refactor(xatu): retain node rule scaffolding

src/xatu/rules.rs

@@ -27,6 +27,16 @@ impl std::fmt::Display for Diagnostic {
     }
 }
 
+#[expect(dead_code, reason = "retained for planned node rules")]
+pub trait NodeRule {
+    type Node;
+
+    const ID: &'static str;
+
+    fn select(data: &ast::NodeData) -> Option<&Self::Node>;
+    fn check(node: &Self::Node) -> Option<&'static str>;
+}
+
 pub trait TokenRule {
     type Token;
 
@@ -53,6 +63,21 @@ pub fn run_rules(file: &ast::ParsedFile) -> Vec<Diagnostic> {
         .collect()
 }
 
+#[expect(dead_code, reason = "retained for planned node rules")]
+fn run_node_rule<R>(node: &ast::Node) -> Option<Finding>
+where
+    R: NodeRule,
+{
+    let typed_node = R::select(&node.data)?;
+    let message = R::check(typed_node)?;
+
+    Some(Finding {
+        rule_id: R::ID,
+        range: node.range,
+        message,
+    })
+}
+
 fn run_token_rule<R>(token: &ast::Token) -> Option<Finding>
 where
     R: TokenRule,