sim: add .dot output for Assignments graph for debugging

crates/fayalite/src/sim.rs

@@ -42,8 +42,12 @@ use bitvec::{bits, order::Lsb0, slice::BitSlice, vec::BitVec, view::BitView};
 use hashbrown::{HashMap, HashSet};
 use num_bigint::BigInt;
 use num_traits::{Signed, ToPrimitive, Zero};
-use petgraph::visit::{
+use petgraph::{
-    GraphBase, IntoNeighbors, IntoNeighborsDirected, IntoNodeIdentifiers, VisitMap, Visitable,
+    data::FromElements,
+    visit::{
+        EdgeRef, GraphBase, IntoEdgeReferences, IntoNeighbors, IntoNeighborsDirected,
+        IntoNodeIdentifiers, IntoNodeReferences, NodeRef, VisitMap, Visitable,
+    },
 };
 use std::{borrow::Cow, collections::BTreeSet, fmt, marker::PhantomData, mem, ops::IndexMut};
 
@@ -384,8 +388,8 @@ impl CompiledExpr<Array> {
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 enum AssignmentOrSlotIndex {
     AssignmentIndex(usize),
-    SmallSlots(StatePartIndex<StatePartKindSmallSlots>),
+    SmallSlot(StatePartIndex<StatePartKindSmallSlots>),
-    BigSlots(StatePartIndex<StatePartKindBigSlots>),
+    BigSlot(StatePartIndex<StatePartKindBigSlots>),
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
@@ -408,6 +412,15 @@ enum AssignmentIO {
     },
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+enum AssignmentsEdge {
+    IO(AssignmentIO),
+    AssignmentImmediatePredecessor {
+        predecessor_assignment_index: usize,
+        assignment_index: usize,
+    },
+}
+
 #[derive(Debug)]
 enum Assignments {
     Accumulating {
@@ -415,6 +428,7 @@ enum Assignments {
     },
     Finalized {
         assignments: Box<[Assignment]>,
+        slots_layout: TypeLayout<InsnsBuildingDone>,
         slot_readers: SlotToAssignmentIndexFullMap,
         slot_writers: SlotToAssignmentIndexFullMap,
         assignment_immediate_predecessors: Box<[Box<[usize]>]>,
@@ -431,13 +445,13 @@ impl Default for Assignments {
 }
 
 impl Assignments {
-    fn finalize(&mut self, slots_len: TypeLen) {
+    fn finalize(&mut self, slots_layout: TypeLayout<InsnsBuildingDone>) {
         let Self::Accumulating { assignments } = self else {
             unreachable!("already finalized");
         };
         let assignments = mem::take(assignments).into_boxed_slice();
-        let mut slot_readers = SlotToAssignmentIndexFullMap::new(slots_len);
+        let mut slot_readers = SlotToAssignmentIndexFullMap::new(slots_layout.len());
-        let mut slot_writers = SlotToAssignmentIndexFullMap::new(slots_len);
+        let mut slot_writers = SlotToAssignmentIndexFullMap::new(slots_layout.len());
         let mut assignment_immediate_predecessors = vec![BTreeSet::new(); assignments.len()];
         let mut assignment_immediate_successors = vec![BTreeSet::new(); assignments.len()];
         for (assignment_index, assignment) in assignments.iter().enumerate() {
@@ -465,6 +479,7 @@ impl Assignments {
         }
         *self = Self::Finalized {
             assignments,
+            slots_layout,
             slot_readers,
             slot_writers,
             assignment_immediate_predecessors: assignment_immediate_predecessors
@@ -489,6 +504,12 @@ impl Assignments {
         };
         assignments
     }
+    fn slots_layout(&self) -> TypeLayout<InsnsBuildingDone> {
+        let Self::Finalized { slots_layout, .. } = self else {
+            unreachable!("Assignments::finalize should have been called");
+        };
+        *slots_layout
+    }
     fn slot_readers(&self) -> &SlotToAssignmentIndexFullMap {
         let Self::Finalized { slot_readers, .. } = self else {
             unreachable!("Assignments::finalize should have been called");
@@ -521,46 +542,131 @@ impl Assignments {
         };
         assignment_immediate_successors
     }
+    fn elements(&self) -> AssignmentsElements<'_> {
+        let SlotToAssignmentIndexFullMap(TypeParts {
+            small_slots,
+            big_slots,
+        }) = self.slot_readers();
+        AssignmentsElements {
+            node_indexes: HashMap::with_capacity(
+                self.assignments().len() + small_slots.len() + big_slots.len(),
+            ),
+            nodes: self.node_references(),
+            edges: self.edge_references(),
+        }
+    }
 }
 
 impl GraphBase for Assignments {
-    type EdgeId = AssignmentIO;
+    type EdgeId = AssignmentsEdge;
     type NodeId = AssignmentOrSlotIndex;
 }
 
+#[derive(Debug, Clone, Copy)]
+enum AssignmentsNodeRef<'a> {
+    Assignment {
+        index: usize,
+        assignment: &'a Assignment,
+    },
+    SmallSlot(StatePartIndex<StatePartKindSmallSlots>, SlotDebugData),
+    BigSlot(StatePartIndex<StatePartKindBigSlots>, SlotDebugData),
+}
+
+impl<'a> NodeRef for AssignmentsNodeRef<'a> {
+    type NodeId = AssignmentOrSlotIndex;
+    type Weight = AssignmentsNodeRef<'a>;
+
+    fn id(&self) -> Self::NodeId {
+        match *self {
+            AssignmentsNodeRef::Assignment {
+                index,
+                assignment: _,
+            } => AssignmentOrSlotIndex::AssignmentIndex(index),
+            AssignmentsNodeRef::SmallSlot(slot, _) => AssignmentOrSlotIndex::SmallSlot(slot),
+            AssignmentsNodeRef::BigSlot(slot, _) => AssignmentOrSlotIndex::BigSlot(slot),
+        }
+    }
+
+    fn weight(&self) -> &Self::Weight {
+        self
+    }
+}
+
+impl<'a> petgraph::visit::Data for &'a Assignments {
+    type NodeWeight = AssignmentsNodeRef<'a>;
+    type EdgeWeight = AssignmentsEdge;
+}
+
+struct AssignmentsElements<'a> {
+    node_indexes: HashMap<AssignmentOrSlotIndex, usize>,
+    nodes: AssignmentsNodes<'a>,
+    edges: AssignmentsEdges<'a>,
+}
+
+impl<'a> Iterator for AssignmentsElements<'a> {
+    type Item = petgraph::data::Element<
+        <&'a Assignments as petgraph::visit::Data>::NodeWeight,
+        <&'a Assignments as petgraph::visit::Data>::EdgeWeight,
+    >;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let Self {
+            node_indexes,
+            nodes,
+            edges,
+        } = self;
+        if let Some(node) = nodes.next() {
+            node_indexes.insert(node.id(), node_indexes.len());
+            return Some(petgraph::data::Element::Node { weight: node });
+        }
+        let edge = edges.next()?;
+        Some(petgraph::data::Element::Edge {
+            source: node_indexes[&edge.source()],
+            target: node_indexes[&edge.target()],
+            weight: *edge.weight(),
+        })
+    }
+}
+
+#[derive(Clone)]
 struct AssignmentsNodeIdentifiers {
     assignment_indexes: std::ops::Range<usize>,
     small_slots: std::ops::Range<u32>,
     big_slots: std::ops::Range<u32>,
 }
 
-impl Iterator for AssignmentsNodeIdentifiers {
+impl AssignmentsNodeIdentifiers {
-    type Item = AssignmentOrSlotIndex;
+    fn internal_iter<'a>(&'a mut self) -> impl Iterator<Item = AssignmentOrSlotIndex> + 'a {
-    fn next(&mut self) -> Option<Self::Item> {
         let Self {
             assignment_indexes,
             small_slots,
             big_slots,
         } = self;
         assignment_indexes
-            .next()
             .map(AssignmentOrSlotIndex::AssignmentIndex)
-            .or_else(|| {
+            .chain(small_slots.map(|value| {
-                small_slots.next().map(|value| {
+                AssignmentOrSlotIndex::SmallSlot(StatePartIndex {
-                    AssignmentOrSlotIndex::SmallSlots(StatePartIndex {
+                    value,
-                        value,
+                    _phantom: PhantomData,
-                        _phantom: PhantomData,
-                    })
                 })
-            })
+            }))
-            .or_else(|| {
+            .chain(big_slots.map(|value| {
-                big_slots.next().map(|value| {
+                AssignmentOrSlotIndex::BigSlot(StatePartIndex {
-                    AssignmentOrSlotIndex::BigSlots(StatePartIndex {
+                    value,
-                        value,
+                    _phantom: PhantomData,
-                        _phantom: PhantomData,
-                    })
                 })
-            })
+            }))
+    }
+}
+
+impl Iterator for AssignmentsNodeIdentifiers {
+    type Item = AssignmentOrSlotIndex;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.internal_iter().next()
+    }
+
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.internal_iter().nth(n)
     }
 }
 
@@ -580,6 +686,44 @@ impl<'a> IntoNodeIdentifiers for &'a Assignments {
     }
 }
 
+struct AssignmentsNodes<'a> {
+    assignments: &'a Assignments,
+    nodes: AssignmentsNodeIdentifiers,
+}
+
+impl<'a> Iterator for AssignmentsNodes<'a> {
+    type Item = AssignmentsNodeRef<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.nodes.next().map(|node| match node {
+            AssignmentOrSlotIndex::AssignmentIndex(index) => AssignmentsNodeRef::Assignment {
+                index,
+                assignment: &self.assignments.assignments()[index],
+            },
+            AssignmentOrSlotIndex::SmallSlot(slot) => AssignmentsNodeRef::SmallSlot(
+                slot,
+                *self.assignments.slots_layout().small_slots.debug_data(slot),
+            ),
+            AssignmentOrSlotIndex::BigSlot(slot) => AssignmentsNodeRef::BigSlot(
+                slot,
+                *self.assignments.slots_layout().big_slots.debug_data(slot),
+            ),
+        })
+    }
+}
+
+impl<'a> IntoNodeReferences for &'a Assignments {
+    type NodeRef = AssignmentsNodeRef<'a>;
+    type NodeReferences = AssignmentsNodes<'a>;
+
+    fn node_references(self) -> Self::NodeReferences {
+        AssignmentsNodes {
+            assignments: self,
+            nodes: self.node_identifiers(),
+        }
+    }
+}
+
 struct AssignmentsNeighborsDirected<'a> {
     assignment_indexes: std::slice::Iter<'a, usize>,
     small_slots: std::collections::btree_set::Iter<'a, StatePartIndex<StatePartKindSmallSlots>>,
@@ -603,13 +747,13 @@ impl Iterator for AssignmentsNeighborsDirected<'_> {
         } else if let retval @ Some(_) = small_slots
             .next()
             .copied()
-            .map(AssignmentOrSlotIndex::SmallSlots)
+            .map(AssignmentOrSlotIndex::SmallSlot)
         {
             retval
         } else if let retval @ Some(_) = big_slots
             .next()
             .copied()
-            .map(AssignmentOrSlotIndex::BigSlots)
+            .map(AssignmentOrSlotIndex::BigSlot)
         {
             retval
         } else {
@@ -664,12 +808,12 @@ impl<'a> IntoNeighborsDirected for &'a Assignments {
                     big_slots: big_slots.iter(),
                 }
             }
-            AssignmentOrSlotIndex::SmallSlots(slot) => AssignmentsNeighborsDirected {
+            AssignmentOrSlotIndex::SmallSlot(slot) => AssignmentsNeighborsDirected {
                 assignment_indexes: slot_map[slot].iter(),
                 small_slots: Default::default(),
                 big_slots: Default::default(),
             },
-            AssignmentOrSlotIndex::BigSlots(slot) => AssignmentsNeighborsDirected {
+            AssignmentOrSlotIndex::BigSlot(slot) => AssignmentsNeighborsDirected {
                 assignment_indexes: slot_map[slot].iter(),
                 small_slots: Default::default(),
                 big_slots: Default::default(),
@@ -678,6 +822,149 @@ impl<'a> IntoNeighborsDirected for &'a Assignments {
     }
 }
 
+impl EdgeRef for AssignmentsEdge {
+    type NodeId = AssignmentOrSlotIndex;
+    type EdgeId = AssignmentsEdge;
+    type Weight = AssignmentsEdge;
+
+    fn source(&self) -> Self::NodeId {
+        match *self {
+            AssignmentsEdge::IO(AssignmentIO::BigInput {
+                assignment_index: _,
+                slot,
+            }) => AssignmentOrSlotIndex::BigSlot(slot),
+            AssignmentsEdge::IO(AssignmentIO::SmallInput {
+                assignment_index: _,
+                slot,
+            }) => AssignmentOrSlotIndex::SmallSlot(slot),
+            AssignmentsEdge::IO(AssignmentIO::BigOutput {
+                assignment_index,
+                slot: _,
+            }) => AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+            AssignmentsEdge::IO(AssignmentIO::SmallOutput {
+                assignment_index,
+                slot: _,
+            }) => AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+            AssignmentsEdge::AssignmentImmediatePredecessor {
+                predecessor_assignment_index,
+                assignment_index: _,
+            } => AssignmentOrSlotIndex::AssignmentIndex(predecessor_assignment_index),
+        }
+    }
+
+    fn target(&self) -> Self::NodeId {
+        match *self {
+            AssignmentsEdge::IO(AssignmentIO::BigInput {
+                assignment_index,
+                slot: _,
+            }) => AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+            AssignmentsEdge::IO(AssignmentIO::SmallInput {
+                assignment_index,
+                slot: _,
+            }) => AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+            AssignmentsEdge::IO(AssignmentIO::BigOutput {
+                assignment_index: _,
+                slot,
+            }) => AssignmentOrSlotIndex::BigSlot(slot),
+            AssignmentsEdge::IO(AssignmentIO::SmallOutput {
+                assignment_index: _,
+                slot,
+            }) => AssignmentOrSlotIndex::SmallSlot(slot),
+            AssignmentsEdge::AssignmentImmediatePredecessor {
+                predecessor_assignment_index: _,
+                assignment_index,
+            } => AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+        }
+    }
+
+    fn weight(&self) -> &Self::Weight {
+        self
+    }
+
+    fn id(&self) -> Self::EdgeId {
+        *self
+    }
+}
+
+struct AssignmentsEdges<'a> {
+    assignments: &'a Assignments,
+    nodes: AssignmentsNodeIdentifiers,
+    outgoing_neighbors: Option<(AssignmentOrSlotIndex, AssignmentsNeighborsDirected<'a>)>,
+}
+
+impl Iterator for AssignmentsEdges<'_> {
+    type Item = AssignmentsEdge;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if let Some((node, outgoing_neighbors)) = &mut self.outgoing_neighbors {
+                if let Some(outgoing_neighbor) = outgoing_neighbors.next() {
+                    return Some(match (*node, outgoing_neighbor) {
+                        (
+                            AssignmentOrSlotIndex::SmallSlot(_) | AssignmentOrSlotIndex::BigSlot(_),
+                            AssignmentOrSlotIndex::SmallSlot(_) | AssignmentOrSlotIndex::BigSlot(_),
+                        ) => unreachable!(),
+                        (
+                            AssignmentOrSlotIndex::AssignmentIndex(predecessor_assignment_index),
+                            AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+                        ) => AssignmentsEdge::AssignmentImmediatePredecessor {
+                            predecessor_assignment_index,
+                            assignment_index,
+                        },
+                        (
+                            AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+                            AssignmentOrSlotIndex::SmallSlot(slot),
+                        ) => AssignmentsEdge::IO(AssignmentIO::SmallOutput {
+                            assignment_index,
+                            slot,
+                        }),
+                        (
+                            AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+                            AssignmentOrSlotIndex::BigSlot(slot),
+                        ) => AssignmentsEdge::IO(AssignmentIO::BigOutput {
+                            assignment_index,
+                            slot,
+                        }),
+                        (
+                            AssignmentOrSlotIndex::SmallSlot(slot),
+                            AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+                        ) => AssignmentsEdge::IO(AssignmentIO::SmallInput {
+                            assignment_index,
+                            slot,
+                        }),
+                        (
+                            AssignmentOrSlotIndex::BigSlot(slot),
+                            AssignmentOrSlotIndex::AssignmentIndex(assignment_index),
+                        ) => AssignmentsEdge::IO(AssignmentIO::BigInput {
+                            assignment_index,
+                            slot,
+                        }),
+                    });
+                }
+            }
+            let node = self.nodes.next()?;
+            self.outgoing_neighbors = Some((
+                node,
+                self.assignments
+                    .neighbors_directed(node, petgraph::Direction::Outgoing),
+            ));
+        }
+    }
+}
+
+impl<'a> IntoEdgeReferences for &'a Assignments {
+    type EdgeRef = AssignmentsEdge;
+    type EdgeReferences = AssignmentsEdges<'a>;
+
+    fn edge_references(self) -> Self::EdgeReferences {
+        AssignmentsEdges {
+            assignments: self,
+            nodes: self.node_identifiers(),
+            outgoing_neighbors: None,
+        }
+    }
+}
+
 struct AssignmentsVisitMap {
     assignments: Vec<bool>,
     slots: DenseSlotSet,
@@ -689,8 +976,8 @@ impl VisitMap<AssignmentOrSlotIndex> for AssignmentsVisitMap {
             AssignmentOrSlotIndex::AssignmentIndex(assignment_index) => {
                 !mem::replace(&mut self.assignments[assignment_index], true)
             }
-            AssignmentOrSlotIndex::SmallSlots(slot) => self.slots.insert(slot),
+            AssignmentOrSlotIndex::SmallSlot(slot) => self.slots.insert(slot),
-            AssignmentOrSlotIndex::BigSlots(slot) => self.slots.insert(slot),
+            AssignmentOrSlotIndex::BigSlot(slot) => self.slots.insert(slot),
         }
     }
 
@@ -699,8 +986,8 @@ impl VisitMap<AssignmentOrSlotIndex> for AssignmentsVisitMap {
             AssignmentOrSlotIndex::AssignmentIndex(assignment_index) => {
                 self.assignments[assignment_index]
             }
-            AssignmentOrSlotIndex::SmallSlots(slot) => self.slots.contains(slot),
+            AssignmentOrSlotIndex::SmallSlot(slot) => self.slots.contains(slot),
-            AssignmentOrSlotIndex::BigSlots(slot) => self.slots.contains(slot),
+            AssignmentOrSlotIndex::BigSlot(slot) => self.slots.contains(slot),
         }
     }
 }
@@ -2725,12 +3012,15 @@ impl Compiler {
                 .compile_expr(instantiated_module, Expr::canonical(expr.base()))
                 .map_ty(Bundle::from_canonical)
                 .field_by_index(expr.field_index()),
-            ExprEnum::VariantAccess(variant_access) => self.compile_expr(
+            ExprEnum::VariantAccess(variant_access) => {
-                instantiated_module,
+                let start = Expr::ty(variant_access.base()).discriminant_bit_width();
-                variant_access.base().cast_to_bits()
+                let len = Expr::ty(expr).bit_width();
-                    [Expr::ty(variant_access.base()).discriminant_bit_width()..]
+                self.compile_expr(
-                    .cast_bits_to(Expr::ty(expr)),
+                    instantiated_module,
-            ),
+                    variant_access.base().cast_to_bits()[start..start + len]
+                        .cast_bits_to(Expr::ty(expr)),
+                )
+            }
             ExprEnum::ArrayIndex(expr) => self
                 .compile_expr(instantiated_module, Expr::canonical(expr.base()))
                 .map_ty(Array::from_canonical)
@@ -4146,7 +4436,13 @@ impl Compiler {
     }
     fn process_assignments(&mut self) {
         self.assignments
-            .finalize(self.insns.state_layout().len().ty);
+            .finalize(self.insns.state_layout().ty.clone().into());
+        println!(
+            "{:#?}",
+            petgraph::dot::Dot::new(&petgraph::graph::DiGraph::<_, _, usize>::from_elements(
+                self.assignments.elements()
+            ))
+        );
         let assignments_order: Vec<_> = match petgraph::algo::toposort(&self.assignments, None) {
             Ok(nodes) => nodes
                 .into_iter()
@@ -4160,11 +4456,11 @@ impl Compiler {
                     "combinatorial logic cycle detected at: {}",
                     self.assignments.assignments()[assignment_index].source_location,
                 ),
-                AssignmentOrSlotIndex::SmallSlots(slot) => panic!(
+                AssignmentOrSlotIndex::SmallSlot(slot) => panic!(
                     "combinatorial logic cycle detected through: {}",
                     self.insns.state_layout().ty.small_slots.debug_data[slot.as_usize()].name,
                 ),
-                AssignmentOrSlotIndex::BigSlots(slot) => panic!(
+                AssignmentOrSlotIndex::BigSlot(slot) => panic!(
                     "combinatorial logic cycle detected through: {}",
                     self.insns.state_layout().ty.big_slots.debug_data[slot.as_usize()].name,
                 ),

crates/fayalite/src/sim/interpreter.rs

@@ -1652,6 +1652,9 @@ impl<K: StatePartKind, BK: InsnsBuildingKind> StatePartLayout<K, BK> {
             _phantom: PhantomData,
         }
     }
+    pub(crate) fn debug_data(&self, index: StatePartIndex<K>) -> &K::DebugData {
+        &self.debug_data[index.as_usize()]
+    }
 }
 
 impl<K: StatePartKind> From<StatePartLayout<K, InsnsBuilding>>

crates/fayalite/src/sim/vcd.rs

@@ -235,6 +235,17 @@ fn write_escaped<W: io::Write>(writer: &mut W, value: impl Display) -> io::Resul
     write!(Wrapper(writer), "{value}")
 }
 
+fn is_unescaped_verilog_identifier(ident: &str) -> bool {
+    // we only allow ascii, so we can just check bytes
+    let Some((&first, rest)) = ident.as_bytes().split_first() else {
+        return false; // empty string is not an identifier
+    };
+    (first.is_ascii_alphabetic() || first == b'_')
+        && rest
+            .iter()
+            .all(|&ch| ch.is_ascii_alphanumeric() || ch == b'_' || ch == b'$')
+}
+
 fn write_vcd_var<W: io::Write>(
     writer: &mut W,
     var_type: &str,
@@ -244,7 +255,12 @@ fn write_vcd_var<W: io::Write>(
 ) -> io::Result<()> {
     write!(writer, "$var {var_type} {size} ")?;
     write_scalar_id(writer, id)?;
-    writeln!(writer, " {name} $end")
+    writer.write_all(b" ")?;
+    if !is_unescaped_verilog_identifier(name) {
+        writer.write_all(b"\\")?;
+    }
+    write_escaped(writer, name)?;
+    writer.write_all(b" $end\n")
 }
 
 impl WriteTrace for TraceUInt {

crates/fayalite/tests/sim.rs

@@ -293,5 +293,204 @@ fn test_shift_register() {
     }
 }
 
-// TODO: add tests for enums
+#[hdl_module(outline_generated)]
+pub fn enums() {
+    #[hdl]
+    let cd: ClockDomain<SyncReset> = m.input();
+    #[hdl]
+    let en: Bool = m.input();
+    #[hdl]
+    let which_in: UInt<2> = m.input();
+    #[hdl]
+    let data_in: UInt<4> = m.input();
+    #[hdl]
+    let which_out: UInt<2> = m.output();
+    #[hdl]
+    let data_out: UInt<4> = m.output();
+
+    #[hdl]
+    struct MyStruct<T> {
+        a: T,
+        b: SInt<2>,
+    }
+
+    #[hdl]
+    enum MyEnum {
+        A,
+        B((UInt<1>, Bool)),
+        C(MyStruct<Array<UInt<1>, 2>>),
+    }
+
+    #[hdl]
+    let the_reg = reg_builder().clock_domain(cd).reset(MyEnum.A());
+
+    #[hdl]
+    if en {
+        #[hdl]
+        if which_in.cmp_eq(0_hdl_u2) {
+            connect(the_reg, MyEnum.A());
+        } else if which_in.cmp_eq(1_hdl_u2) {
+            connect(the_reg, MyEnum.B((data_in[0].cast_to_static(), data_in[1])));
+        } else {
+            connect(
+                the_reg,
+                MyEnum.C(
+                    #[hdl]
+                    MyStruct {
+                        a: #[hdl]
+                        [data_in[0].cast_to_static(), data_in[1].cast_to_static()],
+                        b: data_in[2..4].cast_to_static(),
+                    },
+                ),
+            );
+        }
+    }
+
+    #[hdl]
+    match the_reg {
+        MyEnum::A => {
+            connect(which_out, 0_hdl_u2);
+            connect(data_out, 0_hdl_u4);
+        }
+        MyEnum::B(v) => {
+            connect(which_out, 1_hdl_u2);
+            connect_any(data_out, v.0 | (v.1.cast_to_static::<UInt<1>>() << 1));
+        }
+        MyEnum::C(v) => {
+            connect(which_out, 2_hdl_u2);
+            connect_any(data_out, v.cast_to_bits());
+        }
+    }
+}
+
+#[cfg(todo)] // FIXME: enum lowering currently broken
+#[hdl]
+#[test]
+fn test_enums() {
+    let _n = SourceLocation::normalize_files_for_tests();
+    let mut sim = Simulation::new(enums());
+    let mut writer = RcWriter::default();
+    sim.add_trace_writer(VcdWriterDecls::new(writer.clone()));
+    sim.write_clock(sim.io().cd.clk, false);
+    sim.write_reset(sim.io().cd.rst, true);
+    sim.write_bool(sim.io().en, false);
+    sim.write_bool_or_int(sim.io().which_in, 0_hdl_u2);
+    sim.write_bool_or_int(sim.io().data_in, 0_hdl_u4);
+    sim.advance_time(SimDuration::from_micros(1));
+    sim.write_clock(sim.io().cd.clk, true);
+    sim.advance_time(SimDuration::from_nanos(100));
+    sim.write_reset(sim.io().cd.rst, false);
+    sim.advance_time(SimDuration::from_nanos(900));
+    #[derive(Debug, PartialEq, Eq)]
+    struct IO {
+        en: bool,
+        which_in: u8,
+        data_in: u8,
+        which_out: u8,
+        data_out: u8,
+    }
+    let io_cycles = [
+        IO {
+            en: false,
+            which_in: 0,
+            data_in: 0,
+            which_out: 0,
+            data_out: 0,
+        },
+        IO {
+            en: true,
+            which_in: 1,
+            data_in: 0,
+            which_out: 0,
+            data_out: 0,
+        },
+        IO {
+            en: false,
+            which_in: 0,
+            data_in: 0,
+            which_out: 1,
+            data_out: 0,
+        },
+        IO {
+            en: true,
+            which_in: 1,
+            data_in: 0xF,
+            which_out: 1,
+            data_out: 0,
+        },
+        IO {
+            en: true,
+            which_in: 1,
+            data_in: 0xF,
+            which_out: 1,
+            data_out: 0x3,
+        },
+        IO {
+            en: true,
+            which_in: 2,
+            data_in: 0xF,
+            which_out: 1,
+            data_out: 0x3,
+        },
+        IO {
+            en: true,
+            which_in: 2,
+            data_in: 0xF,
+            which_out: 2,
+            data_out: 0xF,
+        },
+    ];
+    for (
+        cycle,
+        expected @ IO {
+            en,
+            which_in,
+            data_in,
+            which_out: _,
+            data_out: _,
+        },
+    ) in io_cycles.into_iter().enumerate()
+    {
+        sim.write_bool(sim.io().en, en);
+        sim.write_bool_or_int(sim.io().which_in, which_in.cast_to_static());
+        sim.write_bool_or_int(sim.io().data_in, data_in.cast_to_static());
+        let io = IO {
+            en,
+            which_in,
+            data_in,
+            which_out: sim
+                .read_bool_or_int(sim.io().which_out)
+                .to_bigint()
+                .try_into()
+                .expect("known to be in range"),
+            data_out: sim
+                .read_bool_or_int(sim.io().data_out)
+                .to_bigint()
+                .try_into()
+                .expect("known to be in range"),
+        };
+        assert_eq!(
+            expected,
+            io,
+            "cycle: {cycle}\nvcd:\n{}",
+            String::from_utf8(writer.take()).unwrap(),
+        );
+        sim.write_clock(sim.io().cd.clk, false);
+        sim.advance_time(SimDuration::from_micros(1));
+        sim.write_clock(sim.io().cd.clk, true);
+        sim.advance_time(SimDuration::from_micros(1));
+    }
+    sim.flush_traces().unwrap();
+    let vcd = String::from_utf8(writer.take()).unwrap();
+    println!("####### VCD:\n{vcd}\n#######");
+    if vcd != include_str!("sim/expected/enums.vcd") {
+        panic!();
+    }
+    let sim_debug = format!("{sim:#?}");
+    println!("#######\n{sim_debug}\n#######");
+    if sim_debug != include_str!("sim/expected/enums.txt") {
+        panic!();
+    }
+}
+
 // TODO: add tests for memories