diff --git a/src/alias.ml b/src/alias.ml
index f6b6e1d6..6149f21a 100644
--- a/src/alias.ml
+++ b/src/alias.ml
@@ -31,21 +31,22 @@ let add_deps store t deps =
 
 type tree = Node of Path.t * tree list
 
-let rec setup_rec_aliases store (Node (dir, children)) =
-  List.map recursive_aliases ~f:(fun make_alias ->
-    let alias = make_alias ~dir in
-    List.iter children ~f:(fun child ->
-      let sub_aliases = setup_rec_aliases store child in
-      add_deps store alias sub_aliases);
-    alias)
+let rec setup_rec_alias store ~make_alias ~prefix ~tree:(Node (dir, children)) =
+  let alias = make_alias ~dir:(Path.append prefix dir) in
+  add_deps store alias (List.map children ~f:(fun child ->
+    setup_rec_alias store ~make_alias ~prefix ~tree:child));
+  alias
 
-let rules store tree =
-  ignore (setup_rec_aliases store tree : t list);
+let setup_rec_aliases store ~prefix ~tree =
+  List.iter recursive_aliases ~f:(fun make_alias ->
+    ignore (setup_rec_alias store ~make_alias ~prefix ~tree : t))
+
+let rules store ~prefix ~tree =
+  setup_rec_aliases store ~prefix ~tree;
   Hashtbl.fold store ~init:[] ~f:(fun ~key:alias ~data:deps acc ->
     let open Build.O in
     let rule =
       Build.path_set !deps >>>
-      Build.create_file ~target:alias (fun _ ->
-        close_out (open_out_bin (Path.to_string alias)))
+      Build.touch alias
     in
     rule :: acc)
diff --git a/src/alias.mli b/src/alias.mli
index f96e0a5d..7742ee0b 100644
--- a/src/alias.mli
+++ b/src/alias.mli
@@ -17,4 +17,8 @@ val add_deps : Store.t -> t -> Path.t list -> unit
 
 type tree = Node of Path.t * tree list
 
-val rules : Store.t -> tree -> (unit, unit) Build.t list
+val rules
+  :  Store.t
+  -> prefix:Path.t
+  -> tree:tree
+  -> (unit, unit) Build.t list
diff --git a/src/gen_rules.ml b/src/gen_rules.ml
index 56c00c60..000fe0a4 100644
--- a/src/gen_rules.ml
+++ b/src/gen_rules.ml
@@ -240,8 +240,8 @@ module Gen(P : Params) = struct
     include Alias
 
     let store = Store.create ()
-    let _add_deps t deps = add_deps store t deps
-    let rules () = rules store P.tree
+    let add_deps t deps = add_deps store t deps
+    let rules () = rules store ~prefix:ctx.build_dir ~tree:P.tree
   end
 
   let all_rules = ref []
@@ -1241,7 +1241,7 @@ module Gen(P : Params) = struct
     let digest_path =
       Path.relative dir (Path.basename (Alias.file alias) ^ "-" ^ digest) in
     let dummy = Build.touch digest_path in
-    Alias._add_deps alias [digest_path];
+    Alias.add_deps alias [digest_path];
     let deps =
       let deps = Dep_conf_interpret.dep_of_list ~dir alias_conf.deps in
       match alias_conf.action with