fix: expand %{deps} in (cat) properly

Signed-off-by: Ali Caglayan <alizter@gmail.com>

src/dune_rules/action_unexpanded.ml

@@ -68,6 +68,9 @@ module Action_expander : sig
     (* Evaluate a path in a position of dependency, such as in [(cat <dep>)] *)
     val dep : String_with_vars.t -> Path.t t
 
+    (* Evaluate paths in the position of dependencies, such as in [(cat <deps>)] *)
+    val deps : String_with_vars.t -> Path.t list t
+
     (* Evaluate a path in a position of optional dependency, such as in [(diff
        <dep_if_exists> ...)] *)
     val dep_if_exists : String_with_vars.t -> Path.t t
@@ -241,6 +244,14 @@ end = struct
         Value.to_path_in_build_or_external v
           ~error_loc:(String_with_vars.loc sw) ~dir:t.dir
 
+      let expand_paths t sw =
+        let+ v, vs = expand t ~mode:At_least_one sw in
+        List.map
+          ~f:
+            (Value.to_path_in_build_or_external
+               ~error_loc:(String_with_vars.loc sw) ~dir:t.dir)
+          (v :: vs)
+
       let expand_string env sw =
         let+ v = expand env ~mode:Single sw in
         Value.to_string v ~dir:(Path.build env.dir)
@@ -304,6 +315,10 @@ end = struct
       let fn = Expander.expand_path env sw in
       register_dep fn ~f:Option.some env acc
 
+    let deps sw env acc =
+      let fn = Expander.expand_paths env sw in
+      register_dep fn ~f:(fun _ -> None) env acc
+
     let dep_if_exists sw env acc =
       Memo.return
         (let fn = Expander.expand_path env sw in
@@ -427,8 +442,8 @@ let rec expand (t : Dune_lang.Action.t) ~context : Action.t Action_expander.t =
     let l = List.concat l in
     O.Echo l
   | Cat xs ->
-    let+ xs = A.all (List.map xs ~f:E.dep) in
-    O.Cat xs
+    let+ xs = A.all (List.map xs ~f:E.deps) in
+    O.Cat (List.concat xs)
   | Copy (x, y) ->
     let+ x = E.dep x
     and+ y = E.target y in

test/blackbox-tests/test-cases/quoting/cat.t/run.t

@@ -13,9 +13,3 @@ arguments.
   > EOF
 
   $ dune build @foo
-  File "dune", line 5, characters 7-14:
-  5 |   (cat %{deps})))
-             ^^^^^^^
-  Error: Variable %{deps} expands to 2 values, however a single value is
-  expected here. Please quote this atom.
-  [1]