Modeling Rust trait objects subtyping relation via OCaml polymorphic variants

Learning
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2

A simple fix might be to ensure that to_string accept all object with at least the right traits:

module Display = struct
  type cap = [`Std_fmt_display | `Core_marker_send ]
  type 'a t = ([> cap ] as 'a) Ocaml_rs_smartptr.Rusty_obj.t
  external to_string : _ t -> string = "rstdlib_display_to_string"
end
module Error = struct
  include Display
  type cap =
    [ `Ocaml_stdlib_error_wrapper_error_wrapper
    | `Std_error_error
    | `Core_fmt_debug
    | Display.cap
    ]
   type 'a t = ([> cap] as 'a) Ocaml_rs_smartptr.Rusty_obj.t
  external source : 'a t -> 'a t option = "rstdlib_error_source"
end
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