Fix: Correctly determine lambda purity

The purity of a lambda definition should always be `Pure`. The purity of
the lambda's body is only relevant when the lambda is called. This
change ensures that defining a lambda does not incorrectly mark the
parent scope as impure.

src/ast/compiler/optimizer.rs

@@ -691,7 +691,8 @@ impl Optimizer {
     fn purity_of(&self, node: &TypedNode) -> Purity {
         match &node.kind {
             BoundKind::Constant(_) | BoundKind::Parameter { .. } | BoundKind::Nop => Purity::Pure,
-            BoundKind::Lambda { body, .. } => self.purity_of(body),
+            // Defining a lambda is pure; only calling it may have side effects.
+            BoundKind::Lambda { .. } => Purity::Pure,
             BoundKind::Get { addr, .. } => match addr {
                 Address::Global(idx) => {
                     if let Some(purity_rc) = &self.global_purity {
@@ -734,6 +735,7 @@ impl Optimizer {
             BoundKind::Set { .. } => Purity::Impure,
             BoundKind::Call { callee, args } => {
                 let callee_purity = match &callee.kind {
+                    BoundKind::Lambda { body, .. } => self.purity_of(body),
                     BoundKind::Get {
                         addr: Address::Global(idx),
                         ..
@@ -1365,4 +1367,19 @@ mod tests {
         );
         assert!(dump.contains("Constant: 42"));
     }
+
+    #[test]
+    fn test_opt_unused_lambda_with_side_effects_dce() {
+        // Die Lambda-Definition selbst ist Pure, auch wenn ihr Body Impure ist (assign).
+        // Daher muss sie gelöscht werden, wenn sie nicht gerufen wird.
+        // 'g' muss definiert sein, damit der Binder nicht abbricht.
+        let source = "(fn [] (do (def g 0) (def f (fn [x] (assign g 1))) 42))";
+        let dump = get_optimized_dump(source, true);
+        assert!(
+            !dump.contains("DefLocal (Name: 'f'"),
+            "Unused lambda with side effects should be removed by DCE. Dump: \n{}",
+            dump
+        );
+        assert!(dump.contains("Constant: 42"));
+    }
 }