Refactor MacroRegistry to use Rc<MacroMap>

This change introduces Rc<MacroMap> to allow for copy-on-write semantics
when defining macros in nested scopes. When a macro is defined in a
scope that is already shared, the MacroMap is cloned before
modification, ensuring that changes to one scope do not affect others
that share the same map.

src/ast/compiler/macros.rs

@@ -24,7 +24,7 @@ type MacroMap = HashMap<Rc<str>, (Vec<Rc<str>>, Node<UntypedKind>)>;
 /// A registry for macro declarations.
 #[derive(Clone)]
 pub struct MacroRegistry {
-    scopes: Vec<MacroMap>,
+    scopes: Vec<Rc<MacroMap>>,
 }
 
 impl Default for MacroRegistry {
@@ -36,12 +36,12 @@ impl Default for MacroRegistry {
 impl MacroRegistry {
     pub fn new() -> Self {
         Self {
-            scopes: vec![HashMap::new()],
+            scopes: vec![Rc::new(HashMap::new())],
         }
     }
 
     pub fn push(&mut self) {
-        self.scopes.push(HashMap::new());
+        self.scopes.push(Rc::new(HashMap::new()));
     }
 
     pub fn pop(&mut self) {
@@ -51,7 +51,9 @@ impl MacroRegistry {
     }
 
     pub fn define(&mut self, name: Rc<str>, params: Vec<Rc<str>>, body: Node<UntypedKind>) {
-        if let Some(current) = self.scopes.last_mut() {
+        if let Some(current_rc) = self.scopes.last_mut() {
+            // Copy-on-Write: If the Rc is shared, clone the map before modifying.
+            let current = Rc::make_mut(current_rc);
             current.insert(name, (params, body));
         }
     }