Improve the error message for the last commit

src/main.ml

@@ -111,7 +111,7 @@ let report_error ?(map_fname=fun x->x) ppf exn ~backtrace =
        - external library %S is required in %s\n\
        This cannot work.\n"
       package
-      (Path.to_string defined_locally_in)
+      (Utils.jbuild_name_in ~dir:(Path.drop_build_context defined_locally_in))
       required_by
       required_by
       (Utils.jbuild_name_in ~dir:required_locally_in)

src/path.ml

@@ -346,6 +346,11 @@ let extract_build_context_dir t =
   else
     None
 
+let drop_build_context t =
+  match extract_build_context t with
+  | None -> t
+  | Some (_, t) -> t
+
 let exists t = Sys.file_exists (to_string t)
 let readdir t = Sys.readdir (to_string t) |> Array.to_list
 let is_directory t = Sys.is_directory (to_string t)

src/path.mli

@@ -83,6 +83,9 @@ val extract_build_context : t -> (string * t) option
 *)
 val extract_build_context_dir : t -> (t * t) option
 
+(** Drop the "_build/blah" prefix *)
+val drop_build_context : t -> t
+
 val is_in_build_dir : t -> bool
 
 val insert_after_build_dir_exn : t -> t -> t