feat: Add SiblingSubgraph::from_node

fixes #152

hugr-core/src/hugr/views/sibling_subgraph.rs

@@ -264,6 +264,22 @@ impl SiblingSubgraph {
         Self::try_new_with_checker(inputs, outputs, hugr, checker)
     }
 
+    /// Create a subgraph from a single node.
+    ///
+    /// `SiblingSubgraph::from_node(n,h)` is equivalent to
+    /// `SiblingSubgraph::try_from_nodes([n],h).unwrap()`.  A single node always
+    /// forms a convex subgraph, so `from_node` is infaliible.
+    ///
+    /// Panics if `node` is not present in `hugr`.
+    pub fn from_node(node: Node, hugr: &impl HugrView) -> Self {
+        // We use a custom ConvexChecker here, but once we have
+        // https://github.com/CQCL/portgraph/issues/152 we have the option
+        // to use the TopoConvexChecker. In that case this line becomes
+        // `Self::try_from_nodes([node], hugr).unwrap()`
+        Self::try_from_nodes_with_checker(vec![node], hugr, &SingleNodeConvexChecker)
+            .expect("invalid node")
+    }
+
     /// An iterator over the nodes in the subgraph.
     pub fn nodes(&self) -> &[Node] {
         &self.nodes
@@ -476,6 +492,21 @@ impl<'g, Base: HugrView> ConvexChecker for TopoConvexChecker<'g, Base> {
     }
 }
 
+struct SingleNodeConvexChecker;
+
+impl ConvexChecker for SingleNodeConvexChecker {
+    fn is_convex(
+        &self,
+        nodes: impl IntoIterator<Item = portgraph::NodeIndex>,
+        _inputs: impl IntoIterator<Item = portgraph::PortIndex>,
+        _outputs: impl IntoIterator<Item = portgraph::PortIndex>,
+    ) -> bool {
+        let nodes = nodes.into_iter().collect_vec();
+        assert_eq!(1, nodes.len());
+        true
+    }
+}
+
 /// The type of all ports in the iterator.
 ///
 /// If the array is empty or a port does not exist, returns `None`.

hugr-passes/src/const_fold.rs

@@ -185,8 +185,7 @@ fn fold_op(
         })
         .unzip();
     let replacement = const_graph(consts, reg);
-    let sibling_graph = SiblingSubgraph::try_from_nodes([op_node], hugr)
-        .expect("Operation should form valid subgraph.");
+    let sibling_graph = SiblingSubgraph::from_node(op_node, hugr);
 
     let simple_replace = SimpleReplacement::new(
         sibling_graph,

hugr-passes/src/lower.rs

@@ -67,7 +67,7 @@ pub fn lower_ops(
     replacements
         .into_iter()
         .map(|(node, replacement)| {
-            let subcirc = SiblingSubgraph::try_from_nodes([node], hugr)?;
+            let subcirc = SiblingSubgraph::from_node(node, hugr);
             let rw = subcirc.create_simple_replacement(hugr, replacement)?;
             let mut repls = hugr.apply_rewrite(rw)?;
             debug_assert_eq!(repls.len(), 1);