Tracking Issue for `ptr_cast_add_auto_to_object` future compatibility warning

[WaffleLapkin]

This is a tracking issue for the ptr_cast_add_auto_to_object lint, which was added in #120248.

This lint detects casts of raw pointers to trait objects, which add auto traits. Adding auto traits to trait objects may cause UB when #![feature(arbitrary_self_types)] is used.

Example

#![feature(arbitrary_self_types)]
trait Trait {
    fn f(self: *const Self)
    where
        Self: Send;
}

impl Trait for *const () {
    fn f(self: *const Self) {
        unreachable!()
    }
}

fn main() {
    let unsend: *const () = &();
    let unsend: *const dyn Trait = &unsend;
    let send_bad: *const (dyn Trait + Send) = unsend as _;
    send_bad.f(); // this crashes, since vtable for `*const ()` does not have an entry for `f`
    //~^ warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
}

In case your usage is sound (e.g. because the trait doesn't have auto trait bounds), you can replace cast with a transmute to suppress the warning:

trait Cat {}
impl Cat for *const () {}

fn main() {
    let unsend: *const () = &();
    let unsend: *const dyn Cat = &unsend;
    let _send: *const (dyn Cat + Send) = unsafe {
        // Safety:
        // - Both types are pointers, to the same trait object (and thus have the same vtable)
        // - `Cat` does not have methods with `Send` bounds
        std::mem::transmute::<*const dyn Cat, *const (dyn Cat + Send)>(unsend)
    };
    // meow
}

[bjorn3]

In case your usage is sound (e.g. because the trait doesn't have auto trait bounds), you can replace cast with a transmute to suppress the warning:

Future compatibility warnings are generally made hard errors after a while, right? I don't think we should suggest suppressing a future compatibility warning.

[WaffleLapkin]

@bjorn3 we only will make cases with the warning into errors. transmuteing is, and will be, allowed (although you do actually need to make sure that your code is sound).