fn admit(support: Support) -> Result<(), Refusal>Expand description
The five verdicts, as an admission decision.
NotAttemptable and Unreachable are structurally unreachable from both
entry points — the first because the two StreamAction sites take
execute_stream and the four constructor-less kinds have no Action
value, the second because the hook is never invoked on a bypassed stream.
They are handled rather than unreachable!()d for the same reason
capability.rs has filtered_earlier: a total function beats a
panicking one on a forwarding task.