Remove object lifetime cast

[BoxyUwU]

Hi there o/

I'm a member of the Rust Language's Types Team; as part of stabilizing the arbitrary_self_types and derive_coerce_pointee features we are changing what raw pointer casts are legal (rust-lang/rust#136702). Specifically, casting *const dyn Trait + 'a to *const dyn Trait + 'b where it is not able to be proven that 'a outlives 'b will become an error.

Without going into too much detail, the justification for this change is that casting trait object's lifetime bound to a lifetime that lives for a longer time could invalidate the VTable of the trait object, allowing for dispatching to methods that should not be callable. See this example from the linked issue:

#![forbid(unsafe_code)]
#![feature(arbitrary_self_types, derive_coerce_pointee)]

use std::any::TypeId;
use std::marker::{CoercePointee, PhantomData};

#[derive(CoercePointee)]
#[repr(transparent)]
struct SelfPtr<T: ?Sized>(*const T);

impl<T: ?Sized> std::ops::Deref for SelfPtr<T> {
    type Target = T;
    fn deref(&self) -> &T {
        panic!("please don't call me, I just want the `Receiver` impl!");
    }
}

trait GetTypeId {
    fn get_type_id(self: SelfPtr<Self>) -> TypeId
    where
        Self: 'static;
}

impl<T: ?Sized> GetTypeId for PhantomData<T> {
    fn get_type_id(self: SelfPtr<Self>) -> TypeId
    where
        Self: 'static,
    {
        TypeId::of::<T>()
    }
}

// no `T: 'static` bound necessary
fn type_id_of<T: ?Sized>() -> TypeId {
    let ptr = SelfPtr(
        &PhantomData::<T> as *const (dyn GetTypeId + '_) as *const (dyn GetTypeId + 'static),
    );
    ptr.get_type_id()
}

Unfortunately, going through the usual "future compatibility warning" process turned out to not be possible as checking lifetime constraints without emitting an error is prohibitively difficult to do in the implementation of the borrow checker. This means that you will never receive an FCW and its associated grace period to update your code to be compatible with the new rules.

I have attempted to update your codebase to the new rules for you so that it will still compile when we make this change. I hope this breakage won't cause you too much trouble and that you might even find the post-migration code to be better than how it was previously :-)

If you have any questions about why this change is required please let me know and I'll do my best to further explain the justification.

---

It wasn't clear to me if the pointer casts here were supposed to be implicitly casting the lifetimes of the trait object to 'static or if all the Collector stuff should really be generic over the lifetime of the trait object. I opted to just transmute the lifetime as a small patch but it may be worth looking into this more

[wvwwvwwv]

@BoxyUwU Wow, thanks for making this PR. Does the Rust dev team check test every published crate when a breaking change is made (just a question :-) how did you find this issue?)?

Anyway your change totally makes sense, and I'll just merge it after fixing the clippy error.

It wasn't clear to me if the pointer casts here were supposed to be implicitly casting the lifetimes of the trait object to 'static or if all the Collector stuff should really be generic over the lifetime of the trait object.

The type is assumed to be 'static; if not, the user would have to use an unsafe method to reach here where the doc of the unsafe method explicitly states that the pointe-e can be dropped at an arbitrary point in the future.

[ShoyuVanilla]

Does the Rust dev team check test every published crate when a breaking change is made (just a question :-) how did you find this issue?)?

Whenever investigating a breaking change, it is run over almost all the crates published on crates.io.
https://rustc-dev-guide.rust-lang.org/tests/crater.html