ast: factor out as_node

Shrink another macro.

src/ast.rs

@@ -97,7 +97,7 @@ fn accept_mut(&mut self, visitor: &mut dyn NodeVisitorMut, reversed: bool) {
 }
 
 /// Node is the base trait of all AST nodes.
-pub trait Node: Acceptor + ConcreteNode + std::fmt::Debug {
+pub trait Node: Acceptor + ConcreteNode + AsNode + std::fmt::Debug {
     /// The type of this node.
     fn typ(&self) -> Type;
 
@@ -154,11 +154,19 @@ fn source<'s>(&self, orig: &'s wstr) -> &'s wstr {
     fn self_memory_size(&self) -> usize {
         std::mem::size_of_val(self)
     }
+}
 
-    /// Convert to the dynamic Node type.
+// Convert to the dynamic Node type.
+pub trait AsNode {
     fn as_node(&self) -> &dyn Node;
 }
 
+impl<T: Node + Sized> AsNode for T {
+    fn as_node(&self) -> &dyn Node {
+        self
+    }
+}
+
 /// Return true if two nodes are the same object.
 #[inline(always)]
 pub fn is_same_node(lhs: &dyn Node, rhs: &dyn Node) -> bool {
@@ -519,9 +527,6 @@ impl Node for $name {
             fn typ(&self) -> Type {
                 Type::$type
             }
-            fn as_node(&self) -> &dyn Node {
-                self
-            }
         }
     };
 }