ArrayGet and Set are not pure

compiler/noirc_evaluator/src/ssa/ir/instruction.rs

@@ -254,7 +254,7 @@ impl Instruction {
                 // In ACIR, a division with a false predicate outputs (0,0), so it cannot replace another instruction unless they have the same predicate
                 bin.operator != BinaryOp::Div
             }
-            Cast(_, _) | Truncate { .. } | Not(_) | ArrayGet { .. } | ArraySet { .. } => true,
+            Cast(_, _) | Truncate { .. } | Not(_) => true,
 
             // These either have side-effects or interact with memory
             Constrain(..)
@@ -266,6 +266,12 @@ impl Instruction {
             | DecrementRc { .. }
             | RangeCheck { .. } => false,
 
+            // These can have different behavior depending on the EnableSideEffectsIf context.
+            // Enabling constant folding for these potentially enables replacing an enabled
+            // array get with one that was disabled. See
+            // https://github.com/noir-lang/noir/pull/4716#issuecomment-2047846328.
+            ArrayGet { .. } | ArraySet { .. } => false,
+
             Call { func, .. } => match dfg[*func] {
                 Value::Intrinsic(intrinsic) => !intrinsic.has_side_effects(),
                 _ => false,