rustlings/exercises/lifetimes/lifetimes4.rs

// lifetimes4.rs
//
// Sometimes, we have structs which hold on to data temporarily. A use-case of
// this could be a routing component which accepts a connection and returns it to
// another recipient. To avoid copying the data, we just accept a reference with
// lifetime and return this reference later.
//
// In the example below, we create a `Router` instance in a limited scope. It
// accepts a connection reference created in the enclosing scope and returns it.
// In theory, this should be possible given that the connection reference outlives
// the scope from which it is returned. However the borrow checker does not
// seem to understand it. What can we do about that?
//
// Execute `rustlings hint lifetimes4` or use the `hint` watch subcommand for a
// hint.

// I AM NOT DONE

struct Router<'a> {
    connection: Option<&'a u64>,
}

impl<'a> Router<'a> {
    fn new() -> Self {
        Self { connection: None }
    }

    fn accept_connection(&mut self, connection: &'a u64) {
        self.connection = Some(connection);
    }

    fn return_connection(&mut self) -> Option<&u64> {
        self.connection.take()
    }
}

fn main() {
    let connection = &123;

    let returned_connection = {
        // Create router within scope.
        let mut router = Router::new();

        // Accept connection which lives longer than the router.
        router.accept_connection(connection);

        // Return connection which **should** live longer than the router.
        router.return_connection()
    };

    if let Some(connection) = returned_connection {
        println!("The connection is {connection}");
    }
}