Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0804: cannot add auto trait `Send` to dyn bound via pointer cast
What it meansAn auto trait cannot be added to the bounds of a dyn Trait type via a pointer cast.
What rustc prints
error[E0804]: cannot add auto trait `Send` to dyn bound via pointer cast --> main.rs:4:5 | 4 | _ = ptr as *const (dyn core::any::Any + Send); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsupported cast | = note: this could allow UB elsewhere = help: use `transmute` if you're sure this is sound
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
An auto trait cannot be added to the bounds of a dyn Trait type via a pointer cast.
Erroneous code example:
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.
From rustc --explain E0804 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
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