Make target or targets optional fields of rule stanzas

Fixes ocaml#2469

Signed-off-by: Nathan Rebours <nathan.p.rebours@gmail.com>

src/dune_file.ml

@@ -1703,6 +1703,7 @@ module Rule = struct
 
     let fields_parser =
       fields_mutually_exclusive
+        ~default:Infer
         [ "targets", decode_static
         ; "target", decode_one_static
         ]

src/dune_lang/dune_lang.ml

@@ -1048,7 +1048,7 @@ module Decoder = struct
   [@@inline never]
 
   let fields_mutually_exclusive
-        ?on_dup fields ((Fields (loc, _, _) : _ context) as ctx) state =
+        ?on_dup ?default fields ((Fields (loc, _, _) : _ context) as ctx) state =
     let res, state =
       traverse ~f:(fun (name, parser) ->
         field_o name ?on_dup parser
@@ -1060,7 +1060,10 @@ module Decoder = struct
         ~f:(function (name, Some x) -> Some(name, x) | (_, None) -> None) with
     | [] ->
       let names = List.map fields ~f:fst in
-      fields_missing_need_exactly_one loc names
+      ( match default with
+        | None -> fields_missing_need_exactly_one loc names
+        | Some default -> default, state
+      )
     | [ (_name, res) ] ->
       res, state
     | (_ :: _ :: _ as results) ->

src/dune_lang/dune_lang.mli

@@ -433,8 +433,11 @@ module Decoder : sig
     -> 'a t
     -> 'a option fields_parser
 
+  (** Parser for mutually exclusive fields. If [default] is provided, allow
+      fields absence. *)
   val fields_mutually_exclusive
     : ?on_dup:(Univ_map.t -> string -> Ast.t list -> unit)
+    -> ?default:'a
     -> (string * 'a t) list
     -> 'a fields_parser
 

test/blackbox-tests/test-cases/rule-target-inferrence/run.t

@@ -9,8 +9,4 @@ But should work with the long form as well:
 
   $ dune build @infer --root long-form
   Entering directory 'long-form'
-  File "dune", line 1, characters 0-57:
-  1 | (rule
-  2 |  (action (write-file b "It should work as well!")))
-  Error: fields targets, target are all missing (exactly one is needed)
-  [1]
+  It should work as well!