diff --git a/kernel/log.cc b/kernel/log.cc
index 8f9ee2a13..5fda6ce14 100644
--- a/kernel/log.cc
+++ b/kernel/log.cc
@@ -309,15 +309,10 @@ void log_formatted_warning(std::string_view prefix, std::string message)
 	}
 }
 
-void log_file_warning(const std::string &filename, int lineno,
-                      const char *format, ...)
+void log_formatted_file_warning(std::string_view filename, int lineno, std::string str)
 {
-	va_list ap;
-	va_start(ap, format);
-	std::string prefix = stringf("%s:%d: Warning: ",
-			filename.c_str(), lineno);
-	log_formatted_warning(prefix, vstringf(format, ap));
-	va_end(ap);
+	std::string prefix = stringf("%s:%d: Warning: ", filename, lineno);
+	log_formatted_warning(prefix, std::move(str));
 }
 
 void log_file_info(const std::string &filename, int lineno,
diff --git a/kernel/log.h b/kernel/log.h
index ba2c1d9dc..bdada8437 100644
--- a/kernel/log.h
+++ b/kernel/log.h
@@ -124,8 +124,6 @@ extern int log_debug_suppressed;
 void set_verific_logging(void (*cb)(int msg_type, const char *message_id, const char* file_path, unsigned int left_line, unsigned int left_col, unsigned int right_line, unsigned int right_col, const char *msg));
 extern void (*log_verific_callback)(int msg_type, const char *message_id, const char* file_path, unsigned int left_line, unsigned int left_col, unsigned int right_line, unsigned int right_col, const char *msg);
 
-// Log with filename to report a problem in a source file.
-void log_file_warning(const std::string &filename, int lineno, const char *format, ...) YS_ATTRIBUTE(format(printf, 3, 4));
 void log_file_info(const std::string &filename, int lineno, const char *format, ...) YS_ATTRIBUTE(format(printf, 3, 4));
 
 [[noreturn]] void log_error(const char *format, ...) YS_ATTRIBUTE(format(printf, 1, 2));
@@ -169,6 +167,14 @@ inline void log_warning_noprefix(FmtString<TypeIdentity<Args>...> fmt, Args... a
 
 void log_experimental(const std::string &str);
 
+// Log with filename to report a problem in a source file.
+void log_formatted_file_warning(std::string_view filename, int lineno, std::string str);
+template <typename... Args>
+void log_file_warning(std::string_view filename, int lineno, FmtString<TypeIdentity<Args>...> fmt, Args... args)
+{
+	log_formatted_file_warning(filename, lineno, fmt.format(args...));
+}
+
 static inline void log_suppressed() {
 	if (log_debug_suppressed && !log_make_debug) {
 		log("<suppressed ~%d debug messages>\n", log_debug_suppressed);