diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -41,7 +41,7 @@
 # define _LIBCPP_ASSERT(expression, message)                                        \
     (__builtin_expect(static_cast<bool>(expression), 1) ?                           \
       (void)0 :                                                                     \
-      ::std::__libcpp_verbose_abort("%s:%d: assertion %s failed: %s", __FILE__, __LINE__, #expression, message))
+      ::std::__libcpp_verbose_abort("%s:%d: assertion %s failed: %s\n", __FILE__, __LINE__, #expression, message))
 #elif !defined(_LIBCPP_ASSERTIONS_DISABLE_ASSUME) && __has_builtin(__builtin_assume)
 # define _LIBCPP_ASSERT(expression, message)                                        \
     (_LIBCPP_DIAGNOSTIC_PUSH                                                        \
diff --git a/libcxx/test/support/check_assertion.h b/libcxx/test/support/check_assertion.h
--- a/libcxx/test/support/check_assertion.h
+++ b/libcxx/test/support/check_assertion.h
@@ -249,6 +249,8 @@
   char const* message = va_arg(list, char const*);
   va_end(list);
 
+  assert(format[__builtin_strlen(format) - 1] == '\n');
+
   if (GlobalMatcher().Matches(file, line, message)) {
     std::exit(DeathTest::RK_MatchFound);
   }