fix(rust): avoid panic when projecting solitary count into empty frame

When the schema of a frame is empty, a count expression is still valid
since it does not refer to any columns. In this scenario, selecting
the "last" column in the schema would previously panic, since there
are none. Fix this by handling this edge case explicitly.

- Closes #16904

crates/polars-plan/src/plans/optimizer/projection_pushdown/projection.rs

@@ -62,18 +62,25 @@ pub(super) fn process_projection(
     // the whole file while we only want the count
     if exprs.len() == 1 && is_count(exprs[0].node(), expr_arena) {
         let input_schema = lp_arena.get(input).schema(lp_arena);
-        // simply select the last column
-        // NOTE: the first can be the inserted index column, so that might not work
-        let (first_name, _) = input_schema.try_get_at_index(input_schema.len() - 1)?;
-        let expr = expr_arena.add(AExpr::Column(ColumnName::from(first_name.as_str())));
-        if !acc_projections.is_empty() {
-            check_double_projection(
-                &exprs[0],
-                expr_arena,
-                &mut acc_projections,
-                &mut projected_names,
-            );
-        }
+        let expr = if input_schema.is_empty() {
+            // If the input schema is empty, we should just project
+            // ourselves
+            exprs[0].node()
+        } else {
+            // simply select the last column
+            // NOTE: the first can be the inserted index column, so that might not work
+            let (first_name, _) = input_schema.try_get_at_index(input_schema.len() - 1)?;
+            let expr = expr_arena.add(AExpr::Column(ColumnName::from(first_name.as_str())));
+            if !acc_projections.is_empty() {
+                check_double_projection(
+                    &exprs[0],
+                    expr_arena,
+                    &mut acc_projections,
+                    &mut projected_names,
+                );
+            }
+            expr
+        };
         add_expr_to_accumulated(expr, &mut acc_projections, &mut projected_names, expr_arena);
         local_projection.push(exprs.pop().unwrap());
         proj_pd.is_count_star = true;

py-polars/tests/unit/test_projections.py

@@ -511,3 +511,14 @@ def test_projection_pushdown_semi_anti_no_selection(
     assert "PROJECT 1/2" in (
         q_a.join(q_b, left_on="a", right_on="b", how=how).explain()
     )
+
+
+def test_projection_empty_frame_len_16904() -> None:
+    df = pl.LazyFrame({})
+
+    q = df.select(pl.len())
+
+    assert "PROJECT */0" in q.explain()
+
+    expect = pl.DataFrame({"len": [0]}, schema_overrides={"len": pl.UInt32()})
+    assert_frame_equal(q.collect(), expect)