fix: Raise instead of panicking when casting a Series to a Struct with the wrong number of fields

crates/polars-core/src/chunked_array/cast.rs

@@ -134,6 +134,7 @@ fn cast_single_to_struct(
     fields: &[Field],
     options: CastOptions,
 ) -> PolarsResult<Series> {
+    polars_ensure!(fields.len() == 1, InvalidOperation: "must specify one field in the struct");
     let mut new_fields = Vec::with_capacity(fields.len());
     // cast to first field dtype
     let mut fields = fields.iter();

py-polars/tests/unit/datatypes/test_struct.py

@@ -11,6 +11,7 @@
 
 import polars as pl
 import polars.selectors as cs
+from polars.exceptions import InvalidOperationError
 from polars.testing import assert_frame_equal, assert_series_equal
 
 if TYPE_CHECKING:
@@ -923,7 +924,7 @@ def test_struct_wildcard_expansion_and_exclude() -> None:
         ),
     ]
 
-    with pytest.raises(pl.exceptions.InvalidOperationError):
+    with pytest.raises(InvalidOperationError):
         df.lazy().select(
             pl.col("meta_data").struct.with_fields(pl.field("*").exclude("user_data"))
         ).collect()
@@ -1222,7 +1223,17 @@ def test_leaf_list_eq_19613(data: Any) -> None:
 def test_nested_object_raises_15237() -> None:
     obj = object()
     df = pl.DataFrame({"a": [obj]})
+    with pytest.raises(InvalidOperationError, match="nested objects are not allowed"):
+        df.select(pl.struct("a"))
+
+
+def test_cast_to_struct_needs_field_14083() -> None:
     with pytest.raises(
-        pl.exceptions.InvalidOperationError, match="nested objects are not allowed"
+        InvalidOperationError, match="must specify one field in the struct"
     ):
-        df.select(pl.struct("a"))
+        pl.Series([1], dtype=pl.Int32).cast(pl.Struct)
+
+    with pytest.raises(
+        InvalidOperationError, match="must specify one field in the struct"
+    ):
+        pl.Series([1], dtype=pl.Int32).cast(pl.Struct({"a": pl.UInt8, "b": pl.UInt8}))