feat!: Infer extension deltas for Case, Cfg, Conditional, DataflowBlo…

…ck, Dfg, TailLoop (#1195)

* Add an ExtensionId TO_BE_INFERRED
* On Case, Cfg, Conditional, DataflowBlock, Dfg, TailLoop,
`Hugr::infer_extensions` replaces TO_BE_INFERRED with the smallest set
of ExtensionIds that's correct wrt. its child nodes (i.e., the union of
the child node deltas). If there are other ExtensionIds alongside
TO_BE_INFERRED these will be kept (allowing a "lower bound" to be
specified for individual nodes)
* `Hugr::infer_extensions` also takes a `remove: bool` which, if true,
modifies deltas of the same OpTypes that do *not* have TO_BE_INFERRED,
by removing ExtensionIds. (Thus, non-inferred deltas act as upper
bounds).

Relates to #640 but does not address the builder where this should be
the default behaviour.

BREAKING CHANGE: ExtensionError moved from hugr::validate to hugr;
Hugr::infer_extensions takes bool parameter

hugr-core/src/extension.rs

@@ -405,6 +405,13 @@ pub enum ExtensionBuildError {
 #[derive(Clone, Debug, Default, Hash, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub struct ExtensionSet(BTreeSet<ExtensionId>);
 
+/// A special ExtensionId which indicates that the delta of a non-Function
+/// container node should be computed by extension inference. See [`infer_extensions`]
+/// which lists the container nodes to which this can be applied.
+///
+/// [`infer_extensions`]: crate::hugr::Hugr::infer_extensions
+pub const TO_BE_INFERRED: ExtensionId = ExtensionId::new_unchecked(".TO_BE_INFERRED");
+
 impl ExtensionSet {
     /// Creates a new empty extension set.
     pub const fn new() -> Self {

hugr-core/src/hugr.rs

@@ -13,7 +13,6 @@ use std::collections::VecDeque;
 use std::iter;
 
 pub(crate) use self::hugrmut::HugrMut;
-use self::validate::ExtensionError;
 pub use self::validate::ValidationError;
 
 pub use ident::{IdentList, InvalidIdentifier};
@@ -25,9 +24,9 @@ use thiserror::Error;
 
 pub use self::views::{HugrView, RootTagged};
 use crate::core::NodeIndex;
-use crate::extension::ExtensionRegistry;
+use crate::extension::{ExtensionRegistry, ExtensionSet, TO_BE_INFERRED};
 use crate::ops::custom::resolve_extension_ops;
-use crate::ops::OpTag;
+use crate::ops::{OpTag, OpTrait};
 pub use crate::ops::{OpType, DEFAULT_OPTYPE};
 use crate::{Direction, Node};
 
@@ -92,15 +91,80 @@ impl Hugr {
         self.validate_no_extensions(extension_registry)?;
         #[cfg(feature = "extension_inference")]
         {
-            self.infer_extensions()?;
+            self.infer_extensions(false)?;
             self.validate_extensions()?;
         }
         Ok(())
     }
 
-    /// Leaving this here as in the future we plan for it to infer deltas
-    /// of container nodes e.g. [OpType::DFG]. For the moment it does nothing.
-    pub fn infer_extensions(&mut self) -> Result<(), ExtensionError> {
+    /// Infers an extension-delta for any non-function container node
+    /// whose current [extension_delta] contains [TO_BE_INFERRED]. The inferred delta
+    /// will be the smallest delta compatible with its children and that includes any
+    /// other [ExtensionId]s in the current delta.
+    ///
+    /// If `remove` is true, for such container nodes *without* [TO_BE_INFERRED],
+    /// ExtensionIds are removed from the delta if they are *not* used by any child node.
+    ///
+    /// The non-function container nodes are:
+    /// [Case], [CFG], [Conditional], [DataflowBlock], [DFG], [TailLoop]
+    ///
+    /// [Case]: crate::ops::Case
+    /// [CFG]: crate::ops::CFG
+    /// [Conditional]: crate::ops::Conditional
+    /// [DataflowBlock]: crate::ops::DataflowBlock
+    /// [DFG]: crate::ops::DFG
+    /// [TailLoop]: crate::ops::TailLoop
+    /// [extension_delta]: crate::ops::OpType::extension_delta
+    /// [ExtensionId]: crate::extension::ExtensionId
+    pub fn infer_extensions(&mut self, remove: bool) -> Result<(), ExtensionError> {
+        fn delta_mut(optype: &mut OpType) -> Option<&mut ExtensionSet> {
+            match optype {
+                OpType::DFG(dfg) => Some(&mut dfg.signature.extension_reqs),
+                OpType::DataflowBlock(dfb) => Some(&mut dfb.extension_delta),
+                OpType::TailLoop(tl) => Some(&mut tl.extension_delta),
+                OpType::CFG(cfg) => Some(&mut cfg.signature.extension_reqs),
+                OpType::Conditional(c) => Some(&mut c.extension_delta),
+                OpType::Case(c) => Some(&mut c.signature.extension_reqs),
+                //OpType::Lift(_) // Not ATM: only a single element, and we expect Lift to be removed
+                //OpType::FuncDefn(_) // Not at present due to the possibility of recursion
+                _ => None,
+            }
+        }
+        fn infer(h: &mut Hugr, node: Node, remove: bool) -> Result<ExtensionSet, ExtensionError> {
+            let mut child_sets = h
+                .children(node)
+                .collect::<Vec<_>>() // Avoid borrowing h over recursive call
+                .into_iter()
+                .map(|ch| Ok((ch, infer(h, ch, remove)?)))
+                .collect::<Result<Vec<_>, _>>()?;
+
+            let Some(es) = delta_mut(h.op_types.get_mut(node.pg_index())) else {
+                return Ok(h.get_optype(node).extension_delta());
+            };
+            if es.contains(&TO_BE_INFERRED) {
+                // Do not remove anything from current delta - any other elements are a lower bound
+                child_sets.push((node, es.clone())); // "child_sets" now misnamed but we discard fst
+            } else if remove {
+                child_sets.iter().try_for_each(|(ch, ch_exts)| {
+                    if !es.is_superset(ch_exts) {
+                        return Err(ExtensionError {
+                            parent: node,
+                            parent_extensions: es.clone(),
+                            child: *ch,
+                            child_extensions: ch_exts.clone(),
+                        });
+                    }
+                    Ok(())
+                })?;
+            } else {
+                return Ok(es.clone()); // Can't neither add nor remove, so nothing to do
+            }
+            let merged = ExtensionSet::union_over(child_sets.into_iter().map(|(_, e)| e));
+            *es = ExtensionSet::singleton(&TO_BE_INFERRED).missing_from(&merged);
+
+            Ok(es.clone())
+        }
+        infer(self, self.root(), remove)?;
         Ok(())
     }
 }
@@ -203,6 +267,16 @@ impl Hugr {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Error)]
+#[error("Parent node {parent} has extensions {parent_extensions} that are too restrictive for child node {child}, they must include child extensions {child_extensions}")]
+/// An error in the extension deltas.
+pub struct ExtensionError {
+    parent: Node,
+    parent_extensions: ExtensionSet,
+    child: Node,
+    child_extensions: ExtensionSet,
+}
+
 /// Errors that can occur while manipulating a Hugr.
 ///
 /// TODO: Better descriptions, not just re-exporting portgraph errors.
@@ -221,7 +295,14 @@ pub enum HugrError {
 
 #[cfg(test)]
 mod test {
-    use super::{Hugr, HugrView};
+    use super::internal::HugrMutInternals;
+    #[cfg(feature = "extension_inference")]
+    use super::ValidationError;
+    use super::{ExtensionError, Hugr, HugrMut, HugrView, Node};
+    use crate::extension::{ExtensionId, ExtensionSet, EMPTY_REG, TO_BE_INFERRED};
+    use crate::types::{FunctionType, Type};
+    use crate::{const_extension_ids, ops, type_row};
+    use rstest::rstest;
 
     #[test]
     fn impls_send_and_sync() {
@@ -240,4 +321,167 @@ mod test {
         let hugr = simple_dfg_hugr();
         assert_matches!(hugr.get_io(hugr.root()), Some(_));
     }
+
+    const_extension_ids! {
+        const XA: ExtensionId = "EXT_A";
+        const XB: ExtensionId = "EXT_B";
+    }
+
+    #[rstest]
+    #[case([], XA.into())]
+    #[case([XA], XA.into())]
+    #[case([XB], ExtensionSet::from_iter([XA, XB]))]
+
+    fn infer_single_delta(
+        #[case] parent: impl IntoIterator<Item = ExtensionId>,
+        #[values(true, false)] remove: bool, // makes no difference when inferring
+        #[case] result: ExtensionSet,
+    ) {
+        let parent = ExtensionSet::from_iter(parent).union(TO_BE_INFERRED.into());
+        let (mut h, _) = build_ext_dfg(parent);
+        h.infer_extensions(remove).unwrap();
+        assert_eq!(h, build_ext_dfg(result).0);
+    }
+
+    #[test]
+    fn infer_removes_from_delta() {
+        let parent = ExtensionSet::from_iter([XA, XB]);
+        let mut h = build_ext_dfg(parent.clone()).0;
+        let backup = h.clone();
+        h.infer_extensions(false).unwrap();
+        assert_eq!(h, backup); // did nothing
+        h.infer_extensions(true).unwrap();
+        assert_eq!(h, build_ext_dfg(XA.into()).0);
+    }
+
+    #[test]
+    fn infer_bad_remove() {
+        let (mut h, mid) = build_ext_dfg(XB.into());
+        let backup = h.clone();
+        h.infer_extensions(false).unwrap();
+        assert_eq!(h, backup); // did nothing
+        let val_res = h.validate(&EMPTY_REG);
+        let expected_err = ExtensionError {
+            parent: h.root(),
+            parent_extensions: XB.into(),
+            child: mid,
+            child_extensions: XA.into(),
+        };
+        #[cfg(feature = "extension_inference")]
+        assert_eq!(
+            val_res,
+            Err(ValidationError::ExtensionError(expected_err.clone()))
+        );
+        #[cfg(not(feature = "extension_inference"))]
+        assert!(val_res.is_ok());
+
+        let inf_res = h.infer_extensions(true);
+        assert_eq!(inf_res, Err(expected_err));
+    }
+
+    fn build_ext_dfg(parent: ExtensionSet) -> (Hugr, Node) {
+        let ty = Type::new_function(FunctionType::new_endo(type_row![]));
+        let mut h = Hugr::new(ops::DFG {
+            signature: FunctionType::new_endo(ty.clone()).with_extension_delta(parent.clone()),
+        });
+        let root = h.root();
+        let mid = add_inliftout(&mut h, root, ty);
+        (h, mid)
+    }
+
+    fn add_inliftout(h: &mut Hugr, p: Node, ty: Type) -> Node {
+        let inp = h.add_node_with_parent(
+            p,
+            ops::Input {
+                types: ty.clone().into(),
+            },
+        );
+        let out = h.add_node_with_parent(
+            p,
+            ops::Output {
+                types: ty.clone().into(),
+            },
+        );
+        let mid = h.add_node_with_parent(
+            p,
+            ops::Lift {
+                type_row: ty.into(),
+                new_extension: XA,
+            },
+        );
+        h.connect(inp, 0, mid, 0);
+        h.connect(mid, 0, out, 0);
+        mid
+    }
+
+    #[rstest]
+    // Base case success: delta inferred for parent equals grandparent.
+    #[case([XA], [TO_BE_INFERRED], true, [XA])]
+    // Success: delta inferred for parent is subset of grandparent
+    #[case([XA, XB], [TO_BE_INFERRED], true, [XA])]
+    // Base case failure: infers [XA] for parent but grandparent has disjoint set
+    #[case([XB], [TO_BE_INFERRED], false, [XA])]
+    // Failure: as previous, but extra "lower bound" on parent that has no effect
+    #[case([XB], [XA, TO_BE_INFERRED], false, [XA])]
+    // Failure: grandparent ok wrt. child but parent specifies extra lower-bound XB
+    #[case([XA], [XB, TO_BE_INFERRED], false, [XA, XB])]
+    // Success: grandparent includes extra XB required for parent's "lower bound"
+    #[case([XA, XB], [XB, TO_BE_INFERRED], true, [XA, XB])]
+    // Success: grandparent is also inferred so can include 'extra' XB from parent
+    #[case([TO_BE_INFERRED], [TO_BE_INFERRED, XB], true, [XA, XB])]
+    // No inference: extraneous XB in parent is removed so all become [XA].
+    #[case([XA], [XA, XB], true, [XA])]
+    fn infer_three_generations(
+        #[case] grandparent: impl IntoIterator<Item = ExtensionId>,
+        #[case] parent: impl IntoIterator<Item = ExtensionId>,
+        #[case] success: bool,
+        #[case] result: impl IntoIterator<Item = ExtensionId>,
+    ) {
+        let ty = Type::new_function(FunctionType::new_endo(type_row![]));
+        let grandparent = ExtensionSet::from_iter(grandparent);
+        let result = ExtensionSet::from_iter(result);
+        let root_ty = ops::Conditional {
+            sum_rows: vec![type_row![]],
+            other_inputs: ty.clone().into(),
+            outputs: ty.clone().into(),
+            extension_delta: grandparent.clone(),
+        };
+        let mut h = Hugr::new(root_ty.clone());
+        let p = h.add_node_with_parent(
+            h.root(),
+            ops::Case {
+                signature: FunctionType::new_endo(ty.clone())
+                    .with_extension_delta(ExtensionSet::from_iter(parent)),
+            },
+        );
+        add_inliftout(&mut h, p, ty.clone());
+        assert!(h.validate_extensions().is_err());
+        let backup = h.clone();
+        let inf_res = h.infer_extensions(true);
+        if success {
+            assert!(inf_res.is_ok());
+            let expected_p = ops::Case {
+                signature: FunctionType::new_endo(ty).with_extension_delta(result.clone()),
+            };
+            let mut expected = backup;
+            expected.replace_op(p, expected_p).unwrap();
+            let expected_gp = ops::Conditional {
+                extension_delta: result,
+                ..root_ty
+            };
+            expected.replace_op(h.root(), expected_gp).unwrap();
+
+            assert_eq!(h, expected);
+        } else {
+            assert_eq!(
+                inf_res,
+                Err(ExtensionError {
+                    parent: h.root(),
+                    parent_extensions: grandparent,
+                    child: p,
+                    child_extensions: result
+                })
+            );
+        }
+    }
 }

hugr-core/src/hugr/validate.rs

@@ -9,7 +9,7 @@ use petgraph::visit::{Topo, Walker};
 use portgraph::{LinkView, PortView};
 use thiserror::Error;
 
-use crate::extension::{ExtensionRegistry, ExtensionSet, SignatureError};
+use crate::extension::{ExtensionRegistry, SignatureError, TO_BE_INFERRED};
 
 use crate::ops::custom::{resolve_opaque_op, CustomOp, CustomOpError};
 use crate::ops::validate::{ChildrenEdgeData, ChildrenValidationError, EdgeValidationError};
@@ -19,6 +19,7 @@ use crate::types::{EdgeKind, FunctionType};
 use crate::{Direction, Hugr, Node, Port};
 
 use super::views::{HierarchyView, HugrView, SiblingGraph};
+use super::ExtensionError;
 
 /// Structure keeping track of pre-computed information used in the validation
 /// process.
@@ -59,6 +60,9 @@ impl Hugr {
     pub fn validate_extensions(&self) -> Result<(), ValidationError> {
         for parent in self.nodes() {
             let parent_op = self.get_optype(parent);
+            if parent_op.extension_delta().contains(&TO_BE_INFERRED) {
+                return Err(ValidationError::ExtensionsNotInferred { node: parent });
+            }
             let parent_extensions = match parent_op.inner_function_type() {
                 Some(FunctionType { extension_reqs, .. }) => extension_reqs,
                 None => match parent_op.tag() {
@@ -743,6 +747,9 @@ pub enum ValidationError {
     /// There are errors in the extension deltas.
     #[error(transparent)]
     ExtensionError(#[from] ExtensionError),
+    /// A node claims to still be awaiting extension inference. Perhaps it is not acted upon by inference.
+    #[error("Node {node:?} needs a concrete ExtensionSet - inference will provide this for Case/CFG/Conditional/DataflowBlock/DFG/TailLoop only")]
+    ExtensionsNotInferred { node: Node },
     /// Error in a node signature
     #[error("Error in signature of node {node:?}: {cause}")]
     SignatureError { node: Node, cause: SignatureError },
@@ -815,15 +822,5 @@ pub enum InterGraphEdgeError {
     },
 }
 
-#[derive(Debug, Clone, PartialEq, Error)]
-#[error("Parent node {parent} has extensions {parent_extensions} that are too restrictive for child node {child}, they must include child extensions {child_extensions}")]
-/// An error in the extension deltas.
-pub struct ExtensionError {
-    parent: Node,
-    parent_extensions: ExtensionSet,
-    child: Node,
-    child_extensions: ExtensionSet,
-}
-
 #[cfg(test)]
 pub(crate) mod test;