hsmc — Hierarchical State Machines for Rust

hsmc lets you write the control logic of a device — a microwave, a LoRa radio, a sensor, a UI flow — as a statechart: nested states with entry/exit actions, transitions, timers, and async work, all declared in one block. The proc macro turns it into Rust code with no heap allocations, no dynamic dispatch, and no interior mutability.

It targets embedded Rust on embassy first, with full tokio support on the side. Same chart, both runtimes.

A first chart — a blinker

use hsmc::{statechart, Duration};

pub struct Ctx {
    pub on:     bool,    // mirrors the LED's current state
    pub blinks: u32,     // count of times we've turned it on
}

statechart! {
    Blinker {
        context: Ctx;
        default(Off);    // start with the LED off

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

impl BlinkerActions for BlinkerActionContext<'_> {
    async fn light_off(&mut self) { self.on = false; }
    async fn light_on(&mut self)  { self.on = true; self.blinks += 1; }
}

Two states, two timer-driven transitions, two entry actions. The runtime arms each state's timer when you enter it and cancels it when you leave; default(Off) picks the initial state. Land in Off, fire light_off, arm the 500 ms timer; when it elapses, transition to On, fire light_on, arm the next 500 ms timer. Repeat.

That's hsmc at its smallest: states, timers, and one entry action per state. No event channel, no async I/O, no hierarchy yet.

Quickstart

Add to Cargo.toml:

[dependencies]
hsmc = { version = "0.6", features = ["embassy"] }   # firmware
# or
hsmc = { version = "0.6", features = ["tokio"] }     # desktop / tests

Run the Blinker under tokio:

#[tokio::main(flavor = "current_thread")]
async fn main() {
    let mut m = Blinker::new(Ctx { on: false, blinks: 0 });
    let _ = m.run().await;
}

That's the whole driver. A chart runs on a single task — flavor = "current_thread" keeps tokio out of the multi-threaded scheduler so you don't have to think about Send bounds on your context, your events, or your during: futures. Normal Rust, no extra wrapping.

Run it under embassy:

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

A bigger chart — events, payloads, async I/O

The Blinker doesn't talk to anything outside itself. A real device does. Here's a radio that receives commands from somewhere (button press, RPC call, ISR), continuously listens for packets while in the Receiving state, and stops when told:

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;
        }
    }
}

Three things here that the Blinker didn't have:

• Events. events: Ev; declares the input enum. External code drives the chart with m.sender().send(Ev::StartRx); in-chart actions can emit(Ev::Whatever) to queue a follow-up. Handlers pattern-match the variant and bind its payload directly: on(PacketRx { rssi, snr }) => record_packet; makes rssi and snr available to the action.
• during: activities. next_packet(lora, rx_buf) is an async fn that runs while Receiving is active and is dropped when the state exits. It gets &mut borrows of just the lora and rx_buf fields of the context — Rust's split-borrow rules let several during:s on the active path run concurrently without a RefCell or Mutex in sight.
• The runtime races them all. m.run() simultaneously polls the during:, listens on the event channel, and waits for the next timer deadline; whichever fires first drives the next transition. The MCU sleeps in between.

For nested-state hierarchies, emit(...), terminate, and on(every D), see the microwave example.

What you get

• Nested states with proper transitions. Exit a deeply nested state and every enclosing state's exits run in the right order, no manual bookkeeping. A transition is just => Target; and the runtime walks the tree (LCA dispatch). Includes entry:/exit: actions, optional default(...) initial transitions, and terminate.
• Events with typed payloads. Pattern-match a variant and bind its fields right in the chart: on(PacketRx { rssi: i16 }) => record_packet;. The compiler type-checks the binding against your event enum.
• Timers as first-class triggers. on(after 5s) => Next; for a one-shot, on(every 100ms) => tick; for periodic. Armed when the declaring state is entered, cancelled when it's exited — no select! plumbing in your code.
• Async work scoped to a state's lifetime. A during: is an async fn that runs while a state is active and is dropped when you leave. Multiple durings on the active path race concurrently and split-borrow disjoint context fields — no RefCell, no Mutex, no unsafe in your code.
• m.run().await is the whole task. It races durings, the event channel, and timer deadlines, and parks the executor between them. No manual loop, no busy-waits, no Send headaches under current_thread tokio or embassy.
• Observability that's one feature flag away. Add features = ["trace-log"] (or trace-defmt / trace-tracing) and every action, transition, timer, and event becomes a log line in whichever framework your crate already uses — defmt for firmware, log for desktop, tracing for structured spans. With no sink feature, the observation calls compile to nothing.
• Deterministic + machine-checked. The journal feature records every action, transition, timer, and event in order; identical input produces a byte-identical journal. The runtime's event queue and timer table are formally verified — Creusot + Why3 + alt-ergo + z3 discharge the proofs (just verify).
• No heap, no dynamic dispatch. Pure monomorphized code. no_std clean by default. Designed to fit on a Cortex-M0+ under embassy. The macro emits six bounded unsafe { get_unchecked } blocks per chart for hot-path table indexing; their preconditions are checked at build time by const-eval, in dev/test by debug_assert!, and under cargo +nightly miri test. Full inventory and audit in docs/004. unsafe-safety-contract.md. Your chart, your action bodies, and your durings stay unsafe-free.

How a chart behaves — the rules

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 5.

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. Transitions — exit to LCA, enter to target

A transition moves the active path from the current innermost state to some target state. The algorithm is uniform regardless of what triggered it (event, timer, or default — see later rules):

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.

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;).

3. 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 step 3 of the transition algorithm does 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 timers don't restart. A's durings keep running.

You cannot enter a state you never left.

4. 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.
• 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.

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

Any state may declare one default(...). It's a transition — same LCA-aware algorithm as rule 2 — 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. Defaults chain: if the resulting target also has a default(...), it fires too, and so on until a state with no default is reached. 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.

A consequence of rule 3 (up-transitions): an ancestor's default(...) only fires when that ancestor is freshly entered. Transitioning up to an ancestor does not re-enter it, so its default does not re-fire.

6. 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 (the handler may execute a transition per rule 2). 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.

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 2 + 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.

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(). Single-thread (flavor = "current_thread").
embassy	Async actions + run(). Embassy's cooperative executor. no_std.
journal	In-memory Vec<TraceEvent> of every observation (replay, byte-deterministic).
trace-defmt	Auto-emit defmt::* log lines for every observation.
trace-log	Auto-emit log::* log lines for every observation.
trace-tracing	Auto-emit tracing::* events with structured fields.

embassy and tokio are mutually exclusive — enabling both is a compile error. The trace-* and journal features are independently composable with each other and with the runtime feature; with no sink feature on, the trace points compile to nothing (zero overhead).

Observability — one journal, multiple outputs

On hsmc's side, turning on observability is a single Cargo feature. On your side, it's whatever you'd already do to wire defmt / log / tracing to an output — hsmc does not initialize a logger (libraries shouldn't; main owns that decision).

If your firmware crate already uses defmt, add the feature and you're done — chart events interleave with the rest of your defmt output:

hsmc = { version = "0.6", features = ["embassy", "trace-defmt"] }

For a desktop crate, pick log or tracing and do that framework's normal one-line init in main:

[dependencies]
hsmc               = { version = "0.5", features = ["tokio", "trace-tracing"] }
tracing-subscriber = "0.3"

#[tokio::main(flavor = "current_thread")]
async fn main() {
    tracing_subscriber::fmt::init();
    let mut m = Blinker::new(Ctx { on: false, blinks: 0 });
    let _ = m.run().await;
}

Swap trace-tracing for trace-log and tracing_subscriber::fmt::init() for env_logger::init() if you prefer the log ecosystem; the rest is identical. With no sink feature enabled at all, the observation calls compile to () — the runtime is byte-identical to a chart with no instrumentation.

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

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.

Examples

Example	Purpose
microwave	Full-feature pure-event machine: entry/exit, timers, emit, terminate.
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.

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
Multi-threaded tokio	flavor = "current_thread" is the supported path — one task per chart

See docs/002. hsmc-semantics-formal.md § "What hsmc deliberately does NOT have" for the rationale on each.

Status

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

License

MIT. Copyright (c) 2026 Stephen Waits <steve@waits.net>. See LICENSE.