Can't cast a pointer of a trait object newtype to a pointer of that trait object.

[theemathas]

I tried this code:

trait Trait {}

struct Wrap(dyn Trait);

fn cast(x: *mut Wrap) {
    x as *mut dyn Trait;
}

I expected the code to compile. Instead, I got the following error, where it appears that the compiler is trying to do an unsize coercion and failing:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `Wrap: Trait` is not satisfied
 --> src/lib.rs:6:5
  |
6 |     x as *mut dyn Trait;
  |     ^ the trait `Trait` is not implemented for `Wrap`
  |
help: this trait has no implementations, consider adding one
 --> src/lib.rs:1:1
  |
1 | trait Trait {}
  | ^^^^^^^^^^^
  = note: required for the cast from `*mut Wrap` to `*mut dyn Trait`

error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
 --> src/lib.rs:6:5
  |
6 |     x as *mut dyn Trait;
  |     ^ doesn't have a size known at compile-time
  |
  = help: within `Wrap`, the trait `Sized` is not implemented for `(dyn Trait + 'static)`, which is required by `Wrap: Sized`
note: required because it appears within the type `Wrap`
 --> src/lib.rs:3:8
  |
3 | struct Wrap(dyn Trait);
  |        ^^^^
  = note: required for the cast from `*mut Wrap` to `*mut dyn Trait`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 2 previous errors

Of note, the following code compiles fine:

trait Trait {}

struct Wrap(dyn Trait);

fn cast(x: *mut Wrap) {
    x as *mut (dyn Trait,);
}

Probably related to #128621

Meta

This issue reproduces on the playground on stable (1.80.0), and nightly (2024-08-02 fd8d6fb).

[theemathas]

Another variant that compiles fine:

trait Trait {}

struct Wrap1(dyn Trait);
struct Wrap2(dyn Trait);

fn cast(x: *mut Wrap1) -> *mut Wrap2 {
    x as *mut Wrap2
}

[theemathas]

Oddly, doing the cast a roundabout way using a generic makes the code compile:

trait Trait {}

struct Wrap<T: ?Sized>(T);

fn cast_specific(x: *mut Wrap<dyn Trait>) -> *mut dyn Trait {
    cast_generic::<dyn Trait>(x) // compiles fine
    // x as *mut dyn Trait // doesn't work
}

fn cast_generic<T: ?Sized>(x: *mut Wrap<T>) -> *mut T {
    x as *mut T
}