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

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)