feat!: Replace GATs with impl Iterator returns (RPITIT) on HugrView (#1660)

This will simplify the update to the next portgraph release (which also
moved to RPITIT), avoiding the need to use boxes around iterators to be
able to name them.

- drops the dependency on `context-iterators`
- drive-by: Cleanup the implementations of `SiblingGraph` / `SiblingMut`
to avoid `collect_vec().into_iter()`s.

BREAKING CHANGE: Removed `HugrView` associated iterator types, replaced
with `impl Iterator` returns.

Author: aborgna-q

Cargo.toml

@@ -27,7 +27,6 @@ portgraph = { version = "0.12.2" }
 insta = { version = "1.34.0" }
 bitvec = "1.0.1"
 cgmath = "0.18.0"
-context-iterators = "0.2.0"
 cool_asserts = "2.0.3"
 criterion = "0.5.1"
 delegate = "0.13.0"

hugr-core/Cargo.toml

@@ -47,7 +47,6 @@ bitvec = { workspace = true, features = ["serde"] }
 enum_dispatch = { workspace = true }
 lazy_static = { workspace = true }
 petgraph = { workspace = true }
-context-iterators = { workspace = true }
 serde_json = { workspace = true }
 delegate = { workspace = true }
 paste = { workspace = true }

hugr-core/src/export.rs

@@ -69,7 +69,7 @@ impl<'a> Context<'a> {
     /// Exports the root module of the HUGR graph.
     pub fn export_root(&mut self) {
         let hugr_children = self.hugr.children(self.hugr.root());
-        let mut children = Vec::with_capacity(hugr_children.len());
+        let mut children = Vec::with_capacity(hugr_children.size_hint().0);
 
         for child in self.hugr.children(self.hugr.root()) {
             children.push(self.export_node(child));
@@ -110,7 +110,7 @@ impl<'a> Context<'a> {
         num_ports: usize,
     ) -> &'a [model::LinkRef<'a>] {
         let ports = self.hugr.node_ports(node, direction);
-        let mut links = BumpVec::with_capacity_in(ports.len(), self.bump);
+        let mut links = BumpVec::with_capacity_in(ports.size_hint().0, self.bump);
 
         for port in ports.take(num_ports) {
             links.push(model::LinkRef::Id(self.get_link_id(node, port)));
@@ -579,7 +579,7 @@ impl<'a> Context<'a> {
         let targets = self.make_ports(output_node, Direction::Incoming, output_op.types.len());
 
         // Export the remaining children of the node.
-        let mut region_children = BumpVec::with_capacity_in(children.len(), self.bump);
+        let mut region_children = BumpVec::with_capacity_in(children.size_hint().0, self.bump);
 
         for child in children {
             region_children.push(self.export_node(child));
@@ -609,7 +609,7 @@ impl<'a> Context<'a> {
     /// Creates a control flow region from the given node's children.
     pub fn export_cfg(&mut self, node: Node) -> model::RegionId {
         let mut children = self.hugr.children(node);
-        let mut region_children = BumpVec::with_capacity_in(children.len() + 1, self.bump);
+        let mut region_children = BumpVec::with_capacity_in(children.size_hint().0 + 1, self.bump);
 
         // The first child is the entry block.
         // We create a source port on the control flow region and connect it to the
@@ -623,16 +623,16 @@ impl<'a> Context<'a> {
         let source = model::LinkRef::Id(self.get_link_id(entry_block, IncomingPort::from(0)));
         region_children.push(self.export_node(entry_block));
 
-        // Export the remaining children of the node, except for the last one.
-        for _ in 0..children.len() - 1 {
-            region_children.push(self.export_node(children.next().unwrap()));
-        }
-
         // The last child is the exit block.
         // Contrary to the entry block, the exit block does not have a dataflow subgraph.
         // We therefore do not export the block itself, but simply use its output ports
         // as the target ports of the control flow region.
-        let exit_block = children.next().unwrap();
+        let exit_block = children.next_back().unwrap();
+
+        // Export the remaining children of the node, except for the last one.
+        for child in children {
+            region_children.push(self.export_node(child));
+        }
 
         let OpType::ExitBlock(_) = self.hugr.get_optype(exit_block) else {
             panic!("expected an `ExitBlock` node as the last child node");
@@ -657,7 +657,7 @@ impl<'a> Context<'a> {
     /// Export the `Case` node children of a `Conditional` node as data flow regions.
     pub fn export_conditional_regions(&mut self, node: Node) -> &'a [model::RegionId] {
         let children = self.hugr.children(node);
-        let mut regions = BumpVec::with_capacity_in(children.len(), self.bump);
+        let mut regions = BumpVec::with_capacity_in(children.size_hint().0, self.bump);
 
         for child in children {
             let OpType::Case(case_op) = self.hugr.get_optype(child) else {

hugr-core/src/extension/op_def.rs

@@ -436,6 +436,7 @@ impl OpDef {
     }
 
     /// Iterate over all miscellaneous data in the [OpDef].
+    #[allow(unused)] // Unused when no features are enabled
     pub(crate) fn iter_misc(&self) -> impl ExactSizeIterator<Item = (&str, &serde_json::Value)> {
         self.misc.iter().map(|(k, v)| (k.as_str(), v))
     }

hugr-core/src/hugr/rewrite/inline_dfg.rs

@@ -208,7 +208,7 @@ mod test {
 
         // Sanity checks
         assert_eq!(
-            outer.children(inner.node()).len(),
+            outer.children(inner.node()).count(),
             if nonlocal { 3 } else { 6 }
         ); // Input, Output, add; + const, load_const, lift
         assert_eq!(find_dfgs(&outer), vec![outer.root(), inner.node()]);
@@ -217,7 +217,7 @@ mod test {
             outer.get_parent(outer.get_parent(add).unwrap()),
             outer.get_parent(sub)
         );
-        assert_eq!(outer.nodes().len(), 11); // 6 above + inner DFG + outer (DFG + Input + Output + sub)
+        assert_eq!(outer.nodes().count(), 11); // 6 above + inner DFG + outer (DFG + Input + Output + sub)
         {
             // Check we can't inline the outer DFG
             let mut h = outer.clone();
@@ -230,7 +230,7 @@ mod test {
 
         outer.apply_rewrite(InlineDFG(*inner.handle()))?;
         outer.validate(&reg)?;
-        assert_eq!(outer.nodes().len(), 8);
+        assert_eq!(outer.nodes().count(), 8);
         assert_eq!(find_dfgs(&outer), vec![outer.root()]);
         let [_lift, add, sub] = extension_ops(&outer).try_into().unwrap();
         assert_eq!(outer.get_parent(add), Some(outer.root()));
@@ -265,8 +265,8 @@ mod test {
 
         let mut h = h.finish_hugr_with_outputs(cx.outputs(), &reg)?;
         assert_eq!(find_dfgs(&h), vec![h.root(), swap.node()]);
-        assert_eq!(h.nodes().len(), 8); // Dfg+I+O, H, CX, Dfg+I+O
-                                        // No permutation outside the swap DFG:
+        assert_eq!(h.nodes().count(), 8); // Dfg+I+O, H, CX, Dfg+I+O
+                                          // No permutation outside the swap DFG:
         assert_eq!(
             h.node_connections(p_h.node(), swap.node())
                 .collect::<Vec<_>>(),
@@ -292,7 +292,7 @@ mod test {
 
         h.apply_rewrite(InlineDFG(*swap.handle()))?;
         assert_eq!(find_dfgs(&h), vec![h.root()]);
-        assert_eq!(h.nodes().len(), 5); // Dfg+I+O
+        assert_eq!(h.nodes().count(), 5); // Dfg+I+O
         let mut ops = extension_ops(&h);
         ops.sort_by_key(|n| h.num_outputs(*n)); // Put H before CX
         let [h_gate, cx] = ops.try_into().unwrap();

hugr-core/src/hugr/rewrite/outline_cfg.rs

@@ -453,8 +453,8 @@ mod test {
         // `add_hugr_with_wires` does not return an InsertionResult, so recover the nodes manually:
         let cfg = cfg.node();
         let exit_node = h.children(cfg).nth(1).unwrap();
-        let tail = h.input_neighbours(exit_node).exactly_one().unwrap();
-        let head = h.input_neighbours(tail).exactly_one().unwrap();
+        let tail = h.input_neighbours(exit_node).exactly_one().ok().unwrap();
+        let head = h.input_neighbours(tail).exactly_one().ok().unwrap();
         // Just sanity-check we have the correct nodes
         assert!(h.get_optype(exit_node).is_exit_block());
         assert_eq!(