set the filename in the lexbuf

src/lib/uTop.ml

@@ -194,6 +194,7 @@ let input_name = "//toplevel//"
 
 let lexbuf_of_string eof str =
   let pos = ref 0 in
+  let lexbuf =
     Lexing.from_function
       (fun buf len ->
         if !pos = String.length str then begin
@@ -205,6 +206,9 @@ let lexbuf_of_string eof str =
           pos := !pos + len;
           len
         end)
+  in
+  Location.init lexbuf input_name;
+  lexbuf
 
 let mkloc loc =
   (loc.Location.loc_start.Lexing.pos_cnum,