E0804 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0804

An auto trait cannot be added to the bounds of a `dyn Trait` type via a pointer cast.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    let ptr: *const dyn core::any::Any = &();
    _ = ptr as *const (dyn core::any::Any + Send);
    }

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

    use core::{mem::transmute, ptr::NonNull};

    trait Trait {
        fn f(&self)
        where
            Self: Send;
    }

    impl Trait for NonNull<()> {
        fn f(&self) {
            unreachable!()
        }
    }

    fn main() {
        let unsend: &dyn Trait = &NonNull::dangling();
        let bad: &(dyn Trait + Send) = unsafe { transmute(unsend) };
        // This crashes, since the vtable for `NonNull as dyn Trait` does
        // not have an entry for `Trait::f`.
        bad.f();
    }

To fix this error, you can use `transmute` rather than pointer casts, but you must ensure that the vtable is valid for the pointer's type before calling a method on the trait object or allowing other code to do so.