fix(rust): List<null> chunked builder should take care of series name (…

…#10642)

crates/polars-core/src/chunked_array/builder/list/boolean.rs

@@ -69,26 +69,3 @@ impl ListBuilderTrait for ListBooleanChunkedBuilder {
         self.fast_explode
     }
 }
-
-impl ListBuilderTrait for LargeListNullBuilder {
-    #[inline]
-    fn append_series(&mut self, _s: &Series) -> PolarsResult<()> {
-        self.push_null();
-        Ok(())
-    }
-
-    #[inline]
-    fn append_null(&mut self) {
-        self.push_null()
-    }
-
-    fn finish(&mut self) -> ListChunked {
-        unsafe {
-            ListChunked::from_chunks_and_dtype_unchecked(
-                "",
-                vec![self.as_box()],
-                DataType::List(Box::new(DataType::Null)),
-            )
-        }
-    }
-}

crates/polars-core/src/chunked_array/builder/list/mod.rs

@@ -4,6 +4,7 @@ mod boolean;
 #[cfg(feature = "dtype-categorical")]
 mod categorical;
 mod dtypes;
+mod null;
 mod primitive;
 
 pub use anonymous::*;
@@ -12,6 +13,7 @@ pub use boolean::*;
 #[cfg(feature = "dtype-categorical")]
 use categorical::*;
 use dtypes::*;
+use null::*;
 use polars_arrow::array::list::AnonymousBuilder;
 use polars_arrow::array::null::MutableNullArray;
 use polars_arrow::prelude::*;
@@ -116,7 +118,7 @@ pub fn get_list_builder(
             list_capacity,
             Some(inner_type_logical.clone()),
         ))),
-        DataType::Null => Ok(Box::new(LargeListNullBuilder::with_capacity(list_capacity))),
+        DataType::Null => Ok(Box::new(ListNullChunkedBuilder::new(name, list_capacity))),
         DataType::List(_) => Ok(Box::new(AnonymousOwnedListBuilder::new(
             name,
             list_capacity,

crates/polars-core/src/chunked_array/builder/list/null.rs

@@ -0,0 +1,38 @@
+use super::*;
+
+pub struct ListNullChunkedBuilder {
+    builder: LargeListNullBuilder,
+    name: String,
+}
+
+impl ListNullChunkedBuilder {
+    pub fn new(name: &str, capacity: usize) -> Self {
+        ListNullChunkedBuilder {
+            builder: LargeListNullBuilder::with_capacity(capacity),
+            name: name.into(),
+        }
+    }
+}
+
+impl ListBuilderTrait for ListNullChunkedBuilder {
+    #[inline]
+    fn append_series(&mut self, _s: &Series) -> PolarsResult<()> {
+        self.builder.push_null();
+        Ok(())
+    }
+
+    #[inline]
+    fn append_null(&mut self) {
+        self.builder.push_null();
+    }
+
+    fn finish(&mut self) -> ListChunked {
+        unsafe {
+            ListChunked::from_chunks_and_dtype_unchecked(
+                &self.name,
+                vec![self.builder.as_box()],
+                DataType::List(Box::new(DataType::Null)),
+            )
+        }
+    }
+}

crates/polars/tests/it/core/series.rs

@@ -36,3 +36,10 @@ fn test_min_max_sorted_desc() {
     assert_eq!(a.max(), Some(4));
     assert_eq!(a.min(), Some(1));
 }
+
+#[test]
+fn test_construct_list_of_null_series() {
+    let s = Series::new("a", [Series::new_null("a1", 1), Series::new_null("a1", 1)]);
+    assert_eq!(s.null_count(), s.len());
+    assert_eq!(s.field().name(), "a");
+}