any = {}, VueI18nLegacy?: any): any {
     __DEV__ ? 'vue-i18n' : ''
   )
 
+  if (__DEV__) {
+    if (__legacyMode && __allowComposition) {
+      warn(getWarnMessage(I18nWarnCodes.NOTICE_DROP_ALLOW_COMPOSITION))
+    }
+  }
+
   function __getInstance<Instance extends VueI18n | Composer>(
     component: ComponentInternalInstance
   ): Instance | null {

packages/vue-i18n-core/src/warnings.ts

@@ -12,7 +12,8 @@ export const I18nWarnCodes = {
   NOT_SUPPORTED_GET_CHOICE_INDEX: inc(), // 11
   COMPONENT_NAME_LEGACY_COMPATIBLE: inc(), // 12
   NOT_FOUND_PARENT_SCOPE: inc(), // 13
-  IGNORE_OBJ_FLATTEN: inc() // 14
+  IGNORE_OBJ_FLATTEN: inc(), // 14
+  NOTICE_DROP_ALLOW_COMPOSITION: inc() // 15
 } as const
 
 type I18nWarnCodes = (typeof I18nWarnCodes)[keyof typeof I18nWarnCodes]
@@ -25,7 +26,8 @@ export const warnMessages: { [code: number]: string } = {
   [I18nWarnCodes.NOT_SUPPORTED_GET_CHOICE_INDEX]: `Not supported 'getChoiceIndex'.`,
   [I18nWarnCodes.COMPONENT_NAME_LEGACY_COMPATIBLE]: `Component name legacy compatible: '{name}' -> 'i18n'`,
   [I18nWarnCodes.NOT_FOUND_PARENT_SCOPE]: `Not found parent scope. use the global scope.`,
-  [I18nWarnCodes.IGNORE_OBJ_FLATTEN]: `Ignore object flatten: '{key}' key has an string value`
+  [I18nWarnCodes.IGNORE_OBJ_FLATTEN]: `Ignore object flatten: '{key}' key has an string value`,
+  [I18nWarnCodes.NOTICE_DROP_ALLOW_COMPOSITION]: `'allowComposition' option will be dropped in the next major version. For more information, please see 👉 https://tinyurl.com/2p97mcze`
 }
 
 export function getWarnMessage(