fix[rust]: error on unknown supertype in fill null (#5002)

polars/polars-lazy/src/dsl/function_expr/fill_null.rs

@@ -1,8 +1,21 @@
 use super::*;
 
 pub(super) fn fill_null(s: &[Series], super_type: &DataType) -> PolarsResult<Series> {
-    let array = s[0].cast(super_type)?;
-    let fill_value = s[1].cast(super_type)?;
+    let array = &s[0];
+    let fill_value = &s[1];
+    if matches!(super_type, DataType::Unknown) {
+        return Err(PolarsError::SchemaMisMatch(
+            format!(
+                "Cannot 'fill_null' a 'Series' of dtype: '{}' with an argument of dtype: '{}'",
+                array.dtype(),
+                fill_value.dtype()
+            )
+            .into(),
+        ));
+    };
+
+    let array = array.cast(super_type)?;
+    let fill_value = fill_value.cast(super_type)?;
 
     if !array.null_count() == 0 {
         Ok(array)

py-polars/tests/unit/test_errors.py

@@ -195,3 +195,16 @@ def test_filter_not_of_type_bool() -> None:
         pl.ComputeError, match="Filter predicate must be of type Boolean, got"
     ):
         df.filter(pl.col("json_val").str.json_path_match("$.a"))
+
+
+def test_fill_null_unknown_supertype() -> None:
+    df = pl.DataFrame(
+        [
+            pl.Series("a", [1, None, 3]),
+            pl.Series("b", ["hello", "at the", "bar"], dtype=pl.Categorical),
+            pl.Series("c", [1, 1, None]),
+        ]
+    )
+
+    with pytest.raises(pl.SchemaError):
+        df.fill_null(0)