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An auto trait cannot be added to the bounds of a `dyn Trait` type via
a pointer cast.
Erroneous code example:
```rust,edition2021,compile_fail,E0804
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:
```rust,edition2021,no_run
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.
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