Function encoded_len 

pub(super) const fn encoded_len<P: PropertyConstraint>(
    properties: &Properties<P>,
    payload: &[u8],
) -> usize

Encoded size of a “packet id + properties + payload” body, in bytes.

This is the shape shared by SUBSCRIBE, UNSUBSCRIBE, SUBACK, and UNSUBACK: a two-byte packet identifier, a properties section, and an opaque payload (subscription entries or reason codes) that the caller interprets.