magicstatemachines/state/

owned.rs

#[cfg(not(feature = "tracing"))]
use crate::StateCopy;
#[cfg(feature = "decompose")]
use crate::{DecomposedData, DecomposedState, RecomposeError};
use crate::{Initial, StateClone, StateMachineImpl, Transition};
use core::marker::PhantomData;
use core::ops::{Deref, DerefMut};
#[cfg(feature = "tracing")]
use core::panic::Location;
use core::pin::Pin;

/// A directly owned runtime implementation `T` whose compile-time state is `S`.
///
/// Without the `tracing` feature, the state marker has no runtime storage and
/// `StateOwned<T, S>` has the same size and alignment as `T`.
/// With `tracing`, the wrapper also stores a `Vec<TraceEntry>` containing the
/// transition history for this value.
///
/// `StateOwned` is the simplest storage representation. Generic implementation
/// methods usually use [`State<SOwned, T, S>`](crate::State) instead, because
/// the same method can then work for owned, boxed, pinned, shared-guard, and
/// discriminated storage. `StateOwned<T, S>` remains useful when you want the
/// direct transparent wrapper explicitly.
///
/// The state marker `S` is compile-time authority, not runtime data. A
/// transition consumes one state token and returns another:
///
/// ```ignore
/// use magicstatemachines::{StateOwned, transition};
/// use test_def::states::{Connected, Disconnected};
///
/// let disconnected: StateOwned<Connection, Disconnected> =
///     StateOwned::new(Connection::new("localhost:8080"));
/// let connected: StateOwned<Connection, Connected> = transition!(disconnected);
/// ```
///
/// When `tracing` is enabled, the same transition appends a diagnostic record.
/// The trace is historical only; it is not consulted to decide which methods
/// are callable. The compiler-enforced state remains the `S` type parameter.
///
/// ```ignore
/// let connected = transition!(disconnected);
/// let entry = &connected.trace()[0];
///
/// assert!(entry.from().type_name().ends_with("::Disconnected"));
/// assert!(entry.to().type_name().ends_with("::Connected"));
/// ```
///
/// ```ignore
/// use magicstatemachines::{State, StateOwned, SOwned, transition};
/// use test_def::states::{Connected, Disconnected};
///
/// let disconnected: StateOwned<Connection, Disconnected> =
///     StateOwned::new(Connection::new("localhost:8080"));
///
/// // Generic APIs usually spell the same owned state like this:
/// let disconnected: State<SOwned, Connection, Disconnected> =
///     State::new(Connection::new("localhost:8080"));
/// // State-specific methods are implemented on `Connection` with arbitrary
/// // self types, so they can accept this generic owned state directly.
/// let connected: State<SOwned, Connection, Connected> =
///     connection_method_that_connects(disconnected);
/// ```
///
/// State tokens are linear and shared ownership is not valid state storage:
///
/// ```compile_fail
/// use std::rc::Rc;
/// use magicstatemachines::{Initial, StateMachineImpl, StateOwned};
///
/// struct Machine;
/// struct Ready;
/// struct Runtime;
/// struct Token;
///
/// impl Initial<Ready> for Machine {}
/// impl StateMachineImpl for Runtime {
///     type Standin = Machine;
///     type Impl = Self;
///     type TransitionToken = Token;
/// }
///
/// let _: StateOwned<Rc<Runtime>, Ready> = StateOwned::new(Rc::new(Runtime));
/// ```
#[cfg_attr(not(feature = "tracing"), repr(transparent))]
pub struct StateOwned<T, S> {
    pub(crate) value: T,
    pub(crate) state: PhantomData<fn() -> S>,
    #[cfg(feature = "tracing")]
    pub(crate) trace: Vec<crate::TraceEntry>,
}

/// Owned state whose runtime value is pinned by an arbitrary pinning pointer.
///
/// This is the direct owned alias for `StateOwned<Pin<P>, S>`. In generic
/// state-machine methods prefer [`State`](crate::State) plus a storage backend
/// such as [`SPinBox`](crate::SPinBox), because that keeps the same method
/// usable for owned, boxed, pinned, and guard-backed states.
pub type SPin<T, S> = StateOwned<Pin<T>, S>;

/// A one-shot callable that completes a state transition.
#[doc(hidden)]
pub struct TransitionCall<T, From, To> {
    state: StateOwned<T, From>,
    #[cfg(feature = "tracing")]
    callsite: &'static Location<'static>,
    to: PhantomData<fn() -> To>,
}

/// Creates a callable transition requiring the definition's arguments.
///
/// This low-level function requires the implementation's private transition
/// capability. It is mainly used by generated code and crate-internal tests.
/// Implementation methods should normally use
/// [`transition!`](macro@crate::transition), which expands to the private
/// helpers generated by [`StateMachineImpl!`](macro@crate::StateMachineImpl).
///
/// The returned [`TransitionCall`] is one-shot. Calling `.call(args)` checks
/// `args` against the declaration's [`Transition::F`] signature and then
/// retags the wrapper from `S` to `Next`. With `tracing`, the callsite captured
/// here is appended to the returned state's trace.
#[must_use]
#[track_caller]
pub fn transition<T, S, Next>(
    state: StateOwned<T, S>,
    _token: T::TransitionToken,
) -> TransitionCall<T, S, Next>
where
    T: StateMachineImpl,
    T::Standin: Transition<S, Next>,
{
    TransitionCall {
        state,
        #[cfg(feature = "tracing")]
        callsite: Location::caller(),
        to: PhantomData,
    }
}

#[cfg(not(feature = "tracing"))]
impl<T, From, To> TransitionCall<T, From, To>
where
    T: StateMachineImpl,
{
    #[doc(hidden)]
    pub fn call<Args>(self, _args: Args) -> StateOwned<T, To>
    where
        T::Standin: Transition<From, To>,
        <T::Standin as Transition<From, To>>::F: crate::TransitionSignature<Args>,
    {
        StateOwned {
            value: self.state.value,
            state: PhantomData,
        }
    }
}

#[cfg(feature = "tracing")]
impl<T, From, To> TransitionCall<T, From, To>
where
    T: StateMachineImpl,
{
    #[doc(hidden)]
    pub fn call<Args>(self, _args: Args) -> StateOwned<T, To>
    where
        T::Standin: Transition<From, To>,
        <T::Standin as Transition<From, To>>::F: crate::TransitionSignature<Args>,
        From: crate::StateTrait,
        To: crate::ConcreteStateTrait,
    {
        let mut trace = self.state.trace;
        trace.push(crate::TraceEntry::new::<From, To>(self.callsite));

        StateOwned {
            value: self.state.value,
            state: PhantomData,
            trace,
        }
    }
}

impl<T, S> StateOwned<T, S> {
    /// Separates the compile-time state token from the runtime data.
    ///
    /// This is a provenance API for temporarily splitting a valid typestate
    /// value into two opaque halves. The [`DecomposedState`] half carries the
    /// authority that the value was in state `S`; the [`DecomposedData`] half
    /// carries the runtime data `T`. Both halves contain the same generated UID,
    /// and [`StateOwned::recompose`] checks that UID before it recreates a
    /// usable `StateOwned<T, S>`.
    ///
    /// This is useful when state proof and runtime data need to pass through
    /// different layers, but neither layer should be able to manufacture a new
    /// valid state by itself. The state half cannot expose or mutate `T`, and
    /// the data half cannot recreate typestate authority without the matching
    /// state half.
    ///
    /// ```
    /// use magicstatemachines::{Initial, StateMachineImpl, StateOwned};
    ///
    /// struct Machine;
    /// struct Runtime {
    ///     value: u32,
    /// }
    /// struct Ready;
    /// struct Token;
    ///
    /// impl Initial<Ready> for Machine {}
    ///
    /// impl StateMachineImpl for Runtime {
    ///     type Standin = Machine;
    ///     type Impl = Self;
    ///     type TransitionToken = Token;
    /// }
    ///
    /// let ready: StateOwned<Runtime, Ready> = StateOwned::new(Runtime { value: 7 });
    /// let (state, data) = ready.decompose();
    ///
    /// // The halves can now move through different code paths. Only the
    /// // original matching pair can be recomposed into a valid state token.
    /// let ready = StateOwned::recompose(state, data).expect("matching decomposition");
    ///
    /// assert_eq!(ready.value, 7);
    /// ```
    ///
    /// This method is available only with `decompose`. The UID generator is
    /// selected separately: `decompose-rand` uses the stable `rand` dependency,
    /// while `decompose nightly-random` uses nightly `std::random` and does not
    /// need `rand`. If `tracing` is also enabled, the trace stays with the state
    /// half and is restored by recomposition.
    #[cfg(feature = "decompose")]
    #[must_use]
    pub fn decompose(self) -> (DecomposedState<S>, DecomposedData<T>) {
        let uid = decompose_uid();

        (
            DecomposedState {
                uid,
                state: PhantomData,
                #[cfg(feature = "tracing")]
                trace: self.trace,
            },
            DecomposedData {
                uid,
                value: self.value,
            },
        )
    }

    /// Recombines state and data produced by the same [`StateOwned::decompose`] call.
    ///
    /// Recomposition restores the exact same state type `S`. After it succeeds,
    /// the returned value can be used like any other `StateOwned<T, S>`: it
    /// dereferences to `T`, can keep transitioning according to the declared
    /// state-machine contract, and, with `tracing`, continues carrying the
    /// existing trace.
    ///
    /// A mismatched UID returns [`RecomposeError`]. The check is deliberately
    /// narrow: it does not inspect `T`, and it does not infer whether two
    /// runtime values happen to be equal. It only verifies that both opaque UID
    /// halves came from the same decomposition. The UID is non-cryptographic and
    /// exists to catch accidental or invalid recombination, not malicious input.
    ///
    /// ```
    /// use magicstatemachines::{Initial, StateMachineImpl, StateOwned};
    ///
    /// struct Machine;
    /// struct Runtime;
    /// struct Ready;
    /// struct Token;
    ///
    /// impl Initial<Ready> for Machine {}
    ///
    /// impl StateMachineImpl for Runtime {
    ///     type Standin = Machine;
    ///     type Impl = Self;
    ///     type TransitionToken = Token;
    /// }
    ///
    /// let ready: StateOwned<Runtime, Ready> = StateOwned::new(Runtime);
    /// let (state, data) = ready.decompose();
    /// let ready: StateOwned<Runtime, Ready> =
    ///     StateOwned::recompose(state, data).expect("matching decomposition");
    ///
    /// let _: StateOwned<Runtime, Ready> = ready;
    /// ```
    #[cfg(feature = "decompose")]
    pub fn recompose(
        state: DecomposedState<S>,
        data: DecomposedData<T>,
    ) -> Result<Self, RecomposeError> {
        if state.uid != data.uid {
            return Err(RecomposeError);
        }

        Ok(Self {
            value: data.value,
            state: PhantomData,
            #[cfg(feature = "tracing")]
            trace: state.trace,
        })
    }

    /// Recorded transitions in call order.
    ///
    /// Each entry stores the source state, destination state, and callsite
    /// captured by the transition wrapper. This is available only with the
    /// `tracing` feature and is intended for diagnostics, not for enforcing
    /// the state machine.
    ///
    /// Cloning a traced state clones the erased state markers in every entry.
    /// The public API of each entry stays the same whether erased markers are
    /// backed by `&'static dyn StateTrait` or by the `dynZST` feature.
    #[cfg(feature = "tracing")]
    #[must_use]
    pub fn trace(&self) -> &[crate::TraceEntry] {
        &self.trace
    }
}

#[cfg(all(feature = "decompose", feature = "nightly-random"))]
fn decompose_uid() -> u64 {
    std::random::random(..)
}

#[cfg(all(
    feature = "decompose",
    not(feature = "nightly-random"),
    feature = "decompose-rand"
))]
fn decompose_uid() -> u64 {
    use rand::TryRngCore;

    rand::rngs::OsRng
        .try_next_u64()
        .expect("OS random source failed while decomposing state")
}

#[cfg(all(
    feature = "decompose",
    not(feature = "nightly-random"),
    not(feature = "decompose-rand")
))]
compile_error!(
    "feature `decompose` requires a random backend: enable `nightly-random` or `decompose-rand`"
);

impl<T, S> StateOwned<T, S>
where
    T: StateMachineImpl,
    T::Standin: Initial<S>,
{
    /// Wraps an implementation in a state declared initial by its definition.
    ///
    /// This only compiles when `T::Standin: Initial<S>`. Use transition
    /// methods generated in the implementation module to reach every later
    /// state.
    #[must_use]
    pub const fn new(value: T) -> Self {
        Self {
            value,
            state: PhantomData,
            #[cfg(feature = "tracing")]
            trace: Vec::new(),
        }
    }
}

impl<T, S> Deref for StateOwned<T, S> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.value
    }
}

impl<T, S> DerefMut for StateOwned<T, S> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.value
    }
}

impl<T, S> Clone for StateOwned<T, S>
where
    T: Clone,
    S: StateClone,
{
    fn clone(&self) -> Self {
        Self {
            value: self.value.clone(),
            state: PhantomData,
            #[cfg(feature = "tracing")]
            trace: self.trace.clone(),
        }
    }
}

#[cfg(not(feature = "tracing"))]
impl<T, S> Copy for StateOwned<T, S>
where
    T: Copy,
    S: StateClone + StateCopy,
{
}

impl<T: core::fmt::Debug, S> core::fmt::Debug for StateOwned<T, S> {
    fn fmt(&self, formatter: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        self.value.fmt(formatter)
    }
}

#[cfg(not(feature = "tracing"))]
pub(super) fn complete_transition<T, From, To>(
    state: StateOwned<T, From>,
    _callsite: super::TransitionCallsite,
) -> StateOwned<T, To> {
    StateOwned {
        value: state.value,
        state: PhantomData,
    }
}

#[cfg(feature = "tracing")]
pub(super) fn complete_transition<T, From, To>(
    state: StateOwned<T, From>,
    callsite: super::TransitionCallsite,
) -> StateOwned<T, To>
where
    From: crate::StateTrait,
    To: crate::ConcreteStateTrait,
{
    let mut trace = state.trace;
    trace.push(crate::TraceEntry::new::<From, To>(callsite));

    StateOwned {
        value: state.value,
        state: PhantomData,
        trace,
    }
}