Code shared from the Rust Playground

playground.rs

#[warn(unused_variables)]

pub struct Letter {
    text: String,
}

impl Letter {
    pub fn new(text: String) -> Self {
        Letter { text }
    }
}

/// An empty pre-stamped envelope.
pub struct EmptyEnvelope {}

/// A closed envelope containing a letter.
pub struct ClosedEnvelope {
    letter: Letter,
}

impl EmptyEnvelope {
    /// Put a letter in the envelope and seal it.
    pub fn wrap(self, letter: Letter) -> ClosedEnvelope {
        ClosedEnvelope { letter }
    }
}

pub struct PickupLorryHandle {
    done: bool,
    // references to lorry's resources
    envelope: Option<ClosedEnvelope>,
}

impl PickupLorryHandle {
     
    /// Give an envelope to the delivery driver.
    pub fn pickup(&mut self, envelope: ClosedEnvelope) {
        self.envelope = Some(envelope)
    }

    /// Tell the driver we don't have anything else for them.
    pub fn done(&mut self) {
        self.done = true;
        println!("sent");
    }
}

impl Drop for PickupLorryHandle {
    fn drop(&mut self) {
        if !self.done {
            self.done();
        }
    }
}

pub fn order_pickup() -> PickupLorryHandle {
    PickupLorryHandle { done: false, envelope: None }
}

pub fn buy_prestamped_envelope() -> EmptyEnvelope {
    EmptyEnvelope {}
}

// in a separate module
fn main() {
    let rustfest_letter = Letter::new(String::from("Dear RustFest"));

    let envelope = buy_prestamped_envelope();
    let closed_envelope = envelope.wrap(rustfest_letter);

    let eth_letter = Letter::new(String::from("Dear ETH"));
    let closed_envelope1 = envelope.wrap(eth_letter);

    let mut lorry = order_pickup();
    lorry.pickup(closed_envelope);
}