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/tower/tower/src/steer
    
    the code of the file until where you have to start completion: //! This module provides functionality to aid managing routing reque
    
    fn call(&mut self, req: Req) -> Self::Future {
        assert!(
            self.not_ready.is_empty(),
            "Steer must wait for all services to be ready. Did you forget to call poll_ready()?"
        );

        let idx = self.router.pick(&req, &self.services[..]);
        let cl = &mut self.services[idx];
        self.not_ready.push_back(idx);
        cl.call(req)
    }