allow null array to be cased to all other types (apache#884)

Can drop this after rebase on commit 81ffa240, first released in 7.0.0

Author: jimexist

arrow/src/compute/kernels/cast.rs

@@ -68,6 +68,44 @@ pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
     }
 
     match (from_type, to_type) {
+        (
+            Null,
+            Boolean
+            | Int8
+            | UInt8
+            | Int16
+            | UInt16
+            | Int32
+            | UInt32
+            | Float32
+            | Date32
+            | Time32(_)
+            | Int64
+            | UInt64
+            | Float64
+            | Date64
+            | List(_)
+            | Dictionary(_, _),
+        )
+        | (
+            Boolean
+            | Int8
+            | UInt8
+            | Int16
+            | UInt16
+            | Int32
+            | UInt32
+            | Float32
+            | Date32
+            | Time32(_)
+            | Int64
+            | UInt64
+            | Float64
+            | Date64
+            | List(_)
+            | Dictionary(_, _),
+            Null,
+        ) => true,
         (Struct(_), _) => false,
         (_, Struct(_)) => false,
         (LargeList(list_from), LargeList(list_to)) => {
@@ -306,7 +344,6 @@ pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
         (Timestamp(_, _), Date64) => true,
         // date64 to timestamp might not make sense,
         (Int64, Duration(_)) => true,
-        (Null, Int32) => true,
         (_, _) => false,
     }
 }
@@ -867,6 +904,44 @@ pub fn cast_with_options(
         return Ok(array.clone());
     }
     match (from_type, to_type) {
+        (
+            Null,
+            Boolean
+            | Int8
+            | UInt8
+            | Int16
+            | UInt16
+            | Int32
+            | UInt32
+            | Float32
+            | Date32
+            | Time32(_)
+            | Int64
+            | UInt64
+            | Float64
+            | Date64
+            | List(_)
+            | Dictionary(_, _),
+        )
+        | (
+            Boolean
+            | Int8
+            | UInt8
+            | Int16
+            | UInt16
+            | Int32
+            | UInt32
+            | Float32
+            | Date32
+            | Time32(_)
+            | Int64
+            | UInt64
+            | Float64
+            | Date64
+            | List(_)
+            | Dictionary(_, _),
+            Null,
+        ) => Ok(new_null_array(to_type, array.len())),
         (Struct(_), _) => Err(ArrowError::CastError(
             "Cannot cast from struct to other types".to_string(),
         )),
@@ -1706,10 +1781,6 @@ pub fn cast_with_options(
                 }
             }
         }
-
-        // null to primitive/flat types
-        (Null, Int32) => Ok(Arc::new(Int32Array::from(vec![None; array.len()]))),
-
         (_, _) => Err(ArrowError::CastError(format!(
             "Casting from {:?} to {:?} not supported",
             from_type, to_type,
@@ -4268,17 +4339,39 @@ mod tests {
     }
 
     #[test]
-    fn test_cast_null_array_to_int32() {
-        let array = Arc::new(NullArray::new(6)) as ArrayRef;
+    fn test_cast_null_array_from_and_to_others() {
+        macro_rules! typed_test {
+            ($ARR_TYPE:ident, $DATATYPE:ident, $TYPE:tt) => {{
+                {
+                    let array = Arc::new(NullArray::new(6)) as ArrayRef;
+                    let expected = $ARR_TYPE::from(vec![None; 6]);
+                    let cast_type = DataType::$DATATYPE;
+                    let cast_array = cast(&array, &cast_type).expect("cast failed");
+                    let cast_array = as_primitive_array::<$TYPE>(&cast_array);
+                    assert_eq!(cast_array.data_type(), &cast_type);
+                    assert_eq!(cast_array, &expected);
+                }
+                {
+                    let array = Arc::new($ARR_TYPE::from(vec![None; 4])) as ArrayRef;
+                    let expected = NullArray::new(4);
+                    let cast_array = cast(&array, &DataType::Null).expect("cast failed");
+                    let cast_array = as_null_array(&cast_array);
+                    assert_eq!(cast_array.data_type(), &DataType::Null);
+                    assert_eq!(cast_array, &expected);
+                }
+            }};
+        }
 
-        let expected = Int32Array::from(vec![None; 6]);
+        typed_test!(Int16Array, Int16, Int16Type);
+        typed_test!(Int32Array, Int32, Int32Type);
+        typed_test!(Int64Array, Int64, Int64Type);
 
-        // Cast to a dictionary (same value type, Utf8)
-        let cast_type = DataType::Int32;
-        let cast_array = cast(&array, &cast_type).expect("cast failed");
-        let cast_array = as_primitive_array::<Int32Type>(&cast_array);
-        assert_eq!(cast_array.data_type(), &cast_type);
-        assert_eq!(cast_array, &expected);
+        typed_test!(UInt16Array, UInt16, UInt16Type);
+        typed_test!(UInt32Array, UInt32, UInt32Type);
+        typed_test!(UInt64Array, UInt64, UInt64Type);
+
+        typed_test!(Float32Array, Float32, Float32Type);
+        typed_test!(Float64Array, Float64, Float64Type);
     }
 
     /// Print the `DictionaryArray` `array` as a vector of strings