use core::marker::PhantomData;

fn main() {
    struct Inspector<'a>(&'a u8);

    impl<'a> Drop for Inspector<'a> {
        fn drop(&mut self) {
            println!("I was only {} days from retirement!", self.0);
        }
    }

    struct Uniqu<T> {
        ptr: *mut T,
        marker: PhantomData<T>,  
    }

    struct World<'a> {
        ptr: Option<Uniqu<Inspector<'a>>>,
        days: u8,
    }

    let mut world = World{ptr: None, days:1};

    let mut inspector = Inspector(&world.days);
    let mut uniqu = Uniqu::<Inspector>{ptr: &mut inspector, marker: PhantomData};
    world.ptr = Some(uniqu);

}

通过引用drop check规则，它不能成功编译。但成功。如果我使用这样的代码：

use core::marker::PhantomData;

fn main() {
    struct Inspector<'a>(&'a u8);

    impl<'a> Drop for Inspector<'a> {
        fn drop(&mut self) {
            println!("I was only {} days from retirement!", self.0);
        }
    }

     struct World<'a> {
         ptr: Option<Box<Inspector<'a>>>,
         days: u8,
     }

    let mut world = World{ptr: None, days:1};
    let mut inspector = Inspector(&world.days);

    world.ptr = Some(Box::new(inspector));

}

只需使用Box而不是Uniqu In World struct，它编译失败：

error[E0597]: `world.days` does not live long enough
  --> src/main.rs:21:35
   |
21 |     let mut inspector = Inspector(&world.days);
   |                                   ^^^^^^^^^^^ borrowed value does not live long enough
...
26 | }
   | -
   | |
   | `world.days` dropped here while still borrowed
   | borrow might be used here, when `world` is dropped and runs the destructor for type `World<'_>`

在这种情况下，drop checker已经检查过了。这就是我所需要的。
在第一种情况下，Drop Checker还没有检查它。PhantomData似乎不工作，为什么？