PhantomData seems not work for dropck in rust - 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?

Answer 1

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.