Static PTR_CAST_ADD_AUTO_TO_OBJECT

pub static PTR_CAST_ADD_AUTO_TO_OBJECT: &'static Lint

The ptr_cast_add_auto_to_object lint detects casts of raw pointers to trait objects, which add auto traits.

Example

let ptr: *const dyn core::any::Any = &();
_ = ptr as *const dyn core::any::Any + Send;

{{produces}}

Explanation

Adding an auto trait can make the vtable invalid, potentially causing UB in safe code afterwards. For 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`
}

Generally you must ensure that vtable is right for the pointer’s type, before passing the pointer to safe code.