[rust] add a converter from concrete nodes to generic Node

rust/yarp/build.rs

@@ -154,6 +154,12 @@ fn write_node(file: &mut File, node: &Node) -> Result<(), Box<dyn std::error::Er
     writeln!(file, "}}")?;
     writeln!(file)?;
     writeln!(file, "impl<'pr> {}<'pr> {{", node.name)?;
+    writeln!(file, "    /// Converts this node to a generic node.")?;
+    writeln!(file, "    #[must_use]")?;
+    writeln!(file, "    pub fn as_node(&self) -> Node<'pr> {{")?;
+    writeln!(file, "        Node::{} {{ pointer: self.pointer, marker: PhantomData }}", node.name)?;
+    writeln!(file, "    }}")?;
+    writeln!(file)?;
     writeln!(file, "    /// Returns the location of this node.")?;
     writeln!(file, "    #[must_use]")?;
     writeln!(file, "    pub fn location(&self) -> Location<'pr> {{")?;

rust/yarp/src/lib.rs

@@ -308,4 +308,20 @@ mod tests {
 
         assert_eq!(visitor.count, 2);
     }
+
+    #[test]
+    fn node_upcast_test() {
+        use super::Node;
+
+        let source = "module Foo; end";
+        let result = parse(source.as_ref());
+
+        let node = result.node();
+        let upcast_node = node.as_program_node().unwrap().as_node();
+        assert!(matches!(upcast_node, Node::ProgramNode { .. }));
+
+        let node = node.as_program_node().unwrap().statements().body().iter().next().unwrap();
+        let upcast_node = node.as_module_node().unwrap().as_node();
+        assert!(matches!(upcast_node, Node::ModuleNode { .. }));
+    }
 }