You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/oso/languages/rust/oso/tests/common
    
    the code of the file until where you have to start completion: #![allow(dead_code)]
use oso::Oso;

pub struct OsoTest {
    pub oso: Oso,
}

impl OsoTest {
    pub fn new() -> Self {
        Self { oso: Oso::new() }

    
    pub fn query_err(&mut self, q: &str) -> String {
        let mut results = self.oso.query(q).unwrap();
        let err = results
            .next()
            .unwrap()
            .expect_err("query should return an error");
        err.to_string()
    }