diff --git a/backend/mirai-console/src/internal/util/CommonUtils.kt b/backend/mirai-console/src/internal/util/CommonUtils.kt
index a28170631..d52b4a8b9 100644
--- a/backend/mirai-console/src/internal/util/CommonUtils.kt
+++ b/backend/mirai-console/src/internal/util/CommonUtils.kt
@@ -41,8 +41,27 @@ internal fun StackFrame.findLoader(): ClassLoader? {
     }.getOrNull()
 }
 
+internal inline fun <T : Any> T?.ifNull(block: () -> T): T {
+    contract { callsInPlace(block, InvocationKind.AT_MOST_ONCE) }
+    return this ?: block()
+}
+
 @PublishedApi
-internal inline fun assertionError(message: () -> String): Nothing {
+internal inline fun assertionError(message: () -> String = { "Reached an unexpected branch." }): Nothing {
     contract { callsInPlace(message, InvocationKind.EXACTLY_ONCE) }
     throw AssertionError(message())
-}
\ No newline at end of file
+}
+
+@PublishedApi
+internal inline fun assertUnreachable(message: () -> String = { "Reached an unexpected branch." }): Nothing {
+    contract { callsInPlace(message, InvocationKind.EXACTLY_ONCE) }
+    throw AssertionError(message())
+}
+
+@MarkerUnreachableClause
+@PublishedApi
+internal inline val UNREACHABLE_CLAUSE: Nothing
+    get() = assertUnreachable()
+
+@DslMarker
+private annotation class MarkerUnreachableClause
\ No newline at end of file