Minor: Add ScalarFunctionArgs::return_type method (#16113)

Author: alamb

datafusion/expr/src/udf.rs

@@ -306,6 +306,14 @@ pub struct ScalarFunctionArgs<'a, 'b> {
     pub return_field: &'b Field,
 }
 
+impl<'a, 'b> ScalarFunctionArgs<'a, 'b> {
+    /// The return type of the function. See [`Self::return_field`] for more
+    /// details.
+    pub fn return_type(&self) -> &DataType {
+        self.return_field.data_type()
+    }
+}
+
 /// Information about arguments passed to the function
 ///
 /// This structure contains metadata about how the function was called

datafusion/functions/src/core/named_struct.rs

@@ -150,7 +150,7 @@ impl ScalarUDFImpl for NamedStructFunc {
     }
 
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
-        let DataType::Struct(fields) = args.return_field.data_type() else {
+        let DataType::Struct(fields) = args.return_type() else {
             return internal_err!("incorrect named_struct return type");
         };
 

datafusion/functions/src/core/struct.rs

@@ -117,7 +117,7 @@ impl ScalarUDFImpl for StructFunc {
     }
 
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
-        let DataType::Struct(fields) = args.return_field.data_type() else {
+        let DataType::Struct(fields) = args.return_type() else {
             return internal_err!("incorrect struct return type");
         };