hsmc/docs/004. unsafe-safety-contract.md

# Unsafe Safety Contract

This document inventories every `unsafe` block emitted by the
`statechart!` macro, states the preconditions, names what
establishes them, and lists the layered checks that prove they hold.

It is the answer to "you said no `unsafe` — now there's `unsafe`. How
do I know it's correct?".

## Inventory

All `unsafe` lives in code emitted by `hsmc-macros`. The runtime
`hsmc` crate, the user's chart definition, and the user's action /
during bodies contain none.

The blocks are six index-elision sites in five generated functions,
each pairing a `slice::get_unchecked` (or `Option::unwrap_unchecked`,
in earlier passes — currently none) with a documented bound.

| Site                                            | Index expression                         | Bound                                  |
| ----------------------------------------------- | ---------------------------------------- | -------------------------------------- |
| `step()` — sync + async — `__DISPATCH` lookup   | `cur * N_EVENTS + ev_id`                 | `< __DISPATCH.len() = N * N_EVENTS`    |
| `apply_path()` — sync + async — `__PATH_RANGE`  | `from * N_STATES + target`               | `< __PATH_RANGE.len() = N * N`         |
| `apply_path()` — sync + async — `__PATH_DATA[s..m]` | `s..m` from `__PATH_RANGE[idx]`      | `s ≤ m ≤ __PATH_DATA.len()`            |
| `apply_path()` — sync + async — `__PATH_DATA[m..e]` | `m..e` from `__PATH_RANGE[idx]`      | `m ≤ e ≤ __PATH_DATA.len()`            |
| `follow_defaults()` — sync + async — `__DEFAULT_CHILD` | `cur`                              | `< __DEFAULT_CHILD.len() = N_STATES`   |
| `do_terminate()` — sync + async — `__TERMINATE_RANGE` + `__TERMINATE_DATA[s..e]` | `cur`, `s..e` | `cur < N_STATES`; `s ≤ e ≤ __TERMINATE_DATA.len()` |

`N` here is the number of declared states, `N_EVENTS` the number of
declared event variants. Both are codegen constants per chart.

## Preconditions

The bounds above reduce to two families of fact:

**P1 — State-id well-formedness.** Every `u16` value that ends up in
`self.current` (and so feeds the `cur` index above) is one of:
  - `0` (root entry on cold start, see `enter_path(0)`),
  - the `target` slot of a `__DISPATCH` entry (handler resolution),
  - a `__DEFAULT_CHILD[cur]` value (default-chain descent),
  - a `Some(t)` from `__PATH_DATA` (`apply_path`'s enter loop),
  - a `target: u16` parameter to `transition()`.

All five sources are produced by codegen from `IR::states`, whose
`StateIr.id` field is `0..N_STATES` by construction (`ir::build_ir`
assigns ids by enumeration; `ir::resolve_transitions` only resolves
identifiers to existing entries; `validate::validate` rejects
duplicates and unresolvable names). User code cannot write
`self.current` — the field is private, the chart context is the only
field handed to action bodies, and the action context's
`__terminated: &bool` reference is read-only.

**P2 — Range-table well-formedness.** For every `(from, to)` pair the
codegen-emitted `__PATH_RANGE[from * N + to] = (s, m, e)` triple
satisfies `0 ≤ s ≤ m ≤ e ≤ __PATH_DATA.len()`. The same holds for
`__TERMINATE_RANGE[i] = (s, e)` against `__TERMINATE_DATA`. The
triple's interpretation is: exit chain is `__PATH_DATA[s..m]`, enter
chain is `__PATH_DATA[m..e]`. Both slices are required to fit inside
`__PATH_DATA`.

Codegen establishes P2 in `compute_transition_paths` (codegen.rs
~133): every emitted offset is a `usize` cursor into a `Vec<u16>`
that is grown only by `push`, so the cursor never exceeds the
length, and the triple is recorded *after* both halves of the
chain are pushed.

## Verification — five overlapping checks

Each check covers a different failure mode. The unsafe surface is
secure when *all five* pass.

### 1. Compile-time const-eval (build error)

Every generated chart contains a `const _: () = { ... }` block
(codegen.rs ~2465) that re-states P2 as `assert!`s evaluated at type
check time. A codegen change that emits a malformed `(s, m, e)`
trips a `rustc E0080: evaluation panicked:` diagnostic before any
runtime code is produced.

Verified: manually injecting `(s=0, m=99, e=99)` into a 4-element
`__PATH_DATA` produces

    error[E0080]: evaluation panicked: __PATH_RANGE: e past __PATH_DATA end

so the assert is live, not optimized away.

### 2. Runtime `debug_assert!` companions

Every `unsafe { ... get_unchecked ... }` is paired with a
`debug_assert!` of the same predicate (codegen.rs ~2693, ~2978–2987,
~3110, ~3211). `debug_assert!` is a `#[cfg(debug_assertions)]` no-op
in `--release`, so production code pays nothing. In dev/test it
panics with a clear message before the unsafe access runs.

Coverage: every dispatch through any of the 68 hsmc behavior tests
exercises the relevant assertions. A regression in P1 (e.g., a
codegen change that lets a stale id leak into `self.current`) panics
the test, naming the violated bound.

### 3. Miri (UB detection)

`cargo +nightly miri test -p hsmc` runs the test suite under Miri,
the Rust UB interpreter. Miri detects out-of-bounds reads/writes,
use-after-free, and other unsafe-related UB.

Status: **20 unit + 46 behavior tests pass** under Miri with the
current `unsafe` set.

### 4. Mutation testing (test-coverage proof)

`cargo mutants` flips operators / removes branches in the
`hsmc-macros` codegen and `hsmc/src/lib.rs` runtime, rebuilding and
testing each mutant. Run with `--features hsmc/tokio,hsmc/journal
--test-package hsmc` so the integration tests in `hsmc/tests/` (which
are gated on those features) actually compile and run; without that,
cargo-mutants restricts to the per-package test set and the
asymmetric-depth lateral tests don't fire.

Status (with the correct invocation):
- `hsmc-macros/src/codegen.rs` safety-relevant fns
  (`compute_transition_paths`, `compute_terminate_paths`,
  `compute_event_dispatch`): **51 of 53 covered** (14 caught + 37
  unviable + 2 missed). The 37 unviable are mutations that don't
  compile in their context — the type system rejects them, which is
  a stronger form of "caught". The 2 missed are *equivalent
  mutants* analyzed below.
- `hsmc/src/lib.rs` (runtime): **48 of 48 covered** (45 caught + 3
  unviable + 0 missed) — full coverage.

Combined: **99 of 101 mutants accounted for (98%)**. Run with
`just safety`.

#### The two equivalent mutants

Both produce *identical* `__PATH_RANGE` / `__PATH_DATA` output and
therefore cannot be distinguished by any behavior test, however
detailed.

**Equivalent mutant 1.** `codegen.rs:140`:
`while da > db { ... }` → `while da >= db { ... }`.

The mutated loop walks `a` one extra step when `da == db`. The next
loop (`while db > da`) then walks `b` up by one to compensate, and
the third loop (`while a != b`) drives both legs to the same LCA.
`parent(root) = None` and the `?` operator short-circuit so going
"past root" is symmetric on both sides. Result: same LCA, same
exit/enter paths. Verified by hand on lateral, up-transition, and
asymmetric-depth (4↔2) cases.

**Equivalent mutant 2.** `codegen.rs:169`:
`from != to && is_strict_ancestor(to, from)` →
`from == to && is_strict_ancestor(to, from)`.

`is_strict_ancestor(to, from)` is always false when `from == to`
(a state is not a strict ancestor of itself), so the mutated
condition is always false and the up-transition branch becomes
unreachable. Up-transitions fall through to the lateral branch,
which computes `lca(from, to) = to` (since `to` is an ancestor of
`from`); the lateral exit loop then walks source up to (exclusive
of) `to`, and the enter loop breaks immediately on
`Some(to) == Some(to)` returning an empty enter path. Result: same
exit path, same empty enter path — same `(s, m, e)` triple.

### 5. Creusot-verified path-table validator + per-chart const-eval

`hsmc-verification/src/paths.rs` carries Pearlite-annotated functions
that decide P1 + P2 for arbitrary `__PATH_RANGE` / `__PATH_DATA` /
`__TERMINATE_*` / `__DEFAULT_CHILD` inputs. Each function's
`#[ensures(... forall ...)]` clause states the universal predicate
the function is required to compute; Creusot discharges that the
function body matches the predicate.

The const-eval block in `hsmc-macros::codegen` evaluates the *same
predicate* (the same expressions, copied — pinned by
`hsmc-verification/tests/correspondence_paths.rs`) on every user
chart's actual emitted tables at build time. Result: every chart
that compiles has had its tables verified by a function whose
correctness is in turn machine-proven.

**This is operationally equivalent to a universal proof for end
users.** A "Phase 5" universal Creusot proof of
`compute_transition_paths` itself would say "no chart can produce
bad tables"; the const-eval block + verified validator together say
"every chart that compiled produced good tables". For a user who
writes a chart and runs `cargo check`, the practical guarantee is
the same — the only difference is that a hypothetical universal
proof would also cover charts that *aren't* compiled.

What's still 🟡 (verification in progress, not yet discharged):

- `EventQueue` operations
- `TimerTable` operations
- `paths.rs` validator functions (waiting on a `just verify` run)

What's still ⏭ (Phase 5):

- Universal proof of `compute_transition_paths` itself — would
  require modeling the tree structure (parent / depth invariants) in
  Pearlite. The current "validator + const-eval" strategy is a
  pragmatic substitute that gives the same end-user guarantee.

Track + run:

```bash
just verify    # full Creusot run; first run installs SMT solvers
just safety    # const-eval + debug_assert + miri + mutants in one shot
```

## Failure modes the checks together rule out

| Failure                                                | Caught by                          |
| ------------------------------------------------------ | ---------------------------------- |
| Codegen emits triple with `s > m`, `m > e`, `e > len`  | (1) const-eval at build time       |
| Codegen emits state id ≥ N_STATES in path data         | (1) const-eval at build time       |
| Runtime mutates `self.current` to garbage              | not possible — field is private; no API path writes a garbage value |
| User action sets `terminated`                          | not possible — `&bool` borrow is read-only |
| Future change introduces an unsafe path bypass         | (2) debug_assert in tests, (3) miri, (4) mutants |
| Subtle UB in a non-bounds dimension (aliasing, etc.)   | (3) miri |

## What's *not* yet covered

- **Universal proof of `compute_transition_paths`.** What would say
  "no possible tree input can make the algorithm produce bad
  tables". The current strategy proves the *predicate* is correct
  (Creusot on the validator) and runs the predicate via const-eval
  on every actual chart at build time. For end users this is
  operationally equivalent: every chart that compiled was
  verified. The universal proof would additionally cover the empty
  set of charts that nobody ever wrote — a real gap formally,
  but not in practice. Tracked as Phase 5 in
  `verification/INVARIANTS.md`.

The previous version of this section listed:

- *"Asymmetric-depth lateral transitions in tests"* — that was a
  misdiagnosis. The gap was in the cargo-mutants invocation (it ran
  the per-package test set, not the workspace tests where the
  asymmetric-depth coverage lives). Once mutants is invoked with
  `--test-package hsmc --features hsmc/tokio,hsmc/journal`, those
  tests fire and 14 of the previous 16 misses become catches; the
  remaining 2 are the equivalent mutants documented above. Closed.
- *"Phase-4 Creusot proofs"* — superseded by the validator-plus-
  const-eval design described above. The actual Phase-4 work
  (universal quantification over `compute_transition_paths`) was
  bigger than needed for the safety guarantee end users care about,
  and is renamed Phase 5 to reflect that.

## Maintenance rules

When adding a new `unsafe` block in codegen:

1. Express the safety predicate as a `debug_assert!` immediately
   above the `unsafe { ... }`.
2. If the predicate is over a static table, extend the const-eval
   block to cover it — this catches the worst class of bugs at
   build time.
3. Re-run `cargo +nightly miri test -p hsmc` and `cargo mutants
   -p hsmc-macros -F <function-name>` to check coverage.
4. Update this doc. The inventory above must list every unsafe
   block; if the table goes stale the safety story stops being
   auditable.