Declarative macro_rules! that fuses the hand-written statem surface into
one invocation: emits the unified event enum, the machine struct + state
cell, start, the Machine impl (dispatch + stay/transition apply-tail), and
the enter dispatch. User keeps the meaningful types, the per-state
successor enums, and the free handler fns.
Pure sugar: every safety property is a property of the emitted code,
checked by rustc, so a declarative macro carries (almost) the proc-macro
guarantee set:
1. forgotten (state,event) pair -> E0004 (total match, no injected _)
2. conflicting row -> unreachable_patterns (macro self-denies; HARD only
in-crate, suppressed cross-crate by in_external_macro -- documented)
3. orphan handler -> dead_code (handlers are user free fns)
4. out-of-set target -> E0599 (per-state successor enums)
Hygiene: bodies can't see the macro's self/cx, so the caller names them
via `context(data, prev, cx)` (shared call-site hygiene).
No separate transitions{} block: the match IS the table, successor enums
ARE the per-state target sets (fused variant, diverges from RFC
edge-lint).
- examples/statem_macro.rs: Door machine via the macro (parallel to the
hand-written examples/statem_fused.rs; diff the two to see the delta).
- in-crate test exercises a machine + anchors the in-crate #2 guarantee.
