Error when unsizing types with thin pointers that turn fat

Code

use std::fmt::Debug;

struct ListItem<T: ?Sized> {
    next: Option<Box<Self>>,
    item: T,
}

fn main() {
    let _ = Box::new(ListItem {
        next: None,
        item: [42, 24],
    }) as Box<ListItem<dyn Debug>>;
}

Current output

error[E0605]: non-primitive cast: `Box<ListItem<[i32; 2]>>` as `Box<ListItem<dyn Debug>>`
  --> src/main.rs:9:13
   |
9  |       let _ = Box::new(ListItem {
   |  _____________^
10 | |         next: None,
11 | |         item: [42, 24],
12 | |     }) as Box<ListItem<dyn Debug>>;
   | |__________________________________^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

For more information about this error, try `rustc --explain E0605`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

error[E????]: type `Box<ListItem<[i32; 2]>>` cannot be unsized to `Box<ListItem<dyn Debug>>`
  --> src/main.rs:9:13
   |
9  |       let _ = Box::new(ListItem {
   |  _____________^
10 | |         next: None,
11 | |         item: [42, 24],
12 | |     }) as Box<ListItem<dyn Debug>>;
   | |__________________________________^
note: the struct `ListItem<[i32; 2]>` would change size if unsized to `ListItem<dyn Debug>`
3  │ struct ListItem<T: ?Sized> {
4  │     next: Option<Box<Self>>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^ this field's type would change from a thin pointer to a fat pointer
5  │     item: T,
6  │ }

For more information about this error, try `rustc --explain E????`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Rationale and extra context

No response

Other cases

// Removing the `next` field makes the code compile:

use std::fmt::Debug;

struct ListItem<T: ?Sized> {
    // next: Option<Box<Self>>,
    item: T,
}

fn main() {
    let _ = Box::new(ListItem {
        // next: None,
        item: [42, 24],
    }) as Box<ListItem<dyn Debug>>;
}

Rust Version

rustc 1.83.0-nightly (6c6d21008 2024-09-22)
binary: rustc
commit-hash: 6c6d210089e4589afee37271862b9f88ba1d7755
commit-date: 2024-09-22
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Anything else?

If more information/explanation is needed, I can provide it, just ask.

Reproduce the diagnostic using the example in src/main.rs and the Rust 1.83.0-nightly version reported in the issue. Compare the current E0605 output with the desired unsizing error, including the note about the next field changing from a thin pointer to a fat pointer. Done when this case reports the more specific diagnostic without regressing the variant where the next field is removed.