PhantomData seems not work for dropck in rust

Question

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);

}

By refer to drop check rule, It can't compiles successfully. But successfully. And if i use such code:

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));

}

Just use Box instead of Uniqu In World struct, it compiles unsuccessfully:

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<'_>`

In this situation, drop checker have checked it. This is what I need.

In first situation, drop checker haven't checked it. PhantomData seems not work, why?

I don't have time for deep inspection now, and I never truly understood dropck, but it looks like you're missing a `Drop` impl in `Uniqu`. — Chayim Friedman, Nov 03 '22 at 11:08

score 0 · Answer 1 · answered Nov 03 '22 at 21:53

In your first example, Uniqu does not implement Drop, so it has no potentially-unsafe drop code to check for.

Box however does implement Drop - so that it can free the memory it has allocated - which is why it does not pass the drop checker.

Adding a trivial Drop implementation to Uniqu in your first code section does cause the compiler to reject the code, as seen in the playground.

PhantomData seems not work for dropck in rust

1 Answers1