I am using mockall crate for unittest.

#[cfg_attr(test, automock)]
pub trait Reader {
    fn next_data(&mut self) -> &Data;
}


impl Reader for TestReader {
    fn next_data(&mut self) -> &Data {
        let data: &Data = &self.data[self.index];
        self.index += 1;
        data
    }
}



fn test_reader() {
    let data_iter: IntoIter<Data> = (some vector Vec<Data>).into_iter();
    let mut reader = MockReader::new();
    reader
        .expect_next_data()
        .times(2)
        .returning(move || data_iter.clone().next().is_some());

This is the error message I receive

error[E0599]: no method named `returning` found for mutable reference `&mut __mock_MockReader_Reader::__next_data::Expectation` in the current scope
   --> src/tests/test_data.rs:177:14
    |
174 | /         reader
175 | |             .expect_next_data()
176 | |             .times(2)
177 | |             .returning(move || data_iter.clone().next().is_some());
    | |             -^^^^^^^^^ method not found in `&mut Expectation`
    | |_____________|
    | 

How should I deal with this? The returning should work based on the mockall document. I have no idea why it says method not found.