Regression: transmuting between raw pointer and Option<NonNull> no longer works

[yshui]

Code

I tried this code:

use std::ptr::NonNull;
pub const fn is_null<'a, T: ?Sized>(ptr: *const T) -> bool {
    unsafe {
        matches!(
            core::mem::transmute::<*const T, Option<NonNull<T>>>(ptr),
            None
        )
    }
}

I expected to see this happen: Code compiles

Instead, this happened:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> src/lib.rs:5:13
  |
5 |             core::mem::transmute::<*const T, Option<NonNull<T>>>(ptr),
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: source type: `*const T` (pointer to `T`)
  = note: target type: `Option<NonNull<T>>` (size can vary because of <T as Pointee>::Metadata)

For more information about this error, try `rustc --explain E0512`.

---

Bisected to 338dff3 #136006

[yshui]

Code snippet comes from an older version of konst.

[yshui]

T: ?Sized is required. And transmuting *const T -> NonNull<T> still works, but Option<NonNull<T>> is affected.

[theemathas]

cc @oli-obk, @wesleywiser (author and reviewer of #136006)