Add InternalAPI annotation

mirai-core/build.gradle.kts

@@ -88,7 +88,6 @@ kotlin {
             implementation("io.ktor:ktor-client-android:$ktorVersion")
 
         }
-        languageSettings.enableLanguageFeature("InlineClasses")
     }
 
     sourceSets["jvmMain"].apply {
@@ -116,6 +115,7 @@ kotlin {
 
     sourceSets.all {
         languageSettings.enableLanguageFeature("InlineClasses")
+        languageSettings.useExperimentalAnnotation("kotlin.Experimental")
 
         dependencies {
             implementation("org.jetbrains.kotlin:kotlin-stdlib")

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/Annotations.kt

+package net.mamoe.mirai.utils
+
+/**
+ * 标记这个类, 类型, 函数, 属性, 字段, 或构造器为一个仅供 Mirai 内部使用的 API.
+ *
+ * 这些 API 可能会在任意时刻更改, 且不会发布任何预警.
+ */
+@Experimental(level = Experimental.Level.ERROR)
+@Target(
+    AnnotationTarget.CLASS,
+    AnnotationTarget.TYPEALIAS,
+    AnnotationTarget.FUNCTION,
+    AnnotationTarget.PROPERTY,
+    AnnotationTarget.FIELD,
+    AnnotationTarget.CONSTRUCTOR
+)
+annotation class InternalAPI
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