Asn1 is BinStructCustomType and it uses erlang asn1 compiler output as implementation.

Asn1 modules and types can be compiled using this command

erlc -o "$OUTPUT_DIR" -I "$OUTPUT_DIR" +noobj -bper +maps +undec_rest TEST-ASN1.asn1

+maps is optional, change -bper to your target encoding (erlang compiler support BER, PER and so on) +undec_rest is required to current BinStructCustomType implementation, but this may be non-ideal and sometimes shadow errors in data, returning :not_enough_bytes instead

defmodule Struct do
  use BinStruct

  alias BinStruct.BuiltIn.Asn1

  field :asn1, { Asn1, asn1_module: :"TEST-ASN1", asn1_type: :"SimpleType" }
end