feat(formatting): format generated diff corrections

bin/print_rules.ml

@@ -71,6 +71,8 @@ let rec encode_action : Action.For_shell.t -> Dune_lang.t =
       ]
   | Ignore (outputs, r) ->
     List [ atom (sprintf "ignore-%s" (Outputs.to_string outputs)); encode_action r ]
+  | If_file_exists (path, action) ->
+    List [ atom "if-file-exists"; Encoder.string path; encode_action action ]
   | Progn l -> List (atom "progn" :: List.map l ~f:encode_action)
   | Concurrent l -> List (atom "concurrent" :: List.map l ~f:encode_action)
   | Echo xs -> List (atom "echo" :: List.map xs ~f:string)

doc/changes/added/16209.md

@@ -0,0 +1,3 @@
+- Format generated corrections before optional text diffs compare or promote
+  them, according to the source's formatting configuration, starting in Dune
+  3.25. (#16209, @rgrinberg)

src/dune_engine/action.ml

@@ -47,6 +47,7 @@ struct
   let ignore_stdout t = Ignore (Stdout, t)
   let ignore_stderr t = Ignore (Stderr, t)
   let ignore_outputs t = Ignore (Outputs, t)
+  let if_file_exists path t = If_file_exists (path, t)
   let progn ts = Progn ts
   let concurrent ts = Concurrent ts
   let echo s = Echo s
@@ -309,6 +310,10 @@ let digest =
     | System command ->
       int d 22;
       string d command
+    | If_file_exists (path, t) ->
+      int d 23;
+      digest_path d ~dir path;
+      loop d t ~dir
   in
   fun d t -> loop d t ~dir:Path.root
 ;;
@@ -320,6 +325,7 @@ let fold_one_step t ~init:acc ~f =
   | Redirect_out (_, _, _, t)
   | Redirect_in (_, _, t)
   | Ignore (_, t)
+  | If_file_exists (_, t)
   | With_accepted_exit_codes (_, t) -> f acc t
   | Progn l | Pipe (_, l) | Concurrent l -> List.fold_left l ~init:acc ~f
   | Run _
@@ -367,6 +373,7 @@ let exists t ~leaf ~extension =
     | Redirect_out (_, _, _, t)
     | Redirect_in (_, _, t)
     | Ignore (_, t)
+    | If_file_exists (_, t)
     | With_accepted_exit_codes (_, t) -> loop t
     | Progn l | Pipe (_, l) | Concurrent l -> List.exists l ~f:loop
     | Extension extension_ -> extension extension_
@@ -423,7 +430,7 @@ let is_useful_to memoize =
     | Setenv (_, _, t) -> loop t
     | Redirect_out (_, _, _, t) -> memoize || loop t
     | Redirect_in (_, _, t) -> loop t
-    | Ignore (_, t) | With_accepted_exit_codes (_, t) -> loop t
+    | Ignore (_, t) | If_file_exists (_, t) | With_accepted_exit_codes (_, t) -> loop t
     | Progn l | Pipe (_, l) | Concurrent l -> List.exists l ~f:loop
     | Echo _ -> false
     | Cat _ -> memoize

src/dune_engine/action_exec.ml

@@ -128,6 +128,8 @@ let rec exec t ~ectx ~eenv : Done_or_more_deps.t Fiber.t =
     redirect_out t ~ectx ~eenv outputs ~perm fn
   | Redirect_in (inputs, fn, t) -> redirect_in t ~ectx ~eenv inputs fn
   | Ignore (outputs, t) -> redirect_out t ~ectx ~eenv ~perm:Normal outputs Dev_null.path
+  | If_file_exists (path, t) ->
+    if Fpath.exists (Path.to_string path) then exec t ~ectx ~eenv else Fiber.return Done
   | Progn ts -> exec_list ts ~ectx ~eenv
   | Concurrent ts ->
     Fiber.parallel_map ts ~f:(fun t ->

src/dune_engine/action_intf.ml

@@ -47,6 +47,7 @@ module type Ast = sig
     | Redirect_out of Outputs.t * target * File_perm.t * t
     | Redirect_in of Inputs.t * path * t
     | Ignore of Outputs.t * t
+    | If_file_exists of path * t
     | Progn of t list
     | Concurrent of t list
     | Echo of string list
@@ -82,6 +83,7 @@ module type Helpers = sig
   val ignore_stdout : t -> t
   val ignore_stderr : t -> t
   val ignore_outputs : t -> t
+  val if_file_exists : path -> t -> t
   val progn : t list -> t
   val concurrent : t list -> t
   val echo : string list -> t

src/dune_engine/action_mapper.ml

@@ -29,6 +29,7 @@ module Make (Src : Action_intf.Ast) (Dst : Action_intf.Ast) = struct
       Redirect_out (outputs, f_target ~dir fn, perm, f t ~dir)
     | Redirect_in (inputs, fn, t) -> Redirect_in (inputs, f_path ~dir fn, f t ~dir)
     | Ignore (outputs, t) -> Ignore (outputs, f t ~dir)
+    | If_file_exists (path, t) -> If_file_exists (f_path ~dir path, f t ~dir)
     | Progn l -> Progn (List.map l ~f:(fun t -> f t ~dir))
     | Concurrent l -> Concurrent (List.map l ~f:(fun t -> f t ~dir))
     | Echo xs -> Echo (List.map xs ~f:(f_string ~dir))

src/dune_lang/format.ml

@@ -78,5 +78,17 @@ let format_to_channel ~version ~src oc =
 ;;
 
 let format_action ~version ~src ~dst =
-  Path.build dst |> Io.with_file_out ~f:(format_to_channel ~version ~src)
+  let dst = Path.build dst in
+  if Path.equal src dst
+  then
+    Temp.with_temp_file
+      ~dir:(Path.parent_exn dst)
+      ~prefix:"dune-format"
+      ~suffix:"output"
+      ~f:(function
+      | Error exn -> raise exn
+      | Ok temporary ->
+        Io.with_file_out temporary ~f:(format_to_channel ~version ~src);
+        Fpath.rename_exn (Path.to_string temporary) (Path.to_string dst))
+  else Io.with_file_out dst ~f:(format_to_channel ~version ~src)
 ;;