Function decode_default_with_codec 

pub fn decode_default_with_codec(
    cx: &mut Cx,
    symbol: &Symbol,
    input: Input,
    read_policy: ReadPolicy,
    position: DecodePosition,
) -> Result<DecodedForm>

Decode input and resolve it to data or a term per the codec’s policy and the requested position, using default DecodeLimits.

This is the position-aware entry point: the codec’s CodecDefaultDecode and position jointly pick the DecodedForm variant.