Better error message when the ppx driver is missing

src/gen_rules.ml

@@ -569,7 +569,7 @@ module Gen(P : Params) = struct
 
   let ppx_drivers = Hashtbl.create 32
 
-  let build_ppx_driver ~dir ~dep_kind ~target pp_names =
+  let build_ppx_driver ~dir ~dep_kind ~target ~main pp_names =
     let mode = Mode.best in
     let compiler = Option.value_exn (Mode.compiler mode) in
     let libs =
@@ -581,9 +581,21 @@ module Gen(P : Params) = struct
        Build.dyn_paths (Build.arr (Lib.archive_files ~mode ~ext_lib:ctx.ext_lib))
        >>>
        Build.run (Dep compiler)
-         [ A "-o"; Target target
-         ; Dyn (Lib.link_flags ~mode)
-         ]);
+         (A "-o" :: Target target
+          :: Dyn (Lib.link_flags ~mode)
+          :: match main with
+          | None -> []
+          | Some x -> [Dep x]));
+    Option.iter main ~f:(fun fn ->
+      add_rule
+        (Build.action ~targets:[fn] (Write_file (fn, {|(* File generated by jbuilder *)
+let () =
+  Printf.eprintf "%s: driver missing!
+If you see this message, it means that you are missing a ppx driver at the
+end of your list of preprocessors. Consult the manual for more details."
+    Sys.argv.(0);
+  exit 1
+|}))));
     libs
 
   let ppx_dir = Path.of_string (sprintf "_build/.ppx/%s" ctx.name)
@@ -594,9 +606,11 @@ module Gen(P : Params) = struct
     match Hashtbl.find ppx_drivers key with
     | Some x -> x
     | None ->
-      let exe = Path.relative ppx_dir (sprintf "%s/ppx.exe" key) in
+      let ppx_dir = Path.relative ppx_dir key in
+      let exe = Path.relative ppx_dir "ppx.exe" in
+      let main = Path.relative ppx_dir "jbuilder_ppx_main.ml" in
       let libs =
-        build_ppx_driver names ~dir ~dep_kind ~target:exe
+        build_ppx_driver names ~dir ~dep_kind ~target:exe ~main:(Some main)
       in
       Hashtbl.add ppx_drivers ~key ~data:(exe, libs);
       (exe, libs)
@@ -1192,6 +1206,7 @@ module Gen(P : Params) = struct
            ~dir
            ~dep_kind
            ~target:(Path.relative dir "as-ppx.exe")
+           ~main:None
          : (unit, Lib.t list) Build.t)
     end;