diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -37,7 +37,7 @@
     (__builtin_expect(static_cast<bool>(expression), 1)                                                                \
          ? (void)0                                                                                                     \
          : _LIBCPP_VERBOSE_ABORT(                                                                                      \
-               "%s:%d: assertion %s failed: %s", __builtin_FILE(), __builtin_LINE(), #expression, message))
+               "%s:%d: assertion %s failed: %s\n", __builtin_FILE(), __builtin_LINE(), #expression, message))
 #elif !defined(_LIBCPP_ASSERTIONS_DISABLE_ASSUME) && __has_builtin(__builtin_assume)
 #  define _LIBCPP_ASSERT(expression, message)                                                                          \
     (_LIBCPP_DIAGNOSTIC_PUSH _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wassume")                                              \
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);
   }