diff --git a/src/stdune/hashtbl.ml b/src/stdune/hashtbl.ml
index abadd237..3e8c41e5 100644
--- a/src/stdune/hashtbl.ml
+++ b/src/stdune/hashtbl.ml
@@ -37,6 +37,24 @@ module Make(H : Hashable.S) = struct
       fold t ~init ~f:(fun ~key ~data acc -> f key data acc)
     let fold  t ~init ~f = foldi t ~init ~f:(fun _ x -> f x)
   end
+
+  let of_list l =
+    let h = create (List.length l) in
+    let rec loop = function
+      | [] -> Result.Ok h
+      | (k, v) :: xs ->
+        begin match find h k with
+        | None -> add h k v; loop xs
+        | Some v' -> Error (k, v', v)
+        end
+    in
+    loop l
+
+  let of_list_exn l =
+    match of_list l with
+    | Result.Ok h -> h
+    | Error (_, _, _) ->
+      Exn.code_error "Hashtbl.of_list_exn duplicate keys" []
 end
 
 open MoreLabels.Hashtbl
diff --git a/src/stdune/hashtbl_intf.ml b/src/stdune/hashtbl_intf.ml
index 3d30d0e1..c09d8d27 100644
--- a/src/stdune/hashtbl_intf.ml
+++ b/src/stdune/hashtbl_intf.ml
@@ -8,4 +8,6 @@ module type S = sig
 
   val fold : 'a t -> init:'b -> f:('a -> 'b -> 'b) -> 'b
   val foldi : 'a t -> init:'b -> f:(key -> 'a -> 'b -> 'b) -> 'b
+
+  val of_list_exn : (key * 'a) list -> 'a t
 end
diff --git a/src/stdune/string.ml b/src/stdune/string.ml
index bc7e87f9..3bb7f50e 100644
--- a/src/stdune/string.ml
+++ b/src/stdune/string.ml
@@ -16,6 +16,8 @@ let compare a b = Ordering.of_int (String.compare a b)
 module T = struct
   type t = StringLabels.t
   let compare = compare
+  let equal (x : t) (y : t) = x = y
+  let hash (s : t) = Hashtbl.hash s
 end
 
 let capitalize   = capitalize_ascii
@@ -201,6 +203,7 @@ let maybe_quoted s =
 
 module Set = Set.Make(T)
 module Map = Map.Make(T)
+module Table = Hashtbl.Make(T)
 
 let enumerate_gen s =
   let s = " " ^ s ^ " " in
diff --git a/src/stdune/string.mli b/src/stdune/string.mli
index a2c9d982..54818776 100644
--- a/src/stdune/string.mli
+++ b/src/stdune/string.mli
@@ -53,3 +53,4 @@ val enumerate_or  : string list -> string
 
 module Set : Set.S with type elt = t
 module Map : Map.S with type key = t
+module Table : Hashtbl.S with type key = t
diff --git a/src/super_context.ml b/src/super_context.ml
index 1cbf8206..e32cfa07 100644
--- a/src/super_context.ml
+++ b/src/super_context.ml
@@ -122,7 +122,7 @@ end = struct
     | Renamed_in of Syntax.Version.t * string
 
   module Map = struct
-    type nonrec 'a t = 'a t String.Map.t
+    type nonrec 'a t = 'a t String.Table.t
 
     let values v                      = Nothing (Kind.Values v)
     let renamed_in ~new_name ~version = Renamed_in (version, new_name)
@@ -161,7 +161,7 @@ end = struct
 
       ; "path-no-dep", deleted_in ~version:(1, 0) Path_no_dep
       ]
-      |> String.Map.of_list_exn
+      |> String.Table.of_list_exn
 
     let create_vars ~(context : Context.t) ~cxx_flags =
       let ocamlopt =
@@ -226,9 +226,9 @@ end = struct
       ; vars
       ]
       |> List.concat
-      |> String.Map.of_list_exn
+      |> String.Table.of_list_exn
 
-    let static_vars = String.Map.of_list_exn static_vars
+    let static_vars = String.Table.of_list_exn static_vars
 
     let rec expand t ~syntax_version ~var =
       let name =
@@ -236,7 +236,7 @@ end = struct
         | Single v -> v
         | Pair (v, _) -> v
       in
-      Option.bind (String.Map.find t name) ~f:(function
+      Option.bind (String.Table.find t name) ~f:(function
         | Nothing v -> Some v
         | Since (v, min_version) ->
           if syntax_version >= min_version then