Better error handling when toolchain isn't defined

src/context.ml

@@ -192,7 +192,7 @@ let create ?host ~implicit ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~me
   let which_cache = Hashtbl.create 128 in
   let which x = which ~cache:which_cache ~path x in
   (match findlib_toolchain with
-   | None -> Future.return Findlib.Config.empty
+   | None -> Future.return (None, Findlib.Config.empty)
    | Some _ ->
      match which "ocamlfind" with
      | None -> assert false
@@ -201,8 +201,8 @@ let create ?host ~implicit ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~me
          (Path.to_string fn) ["printconf"; "conf"]
        >>| fun s ->
        let path = Path.absolute s in
-       Findlib.Config.load path)
-  >>= fun findlib_cfg ->
+       (Some path, Findlib.Config.load path))
+  >>= fun (findlib_conf, findlib_cfg) ->
   let get_findlib_var =
     let predicates =
       match findlib_toolchain with
@@ -211,6 +211,15 @@ let create ?host ~implicit ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~me
     in
     fun var -> Findlib.Config.get findlib_cfg ~predicates ~var
   in
+  let () =
+    Option.iter findlib_toolchain ~f:(fun toolchain ->
+      match get_findlib_var "ocamlc", findlib_conf with
+      | "", _ -> ()
+      | _, None -> ()
+      | _, Some fc ->
+        die "@{<error>Error@}: Toolchain %s isn't defined found in %a \
+             (context: %s)" toolchain Path.pp fc name)
+  in
   let which prog =
     let s = get_findlib_var prog in
     if s = "" then

src/findlib.ml

@@ -122,7 +122,7 @@ module Config = struct
   type t = Vars.t
 
   let empty = String_map.empty
-  
+
   let load path =
     let files =
       let path_d = Path.extend_basename path ~suffix:".d" in