Unable to use optional operator ? in binding rule queries

[ggiraldez]

It's not possible to use the ? operator in binding rule queries. The reason is the query engine will not create a capture key when the node is absent and this causes a panic when the graph builder attempts to evaluate the capture value.

Eg. the rule for Solidity:

@mapping [MappingType
    @key [MappingKey [MappingKeyType] @key_name [Identifier]?]
] {
  node @mapping.defs

  if some @key_name {
    node @key.def
    attr (@key.def) node_definition = @key_name
    attr (@key.def) definiens_node = @key
    edge @key.def -> @mapping.defs
  }
}

Will panic here with:

thread 'bindings_output::generated::mappings::named_parameters' panicked at /home/ggiraldez/Projects/slang/crates/metaslang/graph_builder/src/execution/lazy.rs:691:41:
no entry found for key

CAVEAT: Whether we want to fix this or not is up for debate. Because our query engine works more like unification than pattern matching, the above rule would be problematic as it will match twice if the Identifier is present: one time capturing it, and a second time without capturing it. This problem gets worse if the query contains multiple optional captures, as the engine will return matches for all possible combinations.

[ggiraldez]

This patch would fix the issue, but as stated in the caveat above, this could potentially result in a more confusing behaviour. It's also unclear to me that the resulting behaviour is useful for implementing the binding rules.

diff --git a/crates/metaslang/graph_builder/src/execution/lazy.rs b/crates/metaslang/graph_builder/src/execution/lazy.rs
index 9b589607b..8a74631ac 100644
--- a/crates/metaslang/graph_builder/src/execution/lazy.rs
+++ b/crates/metaslang/graph_builder/src/execution/lazy.rs
@@ -688,8 +688,12 @@ impl ast::Capture {
         &self,
         exec: &mut ExecutionContext<'_, '_, '_, KT>,
     ) -> Result<LazyValue, ExecutionError> {
-        let capture = &exec.mat.captures[&*self.name.0];
-        Ok(Value::from_nodes(exec.graph, capture.iter().cloned(), self.quantifier).into())
+        let value = if let Some(capture) = &exec.mat.captures.get(&*self.name.0) {
+            Value::from_nodes(exec.graph, capture.iter().cloned(), self.quantifier)
+        } else {
+            Value::from_nodes(exec.graph, std::iter::empty().into_iter(), self.quantifier)
+        };
+        Ok(value.into())
     }
 }