fix: Preserve offset for CFG edges when serializing to JSON

hugr-core/src/hugr/serialize.rs

@@ -8,6 +8,7 @@ use thiserror::Error;
 use crate::core::NodeIndex;
 use crate::hugr::Hugr;
 use crate::ops::OpType;
+use crate::types::EdgeKind;
 use crate::{Node, PortIndex};
 use portgraph::hierarchy::AttachError;
 use portgraph::{Direction, LinkError, PortView};
@@ -215,7 +216,9 @@ impl TryFrom<&Hugr> for SerHugrLatest {
             let op = hugr.get_optype(node);
             let is_value_port = offset < op.value_port_count(dir);
             let is_static_input = op.static_port(dir).is_some_and(|p| p.index() == offset);
-            let offset = (is_value_port || is_static_input).then_some(offset as u32);
+            let other_port_is_not_order = op.other_port_kind(dir) != Some(EdgeKind::StateOrder);
+            let offset = (is_value_port || is_static_input || other_port_is_not_order)
+                .then_some(offset as u32);
             (node_rekey[&node], offset)
         };
 

hugr-core/src/hugr/serialize/test.rs

@@ -5,6 +5,7 @@ use crate::builder::{
     Container, DFGBuilder, Dataflow, DataflowHugr, DataflowSubContainer, HugrBuilder,
     ModuleBuilder, endo_sig, inout_sig, test::closed_dfg_root_hugr,
 };
+use crate::envelope::{EnvelopeConfig, read_envelope, write_envelope};
 use crate::extension::ExtensionRegistry;
 use crate::extension::prelude::Noop;
 use crate::extension::prelude::{bool_t, qb_t, usize_t};
@@ -16,15 +17,21 @@ use crate::hugr::validate::ValidationError;
 use crate::hugr::views::ExtractionResult;
 use crate::ops::custom::{ExtensionOp, OpaqueOp, OpaqueOpError};
 use crate::ops::{self, DFG, Input, Module, Output, Value, dataflow::IOTrait};
+use crate::package::Package;
 use crate::std_extensions::arithmetic::float_types::float64_type;
 use crate::std_extensions::arithmetic::int_types::{ConstInt, INT_TYPES};
 use crate::std_extensions::logic::LogicOp;
+use crate::std_extensions::std_reg;
+use crate::test_file;
 use crate::types::type_param::TypeParam;
 use crate::types::{
     FuncValueType, PolyFuncType, PolyFuncTypeRV, Signature, SumType, Type, TypeArg, TypeBound,
     TypeRV,
 };
 use crate::{OutgoingPort, Visibility, type_row};
+use std::fs::File;
+use std::io::{BufReader, Cursor};
+
 use std::sync::LazyLock;
 
 use itertools::Itertools;
@@ -625,6 +632,25 @@ fn std_extensions_valid() {
     }
 }
 
+#[test]
+#[cfg_attr(miri, ignore)] // Opening files is not supported in (isolated) miri
+// https://github.com/CQCL/hugr/issues/2600
+fn cfg_edge_ordering() {
+    let pkg: Package = Package::load(
+        BufReader::new(File::open(test_file!("issue-2600.hugr")).unwrap()),
+        None,
+    )
+    .unwrap();
+    pkg.validate().unwrap();
+
+    let mut data1: Vec<u8> = Vec::new();
+    let _ = write_envelope(&mut data1, &pkg, EnvelopeConfig::text());
+
+    let buff1 = Cursor::new(data1);
+    let (_, pkg1) = read_envelope(buff1, &std_reg()).unwrap();
+    pkg1.validate().unwrap();
+}
+
 mod proptest {
     use super::check_testing_roundtrip;
     use super::{NodeSer, SimpleOpDef};

hugr-py/src/hugr/hugr/base.py

@@ -971,7 +971,7 @@ def _serialize_link(
     def _constrain_offset(self, p: P) -> PortOffset | None:
         """Constrain an offset to be a valid encoded port offset.
 
-        Order edges and control flow edges should be encoded without an offset.
+        Order edges should be encoded without an offset.
         """
         if p.offset < 0:
             assert p.offset == -1, "Only order edges are allowed with offset < 0"