Refactor NodeIdentity to use unique IDs

The `NodeIdentity` struct has been refactored to use a unique `id` field
generated by an atomic counter. This ensures that each `NodeIdentity`
instance is distinct, even if it has the same `SourceLocation`. The
`location` field is now optional, allowing for anonymous nodes.

This change improves the reliability of identity comparisons and
provides a more robust way to manage AST node identities.

src/ast/types.rs

@@ -14,14 +14,54 @@ pub struct SourceLocation {
     pub col: u32,
 }
 
-/// Shared identity for nodes (Location, etc.)
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+/// Shared identity for nodes. The identity is defined by the object instance (unique ID).
+/// SourceLocation is kept as optional metadata.
+#[derive(Debug, Clone)]
 pub struct NodeIdentity {
-    pub location: SourceLocation,
+    pub id: u64,
+    pub location: Option<SourceLocation>,
+}
+
+impl PartialEq for NodeIdentity {
+    fn eq(&self, other: &Self) -> bool {
+        self.id == other.id
+    }
+}
+
+impl Eq for NodeIdentity {}
+
+impl std::hash::Hash for NodeIdentity {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.id.hash(state);
+    }
 }
 
 pub type Identity = Rc<NodeIdentity>;
 
+static NODE_ID_COUNTER: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(1);
+
+impl NodeIdentity {
+    pub fn next_id() -> u64 {
+        NODE_ID_COUNTER.fetch_add(1, std::sync::atomic::Ordering::SeqCst)
+    }
+
+    /// Creates a new unique identity at the given location.
+    pub fn new(location: SourceLocation) -> Identity {
+        Rc::new(NodeIdentity {
+            id: Self::next_id(),
+            location: Some(location),
+        })
+    }
+
+    /// Creates a new unique identity without location metadata (for synthetic nodes).
+    pub fn anonymous() -> Identity {
+        Rc::new(NodeIdentity {
+            id: Self::next_id(),
+            location: None,
+        })
+    }
+}
+
 /// Interned string identifier
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct Keyword(pub u32);