Extension Inference for functions relies on builder

[acl-cqc]

When defining a function, the builder fixes solutions for the extensions of the Input and Output child of the FuncDefn:

hugr/src/builder/build_traits.rs

Lines 103 to 107 in 762839d

	let db = DFGBuilder::create_with_io(
	self.hugr_mut(),
	f_node,
	signature.signature,
	Some(signature.input_extensions),

(note last line)

hugr/src/builder/dataflow.rs

Lines 53 to 58 in 41e15da

	.add_node_with_parent(parent, NodeType::new(input, input_extensions.clone()))?;
	base.as_mut().add_node_with_parent(
	parent,
	NodeType::new(
	output,
	input_extensions.map(|inp| inp.union(&signature.extension_reqs)),

(note the two input_extensions)

The relationship between Input and Output is not modelled by inference - they should have a Plus constraint, but Plus constraints are only generated here:

hugr/src/extension/infer.rs

Lines 317 to 330 in 41e15da

	match node_type.io_extensions() {
	// Input extensions are open
	None => {
	let c = if let Some(sig) = node_type.op_signature() {
	let delta = sig.extension_reqs;
	if delta.is_empty() {
	Constraint::Equal(m_input)
	} else {
	Constraint::Plus(delta, m_input)
	}
	} else {
	Constraint::Equal(m_input)
	};
	self.add_constraint(m_output, c);

(note this only applies if the node has an op_signature, which FuncDefn doesn't, and only if extension-annotations need to be inferred; if they are pre-specified, inference just accepts them)....and neither is it checked by validation, as the only checks on Input/Output are done here:

hugr/src/hugr/validate.rs

Lines 205 to 212 in 41e15da

	if node_type.tag() != OpTag::FuncDefn {
	// If this is a container with I/O nodes, check that the extension they
	// define match the extensions of the container.
	if let Some([input, output]) = self.hugr.get_io(node) {
	self.extension_validator
	.validate_io_extensions(node, input, output)?;
	}
	}

(note FuncDefns are explicitly excluded, because validate_io_children would use the wrong delta for them).

Thus, it should be possible to construct a Hugr (not using the builder) where the body of the function adds extension-requirements not specified in the function delta, and to get this past validation (possibly past inference too, but otherwise by hand-specifying extensions). Step 1, obviously, produce that test case ;-)

[acl-cqc]

May be best to do this after #692 where the builder changes to pass None (no extensions specified) to create_with_io

[acl-cqc]

I think we can say this was done in #1142 / #1183