fn subgroup_context(header: &AnySubgroupHeader) -> SubgroupContextExpand description
The identity fields a subgroup header contributes to its objects.
Every field comes from an AnySubgroupHeader accessor, so the
per-draft knowledge behind them — which stream types leave the subgroup
ID to the first object, which drafts fold a mode field into the
header-type octet, which drafts may omit the publisher priority — stays
in the codec beside the decoders that define it. This crate keeps no
copy of it, which is the point: a local copy (a mask constant and a
per-draft match) drifts from the decoders it copies.
Where a draft encodes the subgroup ID implicitly as the first object’s
ID, the codec stores zero and AnySubgroupHeader::subgroup_id
reports None rather than that zero.