diff --git a/infer/src/bufferoverrun/bufferOverrunDomain.ml b/infer/src/bufferoverrun/bufferOverrunDomain.ml
index 99528e9c4..a6dc9a593 100644
--- a/infer/src/bufferoverrun/bufferOverrunDomain.ml
+++ b/infer/src/bufferoverrun/bufferOverrunDomain.ml
@@ -12,6 +12,7 @@ open AbsLoc
 open! AbstractDomain.Types
 module F = Format
 module L = Logging
+module OndemandEnv = BufferOverrunOndemandEnv
 module Relation = BufferOverrunDomainRelation
 module SPath = Symb.SymbolPath
 module Trace = BufferOverrunTrace
@@ -22,90 +23,6 @@ type eval_sym_trace =
   ; trace_of_sym: Symb.Symbol.t -> Trace.Set.t
   ; eval_locpath: PowLoc.eval_locpath }
 
-module OndemandEnv = struct
-  module FormalTyps = Caml.Map.Make (Pvar)
-
-  type t =
-    { typ_of_param_path: SPath.partial -> Typ.t option
-    ; may_last_field: SPath.partial -> bool
-    ; entry_location: Location.t
-    ; integer_type_widths: Typ.IntegerWidths.t }
-
-  let mk pdesc =
-    let formal_typs =
-      List.fold (Procdesc.get_pvar_formals pdesc) ~init:FormalTyps.empty
-        ~f:(fun acc (formal, typ) -> FormalTyps.add formal typ acc )
-    in
-    fun tenv integer_type_widths ->
-      let rec typ_of_param_path = function
-        | SPath.Pvar x ->
-            FormalTyps.find_opt x formal_typs
-        | SPath.Deref (_, x) ->
-            Option.map (typ_of_param_path x) ~f:(fun typ ->
-                match typ.Typ.desc with
-                | Tptr (typ, _) ->
-                    typ
-                | Tarray {elt} ->
-                    elt
-                | Tvoid ->
-                    Typ.void
-                | _ ->
-                    L.(die InternalError) "Untyped expression is given." )
-        | SPath.Field (fn, x) ->
-            let lookup = Tenv.lookup tenv in
-            Option.map (typ_of_param_path x) ~f:(Typ.Struct.fld_typ ~lookup ~default:Typ.void fn)
-        | SPath.Callsite {ret_typ} ->
-            Some ret_typ
-      in
-      let is_last_field fn (fields : Typ.Struct.field list) =
-        Option.value_map (List.last fields) ~default:false ~f:(fun (last_fn, _, _) ->
-            Typ.Fieldname.equal fn last_fn )
-      in
-      let rec may_last_field = function
-        | SPath.Pvar _ | SPath.Deref _ | SPath.Callsite _ ->
-            true
-        | SPath.Field (fn, x) ->
-            may_last_field x
-            && Option.value_map ~default:true (typ_of_param_path x) ~f:(fun parent_typ ->
-                   match parent_typ.Typ.desc with
-                   | Tstruct typename ->
-                       let opt_struct = Tenv.lookup tenv typename in
-                       Option.value_map opt_struct ~default:false ~f:(fun str ->
-                           is_last_field fn str.Typ.Struct.fields )
-                   | _ ->
-                       true )
-      in
-      let entry_location = Procdesc.Node.get_loc (Procdesc.get_start_node pdesc) in
-      {typ_of_param_path; may_last_field; entry_location; integer_type_widths}
-
-
-  let canonical_path typ_of_param_path path =
-    let module KnownFields = Caml.Map.Make (struct
-      type t = Typ.t option * Typ.Fieldname.t [@@deriving compare]
-    end) in
-    let rec helper path =
-      match path with
-      | SPath.Pvar _ | SPath.Callsite _ ->
-          (None, KnownFields.empty)
-      | SPath.Deref (deref_kind, ptr) ->
-          let ptr_opt, known_fields = helper ptr in
-          (Option.map ptr_opt ~f:(fun ptr -> SPath.deref ~deref_kind ptr), known_fields)
-      | SPath.Field (fn, struct_path) -> (
-          let struct_path_opt, known_fields = helper struct_path in
-          let key = (typ_of_param_path (Option.value ~default:struct_path struct_path_opt), fn) in
-          match KnownFields.find_opt key known_fields with
-          | Some _ as canonicalized ->
-              (canonicalized, known_fields)
-          | None ->
-              let field_path =
-                Option.value_map struct_path_opt ~default:path ~f:(fun struct_path ->
-                    SPath.field struct_path fn )
-              in
-              (None, KnownFields.add key field_path known_fields) )
-    in
-    Option.value (helper path |> fst) ~default:path
-end
-
 module Val = struct
   type t =
     { itv: Itv.t
diff --git a/infer/src/bufferoverrun/bufferOverrunOndemandEnv.ml b/infer/src/bufferoverrun/bufferOverrunOndemandEnv.ml
new file mode 100644
index 000000000..4cd06ad55
--- /dev/null
+++ b/infer/src/bufferoverrun/bufferOverrunOndemandEnv.ml
@@ -0,0 +1,91 @@
+(*
+ * Copyright (c) 2018-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *)
+
+open! IStd
+module L = Logging
+module SPath = Symb.SymbolPath
+module FormalTyps = Caml.Map.Make (Pvar)
+
+type t =
+  { typ_of_param_path: SPath.partial -> Typ.t option
+  ; may_last_field: SPath.partial -> bool
+  ; entry_location: Location.t
+  ; integer_type_widths: Typ.IntegerWidths.t }
+
+let mk pdesc =
+  let formal_typs =
+    List.fold (Procdesc.get_pvar_formals pdesc) ~init:FormalTyps.empty ~f:(fun acc (formal, typ) ->
+        FormalTyps.add formal typ acc )
+  in
+  fun tenv integer_type_widths ->
+    let rec typ_of_param_path = function
+      | SPath.Pvar x ->
+          FormalTyps.find_opt x formal_typs
+      | SPath.Deref (_, x) ->
+          Option.map (typ_of_param_path x) ~f:(fun typ ->
+              match typ.Typ.desc with
+              | Tptr (typ, _) ->
+                  typ
+              | Tarray {elt} ->
+                  elt
+              | Tvoid ->
+                  Typ.void
+              | _ ->
+                  L.(die InternalError) "Untyped expression is given." )
+      | SPath.Field (fn, x) ->
+          let lookup = Tenv.lookup tenv in
+          Option.map (typ_of_param_path x) ~f:(Typ.Struct.fld_typ ~lookup ~default:Typ.void fn)
+      | SPath.Callsite {ret_typ} ->
+          Some ret_typ
+    in
+    let is_last_field fn (fields : Typ.Struct.field list) =
+      Option.value_map (List.last fields) ~default:false ~f:(fun (last_fn, _, _) ->
+          Typ.Fieldname.equal fn last_fn )
+    in
+    let rec may_last_field = function
+      | SPath.Pvar _ | SPath.Deref _ | SPath.Callsite _ ->
+          true
+      | SPath.Field (fn, x) ->
+          may_last_field x
+          && Option.value_map ~default:true (typ_of_param_path x) ~f:(fun parent_typ ->
+                 match parent_typ.Typ.desc with
+                 | Tstruct typename ->
+                     let opt_struct = Tenv.lookup tenv typename in
+                     Option.value_map opt_struct ~default:false ~f:(fun str ->
+                         is_last_field fn str.Typ.Struct.fields )
+                 | _ ->
+                     true )
+    in
+    let entry_location = Procdesc.Node.get_loc (Procdesc.get_start_node pdesc) in
+    {typ_of_param_path; may_last_field; entry_location; integer_type_widths}
+
+
+let canonical_path typ_of_param_path path =
+  let module KnownFields = Caml.Map.Make (struct
+    type t = Typ.t option * Typ.Fieldname.t [@@deriving compare]
+  end) in
+  let rec helper path =
+    match path with
+    | SPath.Pvar _ | SPath.Callsite _ ->
+        (None, KnownFields.empty)
+    | SPath.Deref (deref_kind, ptr) ->
+        let ptr_opt, known_fields = helper ptr in
+        (Option.map ptr_opt ~f:(fun ptr -> SPath.deref ~deref_kind ptr), known_fields)
+    | SPath.Field (fn, struct_path) -> (
+        let struct_path_opt, known_fields = helper struct_path in
+        let key = (typ_of_param_path (Option.value ~default:struct_path struct_path_opt), fn) in
+        match KnownFields.find_opt key known_fields with
+        | Some _ as canonicalized ->
+            (canonicalized, known_fields)
+        | None ->
+            let field_path =
+              Option.value_map struct_path_opt ~default:path ~f:(fun struct_path ->
+                  SPath.field struct_path fn )
+            in
+            (None, KnownFields.add key field_path known_fields) )
+  in
+  Option.value (helper path |> fst) ~default:path