Constant STREAM_COUNT_UNKNOWN 

pub const STREAM_COUNT_UNKNOWN: u64 = u64::MAX; // 18_446_744_073_709_551_615u64

The value super::PublishDone::stream_count carries when the publisher cannot state an exact count.

Draft-20 Section 10.12: “If the publisher is unable to set Stream Count to the exact number of streams opened for the subscription, it MUST set Stream Count to 2^64 - 1.” Draft-19 said 2^62 - 1, which is where the QUIC varint tops out; MoQT’s own varint (Section 1.4.1) is a leading-ones-length prefix reaching a full 64 bits in nine bytes, so this value is encodable at all — as ff followed by eight ff bytes.

The sentinel is now indistinguishable from a well-formed exact count. With 2^62 - 1 there was headroom above the marker; there is none above this one, so a publisher that really opened 2^64 - 1 streams cannot say so and a receiver cannot tell the two apart. The draft does not remark on it. Not a practical problem, and worth knowing before writing a comparison against this constant.