Union cast failed

[simensgreen]

Expected Behaviour

Can cast [f32; 5] as Data { a: f32, b: [f32; 3], c: f32 }

Example & Steps To Reproduce

#[repr(C)]
#[derive(Clone, Copy)]
struct Data {
    a: f32,
    b: [f32; 3],
    c: f32,
}

union DataOrArray {
    arr: [f32; 5],
    str: Data,
}

impl DataOrArray {
    fn arr(self) -> [f32; 5] { unsafe { self.arr } }
    fn new(arr: [f32; 5]) -> Self { Self { arr } }
}

in shader:

let dora = DataOrArray::new([0.0, 0.0, 0.0, 0.0, 0.0]);
let arr = dora.arr();

Produces:

error: cannot cast between pointer types
         from `*struct types::Data { a: f32, b: [f32; 3], c: f32 }`
           to `*[f32; 5]`

System Info

• Rust: rustc 1.84.0-nightly (b19329a37 2024-11-21)
• OS: MacOS 15.3.2
• GPU: M2 Max
• rust-gpu: 698f10a

[LegNeato]

Can you try latest master? I landed 2f915a0 which fixed issues with a similar message (#46) and I think this is fixed by it too.

[simensgreen]

Nope, even at commit 2f915a0 error still appears

[simensgreen]

I tried trick it with core::mem::transmute:

    pub fn arr(self) -> [f32; 5] {
        unsafe { core::mem::transmute(self) }
    }

Error:

  error: cannot memcpy dynamically sized data
    --> solver/src/types/data_or_array.rs:16:18
     |
  16 |         unsafe { core::mem::transmute(self) }
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^

[simensgreen]

I created repo to test this error, but it failed to compile #242

[LegNeato]

Ok great, I'll take a look.

[LegNeato]

Wasn't able to find the root cause. This is going to churn shortly with some bigger qptr work so I am hesitant to spend more time until that lands

[simensgreen]

@LegNeato Perhaps I can assist you with the QPTR, but I require further information. Could you please elaborate on what it is and from where it originates?

[nazar-pc]

I have a simple example with an array that results in the same error:

  error: cannot cast between pointer types
         from `*[[u32; 4]; 4]`
           to `*[u32; 16]`
     --> /home/nazar-pc/.rustup/toolchains/nightly-2024-11-22-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:436:6
      |
  436 |     }
      |      ^
      |
  note: used from within `<core::ptr::non_null::NonNull<[[u32; 4]; 4]>>::cast::<[u32; 16]>`
     --> /home/nazar-pc/.rustup/toolchains/nightly-2024-11-22-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:436:6
      |
  436 |     }
      |      ^
  note: called by `<ab_proof_of_space_gpu::shaders::chacha8::ChaCha8Block as core::ops::deref::DerefMut>::deref_mut`
     --> src/shaders/chacha8.rs:25:18
      |
  25  |         unsafe { NonNull::from_mut(&mut self.0).cast::<[u32; 16]>().as_mut() }
      |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Not sure if the root cause is the same, but it looks similar.

#[repr(C, align(16))]
pub struct ChaCha8Block(pub [[u32; 4]; 4]);

impl Deref for ChaCha8Block {
    type Target = [u32; 16];

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        // SAFETY: Same size and alignment
        unsafe { NonNull::from_ref(&self.0).cast::<[u32; 16]>().as_ref() }
    }
}

impl DerefMut for ChaCha8Block {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut Self::Target {
        // SAFETY: Same size and alignment
        unsafe { NonNull::from_mut(&mut self.0).cast::<[u32; 16]>().as_mut() }
    }
}

UPD: Tried #269 in its current state, same issue.

UPD2: Even simpler piece of code:

for (d, &initial) in data.iter_mut().zip(&self.data) {
    *d = d.wrapping_add(initial);
}

Fails with:

  error: cannot cast between pointer types
         from `*[u32; 16]`
           to `*[u32]`

Both data and self.data are [u32; 16].