hsmc — Hierarchical State Machines for Rust

hsmc is a declarative, proc-macro-based statechart crate for Rust. It targets embedded Rust (embassy) as the primary runtime and supports tokio via LocalSet. Machines are compiled from a statechart! block into monomorphized code with no dynamic dispatch, no interior mutability, and no heap allocations by default.

statechart! {
    Radio {
        context: Ctx;
        events:  Ev;
        default(Idle);

        state Idle {
            on(StartRx) => Receiving;
        }
        state Receiving {
            during: next_packet(lora, rx_buf);
            on(PacketRx { rssi: i16, snr: i16 }) => record_packet;
            on(StopRx) => Idle;
        }
    }
}

What it does

• Harel-style hierarchical states. Nested states with entry:/exit: actions, LCA-aware transitions, optional default(...) (a transition that may target any state — descend, redirect to a sibling, bounce to an ancestor — with a compile-time cycle check), terminate.
• Event-driven transitions and actions. Typed payload bindings: on(PacketRx { rssi: i16 }) => handler;.
• Timer triggers. on(after Duration::from_secs(5)) => Next; and on(every Duration::from_millis(100)) => tick;.
• during: activities. Async functions that run while a state is active and produce events. Multiple durings can borrow disjoint fields of the context concurrently via Rust's native split borrow — no RefCell, no Mutex, no UnsafeCell in user code.
• m.run().await is the whole task body. Races active durings, the external event channel, and timer deadlines. The MCU sleeps between events (embassy's cooperative executor parks all futures).
• Unified observability. One observation per atom of execution fans out at compile time to a journal (Vec<TraceEvent>) and/or textual trace backends (defmt, log, tracing) — they share a single vocabulary, so they cannot diverge. With no sink feature on, every site expands to () (zero overhead). See Observability.
• Deterministic + mechanically verified. The journal feature records every action, transition, timer, and event in order — same inputs, byte-identical journal. The runtime queue and timer table carry Pearlite specs that Creusot + Why3 + alt-ergo + z3 discharge mechanically (just verify). The integration suite pins every spec rule with byte-equal expected-vs-actual journals, trace-format regression checks, and per-feature behavior tests.

How a chart behaves — the building blocks

Eight rules, each building on the previous. Internalize these and every edge case below falls out — you don't have to memorize anything else.

The full canonical reference lives at docs/002. hsmc-semantics-formal.md; this section is the on-ramp.

1. The active path — you are in every state from root to the innermost active state

At any moment, the machine is in a path of states from the root down to one innermost active state. If you're in C inside B inside A, you're simultaneously in Root, A, B, and C. The root is always active until termination.

The innermost active state is usually a leaf, but it can also be a composite that has children but no default(...) — see rule 2.

This is the most important rule. Every rule below is downstream of it. If a behavior somewhere else seems weird, come back here — it's probably the resolution.

2. default(...) — a transition that fires immediately on entry

Any state may declare one default(...). It's a transition — same LCA-aware algorithm as any event-triggered transition (rule 4) — that fires immediately after the declaring state's entry actions finish (and its durings start). The target may be any state in the chart: a child, a sibling, an ancestor, anywhere. The compiler rejects cycles in the default graph, so the chain is always finite.

The classic case is a composite descending to a leaf:

state Root { default(A); state A { default(B); state B {} } }

Entering Root fires its default → enter A → fires its default → enter B. The path lands at B.

But because default is a real transition, you can also redirect:

state Foyer { default(LivingRoom); }   // sibling — entering Foyer
state LivingRoom {}                     // exits Foyer to LivingRoom

Entering Foyer fires its default Foyer → LivingRoom. LCA = parent of both; Foyer's exits run, LivingRoom's entries run. Foyer was entered and exited as part of one tick.

If a state has no default(...), nothing happens after its entries — the state itself is the resting innermost active state until something explicitly transitions away. This is the "microwave" pattern: a Standby state with sub-modes that the chart enters only on demand.

3. Event bubbling — innermost first, first handler wins

An event arrives. Search starts at the innermost active state. If that state has a handler, it runs and the event is consumed. If not, walk up to the parent and check there. Repeat. If you reach the root with no handler, the event is silently discarded.

Three corollaries:

• Innermost shadows ancestor for the same trigger.
• Multiple handlers in one state on the same trigger fire in declaration order; the transition (if any) fires last.
• Timers don't bubble. A timer belongs to the state that declared it; it fires only in that state's scope.

4. Transitions — exit to LCA, enter to target

Any transition's algorithm is uniform:

1. Find the LCA (lowest common ancestor) of current and target.
2. Walk up from current, exiting each state, until you hit the LCA. The LCA itself is not exited.
3. Walk down from the LCA to target, entering each state. The LCA is not re-entered.
4. If the target declares a default(...), fire it (rule 2). Defaults chain — each link is itself a full transition — until you reach a state with no default. The compiler rejects cycles.

The LCA always exists because root is always an upper bound. State names are globally unique across the chart, so a transition can target any state — siblings, cousins, ancestors, the root itself (on(Trig) => MyChart;).

5. The up-transition rule — never re-enter what you never left

If your transition target is already on the active path (i.e., it's an ancestor of the innermost state), then steps 3 and 4 of the transition algorithm do nothing. You exit the states strictly between current and target; the target itself is not re-entered.

So from leaf C inside B inside A, an on(Up) => A; exits C then B. A's entry actions don't fire. A's default does not fire (it only fires when A is freshly entered, which it wasn't). A's timers don't restart. A's durings keep running.

You cannot enter a state you never left.

6. Entry / exit ordering

Direction is set by the path:

• Entries fire outer-to-inner: when entering down through a path, the outermost state's entries fire first, then the next, then the innermost's. Within a single state, entry actions fire in declaration order. After a state's entries finish, its during: activities start. Then, if the state declared a default(...), it fires as a transition (rule 2) — chains continue until a state with no default.
• Exits fire inner-to-outer: innermost first, then parent, then grandparent. Within a single state, exit actions fire in declaration order. Before a state's exits begin, its during: activities are cancelled.

So a full transition reads: cancel durings on the way out → run exits on the way out → run entries on the way in → start durings on the way in.

7. Timers — armed by state lifecycle

A timer (on(after D) => … or on(every D) => …) is armed when its declaring state is entered, cancelled when that state is exited. Descending into a child is not an exit, so the parent's timers keep running while you're in a child. Re-entering a state restarts its timers from zero.

8. emit / during / termination — all built on the rules above

• emit(ev) from inside an action queues an event. The runtime finishes the current event's handling — including every entry/exit in the resulting transition — before dequeuing the new event. No re-entrant dispatch.
• during: is an async future tied to its state's lifecycle. It starts after the state's entry actions finish and is cancelled (the future is dropped) before the state's exit actions begin. Multiple durings on the active path race; each borrows disjoint &mut fields of the context (Rust's split-borrow checker enforces this at compile time).
• Termination is just rules 6 + 4 with target = "outside the chart": exit the entire active path inner-to-outer, then stop. Pending events drop. In-flight durings drop at their next .await.

That's the whole behavior model. The full semantics doc spells out edge cases (queue overflow surfacing, same-tick timer ties, self-transitions on composites) but the rules above cover the overwhelming majority of charts.

What hsmc deliberately does NOT have

The bet is on simple, consistent semantics. These features are omitted on purpose; for each, what to do instead:

Missing feature	Use instead
Guard conditions on transitions	Split source into two states, or have an action emit() a follow-up event for the chart to branch on
Orthogonal / parallel regions	Two charts as two tasks connected by channels, or multiple during: activities in one state
History states	Store last child explicitly in your context; transition to it on re-entry
Deferred events	Handle where they arrive (drop/log if not relevant), or have producer check state
Internal transitions	action(Trig) => fn; (no transition) — exactly that
State-local context	All state in the root context; durings borrow disjoint fields
Event priorities beyond FIFO	Higher-priority events on a separate channel into a separate, faster machine
Runtime statechart modification	Design every variation up front as states

Quickstart

Add to Cargo.toml:

[dependencies]
hsmc = { version = "0.5", features = ["embassy"] }  # or ["tokio"]

Minimal timer-only machine:

use hsmc::{statechart, Duration};

pub struct Ctx { pub ticks: u32 }

statechart! {
    Blinker {
        context: Ctx;
        default(On);

        state On {
            entry:  light_on;
            on(after Duration::from_millis(500)) => Off;
        }
        state Off {
            on(after Duration::from_millis(500)) => On;
        }
    }
}

impl BlinkerActions for BlinkerActionContext<'_> {
    async fn light_on(&mut self) { self.ticks += 1; }
}

Drive it under tokio:

let mut m = Blinker::new(Ctx { ticks: 0 });
tokio::task::LocalSet::new().run_until(m.run()).await.unwrap();

Or under embassy:

static CHANNEL: Channel<CriticalSectionRawMutex, __NoEvents, 8> = Channel::new();
// (timer-only machines don't need a channel; omit `events:` and use `Blinker::new(ctx)`.)

#[embassy_executor::task]
async fn blink_task() {
    let mut m = Blinker::new(Ctx { ticks: 0 });
    let _ = m.run().await;
}

during: — async activities scoped to a state

A during: runs while its state is on the active path. It's a plain async function that takes &mut references to specific fields of the root context and returns one event:

state Receiving {
    during: next_packet(lora, rx_buf);
    on(PacketRx { rssi: i16 }) => count_packet;
    on(StopRx) => Idle;
}

async fn next_packet(
    lora:   &mut LoRaDriver,
    rx_buf: &mut [u8; 256],
) -> RadioInput {
    lora.rx(rx_buf).await.into()
}

The macro emits next_packet(&mut ctx.lora, &mut ctx.rx_buf) and Rust's native split-borrow verifies the fields don't overlap with any other concurrent during on the active path. Overlapping fields produce a compile-time error.

Multiple concurrent durings

A state (or any ancestor on its path) may declare more than one during:. The run loop races all of them against the external channel and the next timer deadline via tokio::select! / embassy_futures::select. Each during borrows disjoint fields, so the split borrow is free.

state Configured {
    during: heartbeat(hb_counter);     // active in every Configured substate
    state Receiving {
        during: next_packet(lora, rx_buf);
    }
}

Cancel-safety contract

A during: future will be dropped at any .await point when a handler fires, a timer expires, an external event arrives, or the state transitions. Write durings as cancel-safe state machines: every .await must be a clean resume point where dropping the future leaves the borrowed fields in a valid, re-enterable state.

Anti-pattern:

async fn bad(acc: &mut u32) -> Ev {
    *acc += 1;                                   // commit happens here
    tokio::time::sleep(Duration::from_ms(100)).await;  // ← may be dropped
    *acc += 1;                                   // may never run
    Ev::Tick
}

Preferred:

async fn good(acc: &mut u32) -> Ev {
    tokio::time::sleep(Duration::from_ms(100)).await;
    *acc = acc.wrapping_add(2);                   // all mutations after the .await
    Ev::Tick
}

Starvation in select-drop semantics

When several durings race, the shortest-to-complete wins every iteration and the others are dropped without making progress. This is expected: firmware I/O activities are naturally cancel-safe (a dropped lora.rx() just stops receiving) and the starved ones would typically be heartbeat-like tasks that deliver their events eventually.

If two durings compete for the same "always ready" resource, combine them into a single during that uses select internally.

Generated API per machine

For statechart! Foo { ... } the macro emits:

Item	What it is
FooState	Enum of every user-declared state.
FooActions	Trait with one async fn per unique action name. User implements.
FooActionContext<'a>	Wrapper passed to actions. Derefs to the root context. emit().
FooCtx<'a>	Type alias for FooActionContext<'a>.
Foo<const QN: usize = 8>	The machine itself.
FooSender	Cloneable handle for cross-task / ISR event injection.
Foo::<8>::STATE_CHART	&'static str ASCII tree of the hierarchy (useful for defmt/docs).

Machine methods

Method	Notes
new(ctx) / new(ctx, channel)	Construct. Under embassy, pass an &'static Channel.
with_queue_capacity::<N>(ctx,...)	Custom event-queue capacity.
async fn run()	Drive the machine to completion (async features).
async fn dispatch(ev)	Push event + drain to quiescence. Same task injection.
fn step(elapsed) -> Option<Duration>	One unit of work (test/polling driver).
fn send(ev)	Raw push, non-draining. For tests.
sender() -> Sender	Clone handle for other tasks / ISRs.
current_state()	The innermost active state.
is_terminated()	True after terminate event.
has_pending_events()	Queue non-empty.
context() / context_mut()	Borrow the root context.
into_context(self)	Consume, return ctx.
STATE_CHART	Const ASCII diagram.

Feature flags

Feature	Effect
(default)	no_std, no async runtime. Drive via step(Duration) + send(ev).
tokio	Async actions + run(). Targets single-thread via LocalSet.
embassy	Async actions + run(). Targets embassy's cooperative executor. no_std.

embassy and tokio are mutually exclusive — enabling both is a compile error.

Examples

Example	Purpose
microwave	Full-feature pure-event machine: entry/exit, timers, emit, terminate.
microwave_tui	TUI variant of microwave.
during_radio	during: activities simulating a radio task with RX + TX loops.
embassy_full	Embassy feature demonstration.

Run cargo run --example during_radio --features tokio to see the during: pattern end-to-end.

Verification

hsmc ships with a Creusot deductive-verification crate (verification/) that proves the runtime's EventQueue and TimerTable correct against Pearlite specs. Why3 + alt-ergo + z3 discharge every VC mechanically. Run:

just verify

The recipe is self-installing (mise pulls in the pinned nightly, opam, Why3, the SMT solvers, and cargo-creusot) so a fresh clone gets to a green proof in one command. See verification/INVARIANTS.md for the spec-rule → proof mapping.

Beyond Creusot, the suite includes:

• Integration tests — full chart behavior and per-feature determinism, with byte-equal expected-vs-actual journals on the det_* deterministic-flow tests, plus trace-format regression tests pinning every observation verb's textual rendering. Run with cargo test --workspace --features tokio,journal.
• cargo mutants — full coverage under cargo mutants --features tokio,journal,trace-log.
• Miri — clean, no UB on the runtime crates.

Observability — one journal, multiple outputs

Every chart emits a single observation per atom of execution. That observation fans out to whichever sinks you compile in:

Feature	Sink
journal	In-memory Vec<TraceEvent> (replay, byte-deterministic)
trace-defmt	defmt::* log lines
trace-log	log::* log lines
trace-tracing	tracing::* events with structured fields

With no sink feature on, observation is fully elided at compile time — zero overhead. Multiple sinks can be enabled simultaneously; the journal IS the observation stream and trace backends are textual renderings of it, so they cannot diverge.

The textual format is logfmt-style — greppable and regex-parseable:

[statechart:Microwave] started chart_hash=0xa1b2c3d4
[statechart:Microwave] entering state=Idle
[statechart:Microwave] action state=Idle action=clear_display kind=entry
[statechart:Microwave] timer-armed state=Idle timer=t0 duration_ns=5000000000
[statechart:Microwave] during-started state=Idle during=poll_sensor
[statechart:Microwave] entered state=Idle
[statechart:Microwave] event-received name=StartButton
[statechart:Microwave] event-delivered name=StartButton handler=Idle
[statechart:Microwave] transition-begin from=Idle to=Heating reason=event:StartButton
[statechart:Microwave] exiting state=Idle
[statechart:Microwave] timer-cancelled state=Idle timer=t0
[statechart:Microwave] during-cancelled state=Idle during=poll_sensor
[statechart:Microwave] action state=Idle action=stop_motor kind=exit
[statechart:Microwave] exited state=Idle
[statechart:Microwave] entering state=Heating
[statechart:Microwave] entered state=Heating
[statechart:Microwave] transition-complete from=Idle to=Heating

The verb vocabulary

Twenty verbs cover every TraceEvent variant. Begin/end markers bracket entry, exit, and transition phases so the actions, timers, and durings running inside each phase are visible.

Verb	Fires when
started	First step of a chart's life — carries chart_hash
entering / entered	Begin / end of a state's entry phase
exiting / exited	Begin / end of a state's exit phase
action	An action fn was called — kind=entry|exit|handler
during-started / during-cancelled	A during: activity started / was dropped
transition-begin / transition-complete	Begin / end of a transition's exit-then-enter sequence
event-received	An event was popped from the queue
event-delivered	A handler ran for the event
event-dropped	The event reached root with no handler
emit-queued / emit-failed	emit() from inside an action — queued or rejected
timer-armed / timer-cancelled / timer-fired	Timer lifecycle
terminate-requested	terminate(...) event matched, exit chain about to run
terminated	Chart finished (always last)

The reason= field on transition-begin records what triggered the transition: an event (event:VariantName), a timer (timer:State/timer-id), or internal. Real-life captures can be diffed against expected behavior the same way the journal can.

Status

Pre-1.0. Per-release breaking changes are documented in CHANGELOG.md.

Out of scope (deliberate): guard conditions, orthogonal/parallel regions, history states, deferred events, internal transitions, state-local context, event priorities beyond FIFO, runtime statechart modification, multi-threaded tokio. See docs/002. hsmc-semantics-formal.md § "What hsmc deliberately does NOT have" for the rationale and what to use instead.

License

Dual-licensed MIT OR Apache-2.0.