Add Arg_spec.Hidden_targets

Same as Hidden_deps: allows to declare targets than don't appear on
the command line.

src/arg_spec.ml

@@ -12,7 +12,8 @@ type 'a t =
   | Target   of Path.t
   | Path     of Path.t
   | Paths    of Path.t list
-  | Hidden_deps of Path.t list
+  | Hidden_deps    of Path.t list
+  | Hidden_targets of Path.t list
   | Dyn      of ('a -> nothing t)
 
 let rec add_deps ts set =
@@ -29,6 +30,7 @@ let rec add_targets ts acc =
   List.fold_left ts ~init:acc ~f:(fun acc t ->
     match t with
     | Target fn  -> fn :: acc
+    | Hidden_targets fns -> List.rev_append fns acc
     | S ts
     | Concat (_, ts) -> add_targets ts acc
     | _ -> acc)
@@ -51,7 +53,7 @@ let expand ~dir ts x =
       List.map fns ~f:(Path.reach ~from:dir)
     | S ts -> List.concat_map ts ~f:loop_dyn
     | Concat (sep, ts) -> [String.concat ~sep (loop_dyn (S ts))]
-    | Target _ -> die "Target not allowed under Dyn"
+    | Target _ | Hidden_targets _ -> die "Target not allowed under Dyn"
     | Dyn _ -> assert false
     | Hidden_deps l ->
       dyn_deps := Pset.union !dyn_deps (Pset.of_list l);
@@ -66,7 +68,7 @@ let expand ~dir ts x =
     | Concat (sep, ts) -> [String.concat ~sep (loop (S ts))]
     | Target fn -> [Path.reach fn ~from:dir]
     | Dyn f -> loop_dyn (f x)
-    | Hidden_deps _ -> []
+    | Hidden_deps _ | Hidden_targets _ -> []
   in
   let l = List.concat_map ts ~f:loop in
   (l, !dyn_deps)

src/arg_spec.mli

@@ -38,7 +38,8 @@ type 'a t =
   | Target   of Path.t
   | Path     of Path.t
   | Paths    of Path.t list
-  | Hidden_deps of Path.t list
+  | Hidden_deps    of Path.t list
+  | Hidden_targets of Path.t list
   (** Register dependencies but produce no argument *)
   | Dyn      of ('a -> nothing t)