refactor: Don't mutate arena by default in Rewriting Visitor

crates/polars-plan/src/plans/optimizer/cse/cse_lp.rs

@@ -364,6 +364,7 @@ pub(crate) fn elim_cmn_subplans(
 
         lp_node.visit(&mut visitor, arena).map(|_| ()).unwrap();
 
+        let lp_node = IRNode::new_mutate(root);
         let mut rewriter = CommonSubPlanRewriter::new(&sp_count, &id_array);
         lp_node.rewrite(&mut rewriter, arena).unwrap();
 

crates/polars-plan/src/plans/optimizer/mod.rs

@@ -236,7 +236,7 @@ More information on the new streaming engine: https://github.com/pola-rs/polars/
     #[cfg(feature = "cse")]
     if comm_subexpr_elim && !members.has_ext_context {
         let mut optimizer = CommonSubExprOptimizer::new();
-        let alp_node = IRNode::new(lp_top);
+        let alp_node = IRNode::new_mutate(lp_top);
 
         lp_top = try_with_ir_arena(lp_arena, expr_arena, |arena| {
             let rewritten = alp_node.rewrite(&mut optimizer, arena)?;

crates/polars-plan/src/plans/visitor/lp.rs

@@ -6,11 +6,21 @@ use crate::prelude::*;
 #[derive(Copy, Clone, Debug)]
 pub struct IRNode {
     node: Node,
+    // Whether it may mutate the arena on rewrite.
+    // If set the Rewriting Treewalker will mutate the arena.
+    mutate: bool,
 }
 
 impl IRNode {
     pub fn new(node: Node) -> Self {
-        Self { node }
+        Self {
+            node,
+            mutate: false,
+        }
+    }
+
+    pub fn new_mutate(node: Node) -> Self {
+        Self { node, mutate: true }
     }
 
     pub fn node(&self) -> Node {
@@ -85,8 +95,13 @@ impl TreeWalker for IRNode {
         }
         let lp = arena.0.get(self.node);
         let lp = lp.with_exprs_and_input(exprs, inputs);
-        arena.0.replace(self.node, lp);
-        Ok(self)
+        if self.mutate {
+            arena.0.replace(self.node, lp);
+            Ok(self)
+        } else {
+            let node = arena.0.add(lp);
+            Ok(IRNode::new_mutate(node))
+        }
     }
 }