Name the shapes the record lowering refuses

node-graph/node-macro/src/validation.rs

@@ -15,6 +15,7 @@ pub fn validate_node_fn(parsed: &ParsedNodeFn) -> syn::Result<()> {
 		validate_lend_fields,
 		validate_record_io,
 		validate_lazy_reads,
+		validate_lowering_supported,
 	];
 
 	for validator in validators {
@@ -24,6 +25,28 @@ pub fn validate_node_fn(parsed: &ParsedNodeFn) -> syn::Result<()> {
 	Ok(())
 }
 
+/// A signature no lowering claims generates no `Node` impl at all, so the node
+/// compiles and is simply missing at runtime. Naming the gap is the only thing
+/// that ends it, since nothing downstream can tell "declined" from "absent".
+fn validate_lowering_supported(parsed: &ParsedNodeFn) {
+	// A record-io refusal already reported its own reason.
+	if crate::codegen::classify::record_shape_checked(parsed).is_err() || crate::codegen::classify::analyze(parsed).is_some() {
+		return;
+	}
+
+	// An async kernel with lazy inputs is refused by `validate_async_source`, which
+	// names the spawn boundary rather than the missing lowering.
+	if parsed.is_async && parsed.fields.iter().any(|field| matches!(field.ty, ParsedFieldType::Node(_))) {
+		return;
+	}
+
+	emit_error!(
+		parsed.fn_name.span(),
+		"no lowering supports this signature, so the node would generate no `Node` impl";
+		help = "a node is one of: attribute io (`Attr<..>` reads or writes), routing (an unbounded generic forwarded whole), a flipped kernel over an owned primary, or one with a ranked `IList` input"
+	);
+}
+
 fn validate_record_io(parsed: &ParsedNodeFn) {
 	let value = crate::codegen::slot_value_type(&parsed.output_type);
 	if let Type::Tuple(tuple) = &value {
@@ -44,6 +67,15 @@ fn validate_record_io(parsed: &ParsedNodeFn) {
 		return;
 	}
 
+	// A shape the record lowering refuses generates no node impl at all, so without a
+	// diagnostic the node compiles and is simply absent from the registry. The lowering
+	// is the authority on what it can serve, so its refusal is the one error reported:
+	// the checks below describe a shape it already accepted.
+	if let Err(reason) = crate::codegen::classify::record_shape_checked(parsed) {
+		emit_error!(parsed.fn_name.span(), "this node declares attribute io the record lowering cannot serve: {}", reason);
+		return;
+	}
+
 	let async_source = crate::codegen::classify::is_async_source(parsed);
 	if async_source && has_reads {
 		emit_error!(parsed.fn_name.span(), "attribute reads are not supported on async source kernels, only writes");