fix(interpreter): Return fresh nested array on array get in ACIR

compiler/noirc_evaluator/src/ssa/interpreter/mod.rs

@@ -928,7 +928,25 @@ impl<'ssa, W: Write> Interpreter<'ssa, W> {
             let element = elements.get(index as usize).ok_or_else(|| {
                 InterpreterError::IndexOutOfBounds { index, length: elements.len() as u32 }
             })?;
-            element.clone()
+
+            // Either return a fresh nested array (in constrained context) or just clone the element.
+            if !self.in_unconstrained_context() {
+                if let Some(array) = element.as_array_or_slice() {
+                    // In the ACIR runtime we expect fresh arrays when accessing a nested array.
+                    // If we do not clone the elements here a mutable array set afterwards could mutate
+                    // not just this returned array but the array we are fetching from in this array get.
+                    Value::ArrayOrSlice(ArrayValue {
+                        elements: Shared::new(array.elements.borrow().to_vec()),
+                        rc: array.rc,
+                        element_types: array.element_types,
+                        is_slice: array.is_slice,
+                    })
+                } else {
+                    element.clone()
+                }
+            } else {
+                element.clone()
+            }
         };
         self.define(result, element)?;
         Ok(())