diff --git a/src/action.ml b/src/action.ml
index e5cf7a8efd02..3c9e9ec43cbf 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -270,53 +270,14 @@ module Unresolved = struct
         | Search s -> Ok (f s))
 end
 
-module Var_expansion = struct
-  type t =
-    | Paths   of Path.t list
-    | Strings of string list
-
-  let length = function
-    | Paths x -> List.length x
-    | Strings x -> List.length x
-
-  let is_multivalued = function
-    | Paths [_] -> false
-    | Strings [_] -> false
-    | _ -> true
-
-  type context = Path.t (* For String_with_vars.Expand_to *)
-
-  let concat = function
-    | [s] -> s
-    | l -> String.concat ~sep:" " l
-
-  let string_of_path ~dir p = Path.reach ~from:dir p
-  let path_of_string dir s = Path.relative dir s
-
-  let to_strings dir = function
-    | Strings l -> l
-    | Paths   l -> List.map l ~f:(string_of_path ~dir)
-
-  let to_string (dir: context) = function
-    | Strings l -> concat l
-    | Paths   l -> concat (List.map l ~f:(string_of_path ~dir))
-
-  let to_path dir = function
-    | Strings l -> path_of_string dir (concat l)
-    | Paths [p] -> p
-    | Paths l ->
-      path_of_string dir (concat (List.map l ~f:(string_of_path ~dir)))
-
-  let to_prog_and_args dir exp : Unresolved.Program.t * string list =
-    let module P = Unresolved.Program in
-    match exp with
-    | Paths   (x::xs) -> (This x, to_strings dir (Paths xs))
-    | Strings (s::xs) -> (P.of_string ~dir s, to_strings dir (Strings xs))
-    | Paths [] | Strings [] -> (Search "", [])
-end
+let var_expansion_to_prog_and_args  dir exp : Unresolved.Program.t * string list =
+  let module P = Unresolved.Program in
+  match (exp : Var_expansion.t) with
+  | Paths   (x::xs) -> (This x, Var_expansion.to_strings dir (Paths xs))
+  | Strings (s::xs) -> ( P.of_string ~dir s
+                       , Var_expansion.to_strings dir (Strings xs))
+  | Paths [] | Strings [] -> (Search "", [])
 
-module VE = Var_expansion
-module To_VE = String_with_vars.Expand_to(VE)
 module SW = String_with_vars
 
 module Unexpanded = struct
@@ -357,37 +318,39 @@ module Unexpanded = struct
       let expand ~generic ~special ~map ~dir ~allow_multivalue ~f = function
         | Left x -> map x
         | Right template ->
-           match To_VE.expand dir template ~f ~allow_multivalue with
-           | Expansion e -> special dir e
-           | String    s -> generic dir s
+          match
+            Var_expansion.Expand.expand dir template ~f ~allow_multivalue
+          with
+          | Expansion e -> special dir e
+          | String    s -> generic dir s
       [@@inlined always]
 
       let string ~dir ~f x =
         expand ~dir ~f x
           ~allow_multivalue:false
           ~generic:(fun _dir x -> x)
-          ~special:VE.to_string
+          ~special:Var_expansion.to_string
           ~map:(fun x -> x)
 
       let strings ~dir ~f x =
         expand ~dir ~f x
           ~allow_multivalue:true
           ~generic:(fun _dir x -> [x])
-          ~special:VE.to_strings
+          ~special:Var_expansion.to_strings
           ~map:(fun x -> [x])
 
       let path ~dir ~f x =
         expand ~dir ~f x
           ~allow_multivalue:false
-          ~generic:VE.path_of_string
-          ~special:VE.to_path
+          ~generic:Var_expansion.path_of_string
+          ~special:Var_expansion.to_path
           ~map:(fun x -> x)
 
       let prog_and_args ~dir ~f x =
         expand ~dir ~f x
           ~allow_multivalue:true
           ~generic:(fun _dir s -> (Program.of_string ~dir s, []))
-          ~special:VE.to_prog_and_args
+          ~special:var_expansion_to_prog_and_args
           ~map:(fun x -> (x, []))
     end
 
@@ -452,7 +415,9 @@ module Unexpanded = struct
 
   module E = struct
     let expand ~generic ~special ~dir ~allow_multivalue ~f template =
-      match To_VE.partial_expand dir template ~allow_multivalue ~f with
+      match
+        Var_expansion.Expand.partial_expand dir template ~allow_multivalue ~f
+      with
       | Expansion e -> Left (special dir e)
       | String s -> Left (generic dir s)
       | Unexpanded x -> Right x
@@ -461,25 +426,25 @@ module Unexpanded = struct
       expand ~dir ~f x
         ~allow_multivalue:false
         ~generic:(fun _dir x -> x)
-        ~special:VE.to_string
+        ~special:Var_expansion.to_string
 
     let strings ~dir ~f x =
       expand ~dir ~f x
         ~allow_multivalue:true
         ~generic:(fun _dir x -> [x])
-        ~special:VE.to_strings
+        ~special:Var_expansion.to_strings
 
     let path ~dir ~f x =
       expand ~dir ~f x
         ~allow_multivalue:false
-        ~generic:VE.path_of_string
-        ~special:VE.to_path
+        ~generic:Var_expansion.path_of_string
+        ~special:Var_expansion.to_path
 
     let prog_and_args ~dir ~f x =
       expand ~dir ~f x
         ~allow_multivalue:true
         ~generic:(fun dir s -> (Unresolved.Program.of_string ~dir s, []))
-        ~special:VE.to_prog_and_args
+        ~special:var_expansion_to_prog_and_args
   end
 
   let rec partial_expand t ~dir ~map_exe ~f : Partial.t =
diff --git a/src/action.mli b/src/action.mli
index 89e0a91faef9..310dcd891546 100644
--- a/src/action.mli
+++ b/src/action.mli
@@ -1,16 +1,5 @@
 open! Import
 
-module Var_expansion : sig
-  type t =
-    | Paths   of Path.t list
-    | Strings of string list
-
-  val to_string : Path.t -> t -> string
-  (** [to_string dir v] convert the variable expansion to a string.
-     If it is a path, the corresponding string will be relative to
-     [dir]. *)
-end
-
 module Outputs : module type of struct include Action_intf.Outputs end
 
 (** result of the lookup of a program, the path to it or information about the
diff --git a/src/inline_tests.ml b/src/inline_tests.ml
index 80f9de61a516..23ebe737284a 100644
--- a/src/inline_tests.ml
+++ b/src/inline_tests.ml
@@ -190,8 +190,7 @@ include Sub_system.Register_end_point(
     in
 
     let extra_vars =
-      String.Map.singleton "library-name"
-        (Action.Var_expansion.Strings [lib.name])
+      String.Map.singleton "library-name" (Var_expansion.Strings [lib.name])
     in
 
     let runner_libs =
@@ -213,7 +212,7 @@ include Sub_system.Register_end_point(
       let target = Path.relative inline_test_dir main_module_filename in
       let source_modules = Module.Name.Map.values source_modules in
       let files ml_kind =
-        Action.Var_expansion.Paths (
+        Var_expansion.Paths (
           List.filter_map source_modules ~f:(fun m ->
             Module.file m ~dir ml_kind))
       in
diff --git a/src/super_context.ml b/src/super_context.ml
index 63a354975df9..1c64c9f0da96 100644
--- a/src/super_context.ml
+++ b/src/super_context.ml
@@ -35,7 +35,7 @@ type t =
   ; artifacts                        : Artifacts.t
   ; stanzas_to_consider_for_install  : (Path.t * Scope.t * Stanza.t) list
   ; cxx_flags                        : string list
-  ; vars                             : Action.Var_expansion.t String.Map.t
+  ; vars                             : Var_expansion.t String.Map.t
   ; chdir                            : (Action.t, Action.t) Build.t
   ; host                             : t option
   ; libs_by_package : (Package.t * Lib.Set.t) Package.Name.Map.t
@@ -82,7 +82,7 @@ let expand_vars t ~scope ~dir ?(extra_vars=String.Map.empty) s =
     | "SCOPE_ROOT" ->
       Some (Path.reach ~from:dir (Scope.root scope))
     | var ->
-      Option.map ~f:(fun e -> Action.Var_expansion.to_string dir e)
+      Option.map ~f:(fun e -> Var_expansion.to_string dir e)
         (match expand_var_no_root t var with
          | Some _ as x -> x
          | None -> String.Map.find extra_vars var))
@@ -248,7 +248,7 @@ let create
       | None -> Path.relative context.ocaml_bin "ocamlopt"
       | Some p -> p
     in
-    let open Action.Var_expansion in
+    let open Var_expansion in
     let make =
       match Bin.make with
       | None   -> Strings ["make"]
@@ -566,7 +566,7 @@ module Action = struct
     ; (* Static deps from ${...} variables. For instance ${exe:...} *)
       mutable sdeps     : Path.Set.t
     ; (* Dynamic deps from ${...} variables. For instance ${read:...} *)
-      mutable ddeps     : (unit, Action.Var_expansion.t) Build.t String.Map.t
+      mutable ddeps     : (unit, Var_expansion.t) Build.t String.Map.t
     }
 
   let add_lib_dep acc lib kind =
@@ -580,8 +580,8 @@ module Action = struct
     acc.ddeps <- String.Map.add acc.ddeps key dep;
     None
 
-  let path_exp path = Action.Var_expansion.Paths   [path]
-  let str_exp  path = Action.Var_expansion.Strings [path]
+  let path_exp path = Var_expansion.Paths   [path]
+  let str_exp  path = Var_expansion.Strings [path]
 
   let map_exe sctx =
     match sctx.host with
@@ -608,7 +608,7 @@ module Action = struct
       ; ddeps     = String.Map.empty
       }
     in
-    let open Action.Var_expansion in
+    let open Var_expansion in
     let expand loc key var = function
       | Some ("exe"     , s) -> Some (path_exp (map_exe (Path.relative dir s)))
       | Some ("path"    , s) -> Some (path_exp          (Path.relative dir s) )
@@ -729,7 +729,7 @@ module Action = struct
     (t, acc)
 
   let expand_step2 ~dir ~dynamic_expansions ~deps_written_by_user ~map_exe t =
-    let open Action.Var_expansion in
+    let open Var_expansion in
     U.Partial.expand t ~dir ~map_exe ~f:(fun loc key ->
       match String.Map.find dynamic_expansions key with
       | Some _ as opt -> opt
@@ -740,9 +740,9 @@ module Action = struct
           Some
             (match deps_written_by_user with
              | [] ->
-                Loc.warn loc "Variable '<' used with no explicit \
-                              dependencies@.";
-                Strings [""]
+               Loc.warn loc "Variable '<' used with no explicit \
+                             dependencies@.";
+               Strings [""]
              | dep :: _ ->
                Paths [dep])
         | "^" -> Some (Paths deps_written_by_user)
diff --git a/src/super_context.mli b/src/super_context.mli
index a762c9609cca..1daed6992acd 100644
--- a/src/super_context.mli
+++ b/src/super_context.mli
@@ -72,7 +72,7 @@ val expand_vars
   :  t
   -> scope:Scope.t
   -> dir:Path.t
-  -> ?extra_vars:Action.Var_expansion.t String.Map.t
+  -> ?extra_vars:Var_expansion.t String.Map.t
   -> String_with_vars.t
   -> string
 
@@ -80,7 +80,7 @@ val expand_and_eval_set
   :  t
   -> scope:Scope.t
   -> dir:Path.t
-  -> ?extra_vars:Action.Var_expansion.t String.Map.t
+  -> ?extra_vars:Var_expansion.t String.Map.t
   -> Ordered_set_lang.Unexpanded.t
   -> standard:(unit, string list) Build.t
   -> (unit, string list) Build.t
@@ -208,7 +208,7 @@ module Action : sig
   val run
     :  t
     -> loc:Loc.t
-    -> ?extra_vars:Action.Var_expansion.t String.Map.t
+    -> ?extra_vars:Var_expansion.t String.Map.t
     -> Action.Unexpanded.t
     -> dir:Path.t
     -> dep_kind:Build.lib_dep_kind
diff --git a/src/var_expansion.ml b/src/var_expansion.ml
new file mode 100644
index 000000000000..8dc6d996d755
--- /dev/null
+++ b/src/var_expansion.ml
@@ -0,0 +1,44 @@
+open Stdune
+
+module T = struct
+  type t =
+    | Paths   of Path.t list
+    | Strings of string list
+
+  let length = function
+    | Paths x -> List.length x
+    | Strings x -> List.length x
+
+  let is_multivalued = function
+    | Paths [_] -> false
+    | Strings [_] -> false
+    | _ -> true
+
+  type context = Path.t (* For String_with_vars.Expand_to *)
+
+  let concat = function
+    | [s] -> s
+    | l -> String.concat ~sep:" " l
+
+  let string_of_path ~dir p = Path.reach ~from:dir p
+
+  let to_string (dir: context) = function
+    | Strings l -> concat l
+    | Paths   l -> concat (List.map l ~f:(string_of_path ~dir))
+end
+
+include T
+
+module Expand = String_with_vars.Expand_to(T)
+
+let path_of_string dir s = Path.relative dir s
+
+let to_strings dir = function
+  | Strings l -> l
+  | Paths   l -> List.map l ~f:(string_of_path ~dir)
+
+let to_path dir = function
+  | Strings l -> path_of_string dir (concat l)
+  | Paths [p] -> p
+  | Paths l ->
+    path_of_string dir (concat (List.map l ~f:(string_of_path ~dir)))
diff --git a/src/var_expansion.mli b/src/var_expansion.mli
new file mode 100644
index 000000000000..09819fcf8f34
--- /dev/null
+++ b/src/var_expansion.mli
@@ -0,0 +1,19 @@
+open Stdune
+
+type t =
+  | Paths   of Path.t list
+  | Strings of string list
+
+val to_string : Path.t -> t -> string
+(** [to_string dir v] convert the variable expansion to a string.
+    If it is a path, the corresponding string will be relative to
+    [dir]. *)
+
+val path_of_string : Path.t -> string -> Path.t
+
+val to_strings : Path.t -> t -> string list
+
+val to_path : Path.t -> t -> Path.t
+
+module Expand : String_with_vars.Expand_intf
+  with type expansion = t and type context = Path.t