refactor: move [Merge_files_into] into rules (#10797)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

bin/print_rules.ml

@@ -105,13 +105,6 @@ let rec encode : Action.For_shell.t -> Dune_lang.t =
     List [ atom "diff"; path file1; target file2 ]
   | Diff { optional = true; file1; file2; mode = _ } ->
     List [ atom "diff?"; path file1; target file2 ]
-  | Merge_files_into (srcs, extras, into) ->
-    List
-      [ atom "merge-files-into"
-      ; List (List.map ~f:path srcs)
-      ; List (List.map ~f:string extras)
-      ; target into
-      ]
   | Pipe (outputs, l) ->
     List (atom (sprintf "pipe-%s" (Outputs.to_string outputs)) :: List.map l ~f:encode)
   | Extension ext -> List [ atom "ext"; ext ]

src/dune_engine/action.ml

@@ -198,7 +198,6 @@ let fold_one_step t ~init:acc ~f =
   | Remove_tree _
   | Mkdir _
   | Diff _
-  | Merge_files_into _
   | Extension _ -> acc
 ;;
 
@@ -246,7 +245,6 @@ let rec is_dynamic = function
   | Remove_tree _
   | Diff _
   | Mkdir _
-  | Merge_files_into _
   | Extension _ -> false
 ;;
 
@@ -297,7 +295,6 @@ let is_useful_to memoize =
     | Remove_tree _ -> false
     | Diff _ -> true
     | Mkdir _ -> false
-    | Merge_files_into _ -> true
     | Run _ -> true
     | Dynamic_run _ -> true
     | System _ -> true

src/dune_engine/action_exec.ml

@@ -481,20 +481,6 @@ let rec exec t ~display ~ectx ~eenv : done_or_more_deps Produce.t =
                Fiber.return ())
       in
       Done)
-  | Merge_files_into (sources, extras, target) ->
-    let+ () =
-      maybe_async (fun () ->
-        let lines =
-          List.fold_left
-            sources
-            ~init:(String.Set.of_list extras)
-            ~f:(fun set source_path ->
-              Io.lines_of_file source_path |> String.Set.of_list |> String.Set.union set)
-        in
-        let target = Path.build target in
-        Io.write_lines target (String.Set.to_list lines))
-    in
-    Done
   | Pipe (outputs, l) -> exec_pipe ~display ~ectx ~eenv outputs l
   | Extension (module A) ->
     let+ () =

src/dune_engine/action_intf.ml

@@ -51,7 +51,6 @@ module type Ast = sig
     | Remove_tree of target
     | Mkdir of target
     | Diff of (path, target) Diff.t
-    | Merge_files_into of path list * string list * target
     | Pipe of Outputs.t * t list
     | Extension of ext
 end

src/dune_engine/action_mapper.ml

@@ -42,11 +42,6 @@ module Make (Src : Action_intf.Ast) (Dst : Action_intf.Ast) = struct
     | Mkdir x -> Mkdir (f_target ~dir x)
     | Diff ({ file1; file2; _ } as diff) ->
       Diff { diff with file1 = f_path ~dir file1; file2 = f_target ~dir file2 }
-    | Merge_files_into (sources, extras, target) ->
-      Merge_files_into
-        ( List.map sources ~f:(f_path ~dir)
-        , List.map extras ~f:(f_string ~dir)
-        , f_target ~dir target )
     | Pipe (outputs, l) -> Pipe (outputs, List.map l ~f:(fun t -> f t ~dir))
     | Extension ext -> Extension (f_ext ~dir ext)
   ;;

src/dune_engine/action_to_sh.ml

@@ -81,14 +81,6 @@ let simplify act =
       :: acc
     | Diff { optional = false; file1; file2; mode = _ } ->
       Run ("diff", [ file1; file2 ]) :: acc
-    | Merge_files_into (srcs, extras, target) ->
-      Sh
-        (Printf.sprintf
-           "{ echo -ne %s; cat %s; } | sort -u > %s"
-           (Filename.quote (List.map extras ~f:(sprintf "%s\n") |> String.concat ~sep:""))
-           (String.quote_list_for_shell srcs)
-           (String.quote_for_shell target))
-      :: acc
     | Pipe (outputs, l) -> Pipe (List.map ~f:block l, outputs) :: acc
     | Extension _ -> Sh "# extensions are not supported" :: acc
   and block act =

src/dune_rules/ocamldep.ml

@@ -14,6 +14,62 @@ end
 
 open Modules_data
 
+module Merge_files_into = struct
+  module Spec = struct
+    type ('src, 'dst) t = 'src list * string list * 'dst
+
+    let name = "merge_files_into"
+    let version = 1
+    let is_useful_to ~memoize:_ = true
+
+    let bimap (paths, extras, dst) path target =
+      List.map paths ~f:path, extras, target dst
+    ;;
+
+    let encode
+      (type src dst)
+      ((sources, extras, target) : (src, dst) t)
+      (input : src -> Dune_sexp.t)
+      (output : dst -> Dune_sexp.t)
+      : Dune_sexp.t
+      =
+      let open Dune_sexp in
+      List
+        [ atom_or_quoted_string name
+        ; List (List.map sources ~f:input)
+        ; List (List.map ~f:atom_or_quoted_string extras)
+        ; output target
+        ]
+    ;;
+
+    let action (sources, extras, target) ~ectx:_ ~eenv:_ =
+      Async.async (fun () ->
+        let lines =
+          List.fold_left
+            sources
+            ~init:(String.Set.of_list extras)
+            ~f:(fun set source_path ->
+              Io.lines_of_file source_path |> String.Set.of_list |> String.Set.union set)
+        in
+        let target = Path.build target in
+        Io.write_lines target (String.Set.to_list lines))
+    ;;
+  end
+
+  let action sources extras target =
+    let module M = struct
+      type path = Path.t
+      type target = Path.Build.t
+
+      module Spec = Spec
+
+      let v = sources, extras, target
+    end
+    in
+    Dune_engine.Action.Extension (module M)
+  ;;
+end
+
 let parse_module_names ~dir ~(unit : Module.t) ~modules words =
   List.concat_map words ~f:(fun m ->
     let m = Module_name.of_string m in
@@ -135,7 +191,7 @@ let deps_of
              (let+ sources, extras = paths in
               (sources, extras), sources)
          in
-         Action.Merge_files_into (sources, extras, all_deps_file))
+         Merge_files_into.action sources extras all_deps_file)
     in
     Action_builder.With_targets.map ~f:Action.Full.make produce_all_deps
     |> Super_context.add_rule sctx ~dir