fix: keep menhir .conflicts files (ocaml#6865)

Add the .conflicts file to targets of menhir stanzas when the
--explain flag is present and menhir is not called with --only-tokens.

src/dune_rules/menhir/menhir_rules.ml

@@ -75,8 +75,9 @@ module Run (P : PARAMS) = struct
 
   let source m = Path.relative (Path.build dir) (m ^ ".mly")
 
-  let targets m ~cmly =
+  let targets m ~cmly ~conflicts =
     let base = [ m ^ ".ml"; m ^ ".mli" ] in
+    let base = if conflicts then (m ^ ".conflicts") :: base else base in
     List.map ~f:(Path.Build.relative dir) (if cmly then (m ^ ".cmly") :: base else base)
   ;;
 
@@ -185,7 +186,7 @@ module Run (P : PARAMS) = struct
      is the three-step process where Menhir is invoked twice and OCaml type
      inference is performed in between. *)
 
-  let process3 base ~cmly (stanza : stanza) : unit Memo.t =
+  let process3 base ~cmly ~conflicts (stanza : stanza) : unit Memo.t =
     let open Memo.O in
     let expanded_flags = expand_flags stanza.flags in
     (* 1. A first invocation of Menhir creates a mock [.ml] file. *)
@@ -234,7 +235,7 @@ module Run (P : PARAMS) = struct
       ; Path (Path.relative (Path.build dir) base)
       ; A "--infer-read-reply"
       ; Dep (Path.build (inferred_mli base))
-      ; Hidden_targets (targets base ~cmly)
+      ; Hidden_targets (targets base ~cmly ~conflicts)
       ]
     >>= rule
   ;;
@@ -244,15 +245,15 @@ module Run (P : PARAMS) = struct
   (* [process3 stanza] converts a Menhir stanza into a set of build rules. This
      is a simpler one-step process where Menhir is invoked directly. *)
 
-  let process1 base ~cmly (stanza : stanza) : unit Memo.t =
+  let process1 base ~cmly ~conflicts (stanza : stanza) : unit Memo.t =
     let open Memo.O in
     let expanded_flags = expand_flags stanza.flags in
     menhir
       [ Command.Args.dyn expanded_flags
       ; Deps (sources stanza.modules)
       ; A "--base"
       ; Path (Path.relative (Path.build dir) base)
-      ; Hidden_targets (targets base ~cmly)
+      ; Hidden_targets (targets base ~cmly ~conflicts)
       ]
     >>= rule
   ;;
@@ -263,26 +264,32 @@ module Run (P : PARAMS) = struct
      either [process3] or [process1], as appropriate. *)
 
   (* Because Menhir processes [--only-tokens] before the [--infer-*] commands,
-     when [--only-tokens] is present, no [--infer-*] command should be used. *)
+     when [--only-tokens] is present, no [--infer-*] command should be used.
+     When Menhir is called with [--only-tokens], it does not generate a [.conflicts]
+     file even the [--explain] flag is passed. Otherwise, a (possibly empty)
+     [.conflicts] file is always generated (if Menhir succeeds in creating a
+     [.ml/.mli] file).
+  *)
 
   let process (stanza : stanza) : unit Memo.t =
     let base = Option.value_exn stanza.merge_into in
-    let ocaml_type_inference_disabled, cmly =
+    let ocaml_type_inference_disabled, cmly, conflicts =
       Ordered_set_lang.Unexpanded.fold_strings
         stanza.flags
-        ~init:(false, false)
-        ~f:(fun pos sw ((only_tokens, cmly) as acc) ->
+        ~init:(false, false, false)
+        ~f:(fun pos sw ((only_tokens, cmly, conflicts) as acc) ->
           match pos with
           | Neg -> acc
           | Pos ->
             (match String_with_vars.text_only sw with
-             | Some "--only-tokens" -> true, cmly
-             | Some "--cmly" -> only_tokens, true
+             | Some "--only-tokens" -> true, cmly, false
+             | Some "--cmly" -> only_tokens, true, conflicts
+             | Some "--explain" -> only_tokens, cmly, not only_tokens
              | Some _ | None -> acc))
     in
     if ocaml_type_inference_disabled || not stanza.infer
-    then process1 base stanza ~cmly
-    else process3 base stanza ~cmly
+    then process1 base stanza ~cmly ~conflicts
+    else process3 base stanza ~cmly ~conflicts
   ;;
 
   (* ------------------------------------------------------------------------ *)