diff --git a/src/libutil/include/nix/util/memory-source-accessor.hh b/src/libutil/include/nix/util/memory-source-accessor.hh
index a04d1d347..98c193800 100644
--- a/src/libutil/include/nix/util/memory-source-accessor.hh
+++ b/src/libutil/include/nix/util/memory-source-accessor.hh
@@ -1,3 +1,6 @@
+#pragma once
+///@file
+
 #include "nix/util/source-path.hh"
 #include "nix/util/fs-sink.hh"
 #include "nix/util/variant-wrapper.hh"