diff --git a/src/analysis/index_format.ml b/src/analysis/index_format.ml
index 092b0de70..c4a19e814 100644
--- a/src/analysis/index_format.ml
+++ b/src/analysis/index_format.ml
@@ -28,7 +28,7 @@ let add tbl uid locs =
     Hashtbl.replace tbl uid (LidSet.union locs locations)
   with Not_found -> Hashtbl.add tbl uid locs
 
-type stat = { mtime : float; size : int }
+type stat = { mtime : float; size : int; source_digest: string option }
 type index = {
   defs : (Shape.Uid.t, LidSet.t) Hashtbl.t;
   approximated : (Shape.Uid.t, LidSet.t) Hashtbl.t;
diff --git a/src/analysis/index_format.mli b/src/analysis/index_format.mli
index 4f18a3660..4d68269c7 100644
--- a/src/analysis/index_format.mli
+++ b/src/analysis/index_format.mli
@@ -7,7 +7,7 @@ val add : ('a, LidSet.t) Hashtbl.t -> 'a -> LidSet.t -> unit
 
 module Stats : Map.S with type key = String.t
 
-type stat = { mtime : float; size : int }
+type stat = { mtime : float; size : int; source_digest: string option }
 type index = {
   defs : (Shape.Uid.t, LidSet.t) Hashtbl.t;
   approximated : (Shape.Uid.t, LidSet.t) Hashtbl.t;
diff --git a/src/analysis/occurrences.ml b/src/analysis/occurrences.ml
index 100f535a2..58d022cf5 100644
--- a/src/analysis/occurrences.ml
+++ b/src/analysis/occurrences.ml
@@ -152,12 +152,13 @@ let check Index_format.{ stats; _ } file =
   let open Index_format in
   match Stats.find_opt file stats with
   | None -> log ~title:"stat_check" "No mtime found for file %S." file; true
-  | Some { mtime; size } ->
+  | Some { size; _ } ->
     try
       let stats = Unix.stat file in
       let equal =
-        Float.equal stats.st_mtime mtime
-        || Int.equal stats.st_size size
+        (* This is fast but approximative. A better option would be to check
+           [mtime] and then [source_digest] if the times differ. *)
+         Int.equal stats.st_size size
       in
       log ~title:"stat_check"
         "File %s has been modified since the index was built." file;