Constant MAX_APPLICATION_ERROR_CODE 

pub(crate) const MAX_APPLICATION_ERROR_CODE: u64 = _; // 4_611_686_018_427_387_903u64

The QUIC varint ceiling, 2^62 - 1.

A stream application error code above it cannot be encoded, and quinn would panic or truncate rather than refuse. Checked here, before anything is sent, so the stream stays usable (Refusal::ErrorCodeOutOfRange).