fix: Fix ColumnNotFound when using pl.element() inside list.eval (

#19438)

crates/polars-lazy/src/physical_plan/exotic.rs

@@ -24,13 +24,13 @@ pub(crate) fn prepare_expression_for_context(
     // create a dummy lazyframe and run a very simple optimization run so that
     // type coercion and simplify expression optimizations run.
     let column = Series::full_null(name, 0, dtype);
-    let mut lf = column
-        .into_frame()
+    let df = column.into_frame();
+    let input_schema = Arc::new(df.schema());
+    let lf = df
         .lazy()
         .without_optimizations()
         .with_simplify_expr(true)
         .select([expr.clone()]);
-    let schema = lf.collect_schema()?;
     let optimized = lf.optimize(&mut lp_arena, &mut expr_arena)?;
     let lp = lp_arena.get(optimized);
     let aexpr = lp
@@ -42,7 +42,7 @@ pub(crate) fn prepare_expression_for_context(
         &aexpr,
         ctxt,
         &expr_arena,
-        &schema,
+        &input_schema,
         &mut ExpressionConversionState::new(true, 0),
     )
 }

py-polars/tests/unit/operations/namespaces/list/test_list.py

@@ -900,3 +900,14 @@ def test_list_concat_struct_19279() -> None:
             [{"s": "d", "i": 3}],
         ]
     }
+
+
+def test_list_eval_element_schema_19345() -> None:
+    assert_frame_equal(
+        (
+            pl.LazyFrame({"a": [[{"a": 1}]]})
+            .select(pl.col("a").list.eval(pl.element().struct.field("a")))
+            .collect()
+        ),
+        pl.DataFrame({"a": [[1]]}),
+    )