Separate out type-safe enforcement in NeighborTable to another commit

This reverts the changes to the coupling-map generation and the indexing
into `NeighborTable` that were designed to make it more type safe in
favour of doing that in a separate commit.

crates/accelerate/src/sabre_swap/mod.rs

@@ -148,12 +148,12 @@ fn obtain_swaps<'a>(
     layout: &'a NLayout,
 ) -> impl Iterator<Item = [VirtualQubit; 2]> + 'a {
     front_layer.iter_active().flat_map(move |&v| {
-        neighbors[v.to_phys(layout)]
+        neighbors.neighbors[v.to_phys(layout).index()]
             .iter()
-            .filter_map(move |p_neighbor| {
-                let neighbor = p_neighbor.to_virt(layout);
-                if neighbor > v || !front_layer.is_active(neighbor) {
-                    Some([v, neighbor])
+            .filter_map(move |&neighbor| {
+                let virtual_neighbor = neighbor.to_virt(layout);
+                if virtual_neighbor > v || !front_layer.is_active(virtual_neighbor) {
+                    Some([v, virtual_neighbor])
                 } else {
                     None
                 }
@@ -196,6 +196,16 @@ fn populate_extended_set(
     }
 }
 
+fn cmap_from_neighor_table(neighbor_table: &NeighborTable) -> DiGraph<(), ()> {
+    DiGraph::<(), ()>::from_edges(neighbor_table.neighbors.iter().enumerate().flat_map(
+        |(u, targets)| {
+            targets
+                .iter()
+                .map(move |v| (NodeIndex::new(u), NodeIndex::new(v.index())))
+        },
+    ))
+}
+
 /// Run sabre swap on a circuit
 ///
 /// Returns:
@@ -261,7 +271,7 @@ pub fn build_swap_map_inner(
         Some(run_in_parallel) => run_in_parallel,
         None => getenv_use_multiple_threads() && num_trials > 1,
     };
-    let coupling_graph = neighbor_table.coupling_graph();
+    let coupling_graph: DiGraph<(), ()> = cmap_from_neighor_table(neighbor_table);
     let outer_rng = match seed {
         Some(seed) => Pcg64Mcg::seed_from_u64(seed),
         None => Pcg64Mcg::from_entropy(),

crates/accelerate/src/sabre_swap/neighbor_table.rs

@@ -15,7 +15,6 @@ use ndarray::prelude::*;
 use numpy::PyReadonlyArray2;
 use pyo3::prelude::*;
 use rayon::prelude::*;
-use rustworkx_core::petgraph::prelude::*;
 
 use crate::nlayout::PhysicalQubit;
 
@@ -32,26 +31,7 @@ use crate::nlayout::PhysicalQubit;
 #[pyclass(module = "qiskit._accelerate.sabre_swap")]
 #[derive(Clone, Debug)]
 pub struct NeighborTable {
-    neighbors: Vec<Vec<PhysicalQubit>>,
-}
-
-impl NeighborTable {
-    /// Regenerate a Rust-space coupling graph from the table.
-    pub fn coupling_graph(&self) -> DiGraph<(), ()> {
-        DiGraph::from_edges(self.neighbors.iter().enumerate().flat_map(|(u, targets)| {
-            targets
-                .iter()
-                .map(move |v| (NodeIndex::new(u), NodeIndex::new(v.index())))
-        }))
-    }
-}
-
-impl std::ops::Index<PhysicalQubit> for NeighborTable {
-    type Output = [PhysicalQubit];
-
-    fn index(&self, index: PhysicalQubit) -> &Self::Output {
-        &self.neighbors[index.index()]
-    }
+    pub neighbors: Vec<Vec<PhysicalQubit>>,
 }
 
 #[pymethods]