EventName::mask() turns a leaf variant's discriminant `v` into
`1 << (v - 1)`, so the enum can hold at most 64 variants in total —
compound "All" variants included, since they consume discriminants
even though they own no bit and push every later leaf further up the
range. The last variant is KmsServiceStopped at 61, leaving 3 slots.
Past the budget, `1u64 << (v - 1)` shifts by 64 or more: debug builds
panic with a shift overflow, release builds silently mask the shift
amount down and hand back a bit that already belongs to another event.
Either failure surfaces far from the line that added the variant.
The existing test_mask_bit_budget_is_not_exhausted only bounds the
events listed in the test-local ALL_EVENT_NAMES, so a variant nobody
remembered to list went unchecked. Add three layers instead:
- A const assertion on LAST_EVENT_NAME_VALUE, anchored on the last
variant, that fails `cargo build` once the discriminant passes 64.
- An exhaustive-match tripwire next to ALL_EVENT_NAMES, so adding a
variant breaks compilation with a non-exhaustive-patterns error that
points at the budget notes.
- Tests pinning that discriminants stay dense and fully listed, that
every leaf mask is non-zero and owns a unique bit (catching the
release-mode wrapped-shift collision), and that the last variant
still lands inside the u64.
Document the budget and the three guards on mask() so the next person
adding an event sees the remaining headroom.
Refs: rustfs/backlog#1572
