sandbox

struct Bomb;

impl Drop for Bomb {
    fn drop(&mut self) {
        unsafe extern "C" {
            #[link_name = "\n\nerror: `mandatory_method()` was not called before drop\n\n"]
            fn mandatory_method_not_called();
        }

        unsafe { mandatory_method_not_called() }
    }
}

pub struct PhantomNoDrop {
    bomb: Bomb,
}

impl PhantomNoDrop {
    pub fn new() -> Self {
        Self { bomb: Bomb }
    }

    pub fn mandatory_method(self) {
        core::mem::forget(self.bomb);
    }
}

struct Undroppable {
    p: PhantomNoDrop,
}

impl Undroppable {
    pub fn mandatory_method(self) {
        self.p.mandatory_method();
    }
}

fn main() {
    let p = PhantomNoDrop::new();
    p.mandatory_method();

    let u = Undroppable {
        p: PhantomNoDrop::new(),
    };
}