feat(formatting): format generated diff corrections

Starting with Dune 3.25, format generated corrections consumed by optional
text diffs before comparing or promoting them. Select Dune's built-in
formatter or OCamlFormat according to the source's formatting configuration,
and use the locked OCamlFormat dev tool when available.

Construct formatted diffs directly during action expansion so formatter
dependencies, environments, and sandbox requirements remain attached to the
action. Apply the behavior to user diff? actions and Cinaps without exposing
generated corrections as separate build targets. Projects using older language
versions retain the previous behavior.

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

Author: rgrinberg

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
@@ -308,6 +309,10 @@ 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
 ;;
@@ -319,6 +324,7 @@ 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 _
@@ -366,6 +372,7 @@ 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_
@@ -422,7 +429,7 @@ 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:(exec ~ectx ~eenv)

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)
 ;;