Change an error into a warning

Otherwise this breaks the build of atd.

src/jbuild_types.ml

@@ -649,15 +649,12 @@ module Executables = struct
     in
     match to_install with
     | [] ->
-      map_validate (field_o "package" string) ~f:(function
-        | None -> Ok ()
-        | Some _ ->
-          Error
-            (sprintf
-               "this field is useless without a (public_name%s ...) field"
-               (if multi then "s" else "")))
-      >>= fun () ->
-      return (t, None)
+      (field_o "package" Sexp.Ast.loc >>= function
+       | None -> return (t, None)
+       | Some loc ->
+         Loc.warn loc "This field is useless without a (public_name%s ...) field."
+           (if multi then "s" else "");
+         return (t, None))
     | files ->
       Pkgs.package_field pkgs >>= fun package ->
       return (t, Some { Install_conf. section = Bin; files; package })

src/loc.ml

@@ -31,3 +31,13 @@ let in_file fn =
   }
 
 let none = in_file "<none>"
+
+let print ppf { start; stop } =
+  let start_c = start.pos_cnum - start.pos_bol in
+  let stop_c  = stop.pos_cnum  - start.pos_bol in
+  Format.fprintf ppf
+    "@{<loc>File \"%s\", line %d, characters %d-%d:@}@\n"
+    start.pos_fname start.pos_lnum start_c stop_c
+
+let warn t fmt =
+  Format.eprintf ("%a@{<warning>Warning@}: " ^^ fmt ^^ "@.") print t

src/loc.mli

@@ -13,3 +13,9 @@ val fail_lex : Lexing.lexbuf -> ('a, unit, string, _) format4 -> 'a
 val in_file : string -> t
 
 val none : t
+
+(** Prints "File ..., line ..., characters ...:\n" *)
+val print : Format.formatter -> t -> unit
+
+(** Prints a warning *)
+val warn : t -> ('a, Format.formatter, unit) format -> 'a

src/main.ml

@@ -77,13 +77,13 @@ let external_lib_deps ?log ~packages () =
 
 let report_error ?(map_fname=fun x->x) ppf exn ~backtrace =
   match exn with
-  | Loc.Error ({ start; stop }, msg) ->
-    let start_c = start.pos_cnum - start.pos_bol in
-    let stop_c  = stop.pos_cnum  - start.pos_bol in
-    Format.fprintf ppf
-      "@{<loc>File \"%s\", line %d, characters %d-%d:@}\n\
-       @{<error>Error@}: %s\n"
-      (map_fname start.pos_fname) start.pos_lnum start_c stop_c msg
+  | Loc.Error (loc, msg) ->
+    let loc =
+      { loc with
+        start = { loc.start with pos_fname = map_fname loc.start.pos_fname }
+      }
+    in
+    Format.fprintf ppf "%a@{<error>Error@}: %s\n" Loc.print loc msg
   | Fatal_error "" -> ()
   | Fatal_error msg ->
     Format.fprintf ppf "%s\n" (String.capitalize_ascii msg)