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: lib.rs
    path of file: ./repos/oso/languages/rust/oso/src
    
    the code of the file until where you have to start completion: //! Oso policy engine for authorization
//!
//! # Overview
//!
//! Oso is a policy engine for authorization that's embedded in your application.
//! It provides a declarative policy language for expressing authorization logic.
//! You define this logic separately from the rest of your application code,
//! but it executes inside the 
    
//! # fn main() -> anyhow::Result<()> {
//! use oso::{Oso, PolarClass};
//!
//! #[derive(Clone, PolarClass)]
//! struct User {
//!     #[polar(attribute)]
//!     pub username: String,
//! }
//!
//! impl User {
//!     fn superuser() -> Vec<String> {
//!         return vec!["alice".to_string(), "charlie".to_string()]
//!     }
//! }
//!
//! let mut oso = Oso::new();
//!
//! oso.register_class(
//!    User::get_polar_class_builder()
//!         .add_class_method("superusers", User::superuser)
//!         .build()
//! )?;
//!
//! oso.load_str(r#"allow(actor: User, _action, _resource) if
//!                     actor.username.ends_with("example.com");"#)?;
//!
//! let user = User {
//!     username: "alice@example.com".to_owned(),
//! };
//!
//! assert!(oso.is_allowed(user, "foo", "bar")?);
//! # Ok(())
//! # }