Implement alias field via temp file

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/dune/simple_rules.ml

@@ -72,12 +72,24 @@ let user_rule sctx ?extra_bindings ~dir ~expander (rule : Rule.t) =
       |> SC.Action.run sctx (snd rule.action) ~loc:(fst rule.action) ~expander
            ~dep_kind:Required ~targets ~targets_dir:dir
     in
-    Option.iter rule.alias ~f:(fun name ->
+    let action =
+      match rule.alias with
+      | None -> action
+      | Some alias ->
         let stamp =
           let action = Some (snd rule.action) in
           stamp ~deps:rule.deps ~extra_bindings ~action
+          |> Digest.generic
+          |> Digest.to_string
+        in
+        let stamp_target = Path.Build.relative dir stamp in
+        let () =
+          let alias = Alias.make alias ~dir in
+          let deps = Path.Set.singleton (Path.build stamp_target) in
+          Rules.Produce.Alias.add_deps alias deps
         in
-        add_alias sctx ~dir ~name ~stamp ~loc:(Some (fst rule.action)) action);
+        Build.progn [action; Build.create_file stamp_target]
+    in
     SC.add_rule_get_targets
       sctx
       (* user rules may have extra requirements, in which case they will be
@@ -86,9 +98,7 @@ let user_rule sctx ?extra_bindings ~dir ~expander (rule : Rule.t) =
       ~sandbox:Sandbox_config.no_special_requirements ~dir ~mode:rule.mode
       ~loc:rule.loc
       ~locks:(interpret_locks ~expander rule.locks)
-      ( SC.Deps.interpret_named sctx ~expander rule.deps
-      |> SC.Action.run sctx (snd rule.action) ~loc:(fst rule.action) ~expander
-           ~dep_kind:Required ~targets ~targets_dir:dir )
+      action
 
 let copy_files sctx ~dir ~expander ~src_dir (def : Copy_files.t) =
   let loc = String_with_vars.loc def.glob in

test/blackbox-tests/test-cases/github2681/run.t

@@ -10,14 +10,8 @@ dependency of the alias.
   >  (alias bar))
   > EOF
   $ dune build @bar --display short
-  File "dune", line 2, characters 9-50:
-  2 |  (action (with-stdout-to foo (echo "hello world")))
-               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  Error: Rule has targets in different directories.
-  Targets:
-  - _build/.aliases/default/bar-fc3cef760337ee5dd7a56722c8e58053
-  - _build/default/foo
-  [1]
+  $ cat _build/default/foo
+  hello world
 
 The field order should not affect the alias field:
   $ cd .. && mkdir field-order && cd field-order