diff --git a/src/dune/file_tree.ml b/src/dune/file_tree.ml
index fbd80dc3fa26..6528b0688141 100644
--- a/src/dune/file_tree.ml
+++ b/src/dune/file_tree.ml
@@ -2,31 +2,29 @@ open! Stdune
 open Import
 
 module File = struct
-  type t =
-    { ino : int
-    ; dev : int
-    }
+  module T = struct
+    type t =
+      { ino : int
+      ; dev : int
+      }
 
-  let to_dyn { ino; dev } =
-    let open Dyn.Encoder in
-    record [ ("ino", Int.to_dyn ino); ("dev", Int.to_dyn dev) ]
+    let to_dyn { ino; dev } =
+      let open Dyn.Encoder in
+      record [ ("ino", Int.to_dyn ino); ("dev", Int.to_dyn dev) ]
 
-  let compare a b =
-    match Int.compare a.ino b.ino with
-    | Eq -> Int.compare a.dev b.dev
-    | ne -> ne
+    let compare a b =
+      match Int.compare a.ino b.ino with
+      | Eq -> Int.compare a.dev b.dev
+      | ne -> ne
+  end
+
+  include T
 
   let dummy = { ino = 0; dev = 0 }
 
   let of_stats (st : Unix.stats) = { ino = st.st_ino; dev = st.st_dev }
 
-  module Map = Map.Make (struct
-    type nonrec t = t
-
-    let compare = compare
-
-    let to_dyn _ = Dyn.opaque
-  end)
+  module Map = Map.Make (T)
 
   let of_source_path p = of_stats (Path.stat (Path.source p))
 end