refactor: make `Step`'s error an associated type

This was suggested in https://codeberg.org/topola/topola/pulls/79.

src/autorouter/autoroute.rs

@@ -87,9 +87,9 @@ impl AutorouteExecutionStepper {
     }
 }
 
-impl<M: AccessMesadata> Step<Autorouter<M>, AutorouteStatus, AutorouterError, ()>
-    for AutorouteExecutionStepper
-{
+impl<M: AccessMesadata> Step<Autorouter<M>, AutorouteStatus, ()> for AutorouteExecutionStepper {
+    type Error = AutorouterError;
+
     fn step(&mut self, autorouter: &mut Autorouter<M>) -> Result<AutorouteStatus, AutorouterError> {
         let Some(curr_ratline) = self.curr_ratline else {
             return Ok(AutorouteStatus::Finished);

src/autorouter/compare_detours.rs

@@ -66,9 +66,11 @@ impl CompareDetoursExecutionStepper {
 
 // XXX: Do we really need this to be a stepper? We don't use at the moment, as sorting functions
 // aren't steppable either. It may be useful for debugging later on tho.
-impl<M: AccessMesadata> Step<Autorouter<M>, CompareDetoursStatus, AutorouterError, (f64, f64)>
+impl<M: AccessMesadata> Step<Autorouter<M>, CompareDetoursStatus, (f64, f64)>
     for CompareDetoursExecutionStepper
 {
+    type Error = AutorouterError;
+
     fn step(
         &mut self,
         autorouter: &mut Autorouter<M>,

src/autorouter/execution.rs

@@ -74,7 +74,9 @@ impl ExecutionStepper {
     }
 }
 
-impl<M: AccessMesadata> Step<Invoker<M>, InvokerStatus, InvokerError, ()> for ExecutionStepper {
+impl<M: AccessMesadata> Step<Invoker<M>, InvokerStatus, ()> for ExecutionStepper {
+    type Error = InvokerError;
+
     fn step(&mut self, invoker: &mut Invoker<M>) -> Result<InvokerStatus, InvokerError> {
         match self.step_catch_err(&mut invoker.autorouter) {
             Ok(InvokerStatus::Running) => Ok(InvokerStatus::Running),

src/bin/topola-egui/activity.rs

@@ -70,9 +70,9 @@ pub enum ActivityStepper {
     Execution(ExecutionStepper),
 }
 
-impl<M: AccessMesadata> Step<ActivityContext<'_, M>, ActivityStatus, ActivityError, ()>
-    for ActivityStepper
-{
+impl<M: AccessMesadata> Step<ActivityContext<'_, M>, ActivityStatus, ()> for ActivityStepper {
+    type Error = ActivityError;
+
     fn step(&mut self, context: &mut ActivityContext<M>) -> Result<ActivityStatus, ActivityError> {
         match self {
             ActivityStepper::Interaction(interaction) => {
@@ -152,9 +152,11 @@ impl ActivityStepperWithStatus {
     }
 }
 
-impl<M: AccessMesadata> Step<ActivityContext<'_, M>, ActivityStatus, ActivityError, ()>
+impl<M: AccessMesadata> Step<ActivityContext<'_, M>, ActivityStatus, ()>
     for ActivityStepperWithStatus
 {
+    type Error = ActivityError;
+
     fn step(&mut self, context: &mut ActivityContext<M>) -> Result<ActivityStatus, ActivityError> {
         let status = self.activity.step(context)?;
         self.maybe_status = Some(status.clone());

src/bin/topola-egui/interaction.rs

@@ -42,7 +42,9 @@ pub enum InteractionStepper {
     // - interactively moving a footprint.
 }
 
-impl Step<InteractionContext, InteractionStatus, InteractionError, ()> for InteractionStepper {
+impl Step<InteractionContext, InteractionStatus, ()> for InteractionStepper {
+    type Error = InteractionError;
+
     fn step(
         &mut self,
         context: &mut InteractionContext,

src/router/astar.rs

@@ -203,14 +203,16 @@ where
     }
 }
 
-impl<G, K, R, S: AstarStrategy<G, K, R>>
-    Step<S, AstarStatus<G, K, R>, AstarError, (K, Vec<G::NodeId>, R)> for Astar<G, K>
+impl<G, K, R, S: AstarStrategy<G, K, R>> Step<S, AstarStatus<G, K, R>, (K, Vec<G::NodeId>, R)>
+    for Astar<G, K>
 where
     G: GraphBase,
     G::NodeId: Eq + Hash,
     for<'a> &'a G: IntoEdges<NodeId = G::NodeId, EdgeId = G::EdgeId> + MakeEdgeRef,
     K: Measure + Copy,
 {
+    type Error = AstarError;
+
     fn step(&mut self, strategy: &mut S) -> Result<AstarStatus<G, K, R>, AstarError> {
         if let Some(curr_node) = self.maybe_curr_node {
             if self.is_probing {

src/router/route.rs

@@ -73,9 +73,9 @@ impl RouteStepper {
     }
 }
 
-impl<'a, R: AccessRules> Step<Router<'a, R>, RouterStatus, AstarError, BandTermsegIndex>
-    for RouteStepper
-{
+impl<'a, R: AccessRules> Step<Router<'a, R>, RouterStatus, BandTermsegIndex> for RouteStepper {
+    type Error = AstarError;
+
     fn step(&mut self, router: &mut Router<R>) -> Result<RouterStatus, AstarError> {
         let navcorder = Navcorder::new(router.layout_mut());
         let target = self.astar.graph.destination();

src/stepper.rs

@@ -1,7 +1,9 @@
-pub trait Step<C, S: TryInto<O>, E, O> {
-    fn step(&mut self, context: &mut C) -> Result<S, E>;
+pub trait Step<C, S: TryInto<O>, O> {
+    type Error;
 
-    fn finish(&mut self, context: &mut C) -> Result<O, E> {
+    fn step(&mut self, context: &mut C) -> Result<S, Self::Error>;
+
+    fn finish(&mut self, context: &mut C) -> Result<O, Self::Error> {
         loop {
             if let Ok(outcome) = self.step(context)?.try_into() {
                 return Ok(outcome);