fix: allow disconnected outputs in SiblingSubgraph::from_node

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

@@ -277,9 +277,6 @@ impl SiblingSubgraph {
     ///
     /// The subgraph signature will be given by signature of the node.
     pub fn from_node(node: Node, hugr: &impl HugrView) -> Self {
-        // TODO once https://github.com/CQCL/portgraph/issues/155
-        // is fixed we can just call try_from_nodes here.
-        // Until then, doing this saves a lot of work.
         let nodes = vec![node];
         let inputs = hugr
             .node_inputs(node)
@@ -288,7 +285,17 @@ impl SiblingSubgraph {
             .collect_vec();
         let outputs = hugr
             .node_outputs(node)
-            .filter_map(|p| hugr.is_linked(node, p).then_some((node, p)))
+            .filter_map(|p| {
+                // accept linked outputs or unlinked value outputs
+                {
+                    hugr.is_linked(node, p)
+                        || hugr
+                            .get_optype(node)
+                            .port_kind(p)
+                            .is_some_and(|k| k.is_value())
+                }
+                .then_some((node, p))
+            })
             .collect_vec();
 
         Self {
@@ -785,9 +792,11 @@ mod tests {
     use cool_asserts::assert_matches;
 
     use crate::builder::inout_sig;
+    use crate::hugr::Rewrite;
     use crate::ops::Const;
     use crate::std_extensions::arithmetic::float_types::{self, ConstF64};
     use crate::std_extensions::logic::{self, LogicOp};
+    use crate::type_row;
     use crate::utils::test_quantum_extension::{self, cx_gate, rz_f64};
     use crate::{
         builder::{
@@ -1155,4 +1164,38 @@ mod tests {
         let subg = SiblingSubgraph::try_new_dataflow_subgraph(&view).unwrap();
         assert_eq!(subg.nodes().len(), 2);
     }
+
+    #[test]
+    fn test_unconnected() {
+        // test a replacement on a subgraph with a discarded output
+        let mut b = DFGBuilder::new(
+            Signature::new(bool_t(), type_row![])
+                // .with_prelude()
+                .with_extension_delta(crate::std_extensions::logic::EXTENSION_ID),
+        )
+        .unwrap();
+        let inw = b.input_wires().exactly_one().unwrap();
+        let not_n = b.add_dataflow_op(LogicOp::Not, [inw]).unwrap();
+        // Unconnected output, discarded
+        let mut h = b.finish_hugr_with_outputs([]).unwrap();
+
+        let subg = SiblingSubgraph::from_node(not_n.node(), &h);
+
+        assert_eq!(subg.nodes().len(), 1);
+        //  TODO create a valid replacement
+        let replacement = {
+            let mut rep_b = DFGBuilder::new(
+                Signature::new_endo(bool_t())
+                    .with_extension_delta(crate::std_extensions::logic::EXTENSION_ID),
+            )
+            .unwrap();
+            let inw = rep_b.input_wires().exactly_one().unwrap();
+
+            let not_n = rep_b.add_dataflow_op(LogicOp::Not, [inw]).unwrap();
+
+            rep_b.finish_hugr_with_outputs(not_n.outputs()).unwrap()
+        };
+        let rep = subg.create_simple_replacement(&h, replacement).unwrap();
+        rep.apply(&mut h).unwrap();
+    }
 }

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);

hugr/Cargo.toml

@@ -44,4 +44,3 @@ bumpalo = { workspace = true, features = ["collections"] }
 [[bench]]
 name = "bench_main"
 harness = false
-