Union cast failed

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

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.

Nope, even at commit 2f915a0 error still appears

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) }
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^

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

Ok great, I'll take a look.

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

@LegNeato

@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?

Fix union field access by representing unions as structs in SPIR-V

Fix union field access by representing unions as structs

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

Details

  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].

This issue is one of the largest contributors of ugly non-idiomatic code in my codebase.

For example, I have a dynamic index into a fixed size array. This typically means I can use slice operators on an array, but because of this issue it doesn't work:

  error: cannot cast between pointer types
         from `*[struct ManuallyDrop<Match> { value: struct Match { .. } }; 288]`
           to `*[struct ManuallyDrop<Match> { value: struct Match { .. } }]`
     --> src/shader/find_matches_in_buckets.rs:348:35
      |
  348 |             let Some(m) = matches.get_mut(local_matches_offset as usize) else {
      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So I have to make explicit bounds check separately and then do array indexing and on top of that hope that compiler will be able to see bounds check and not insert conditional panic on array access (which with rust-gpu I don't have 100% confidence in).

P.S. No idea where it found ManuallyDrop, I'm using MaybeUninit there instead 🤔

I think this is a hard blocker for me for now. I can't really use new types until this is fixed because I need to cast arrays of u32s from shared memory to arrays of use-case specific pointers that are just wrappers over u32 for any type safety.

Without pointer casting everything is just u32 and it is very difficult to make sense of anything. I can't write sane misuse resistant code like this, it feels more like writing in assembler without types.

I eventually ended up with an actual union situation myself, needing it for shared memory reuse between two parts of the fused shader.

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Union cast failed #241

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Union cast failed #241

Description

Expected Behaviour

Example & Steps To Reproduce

System Info

Activity

LegNeato commented on Apr 23, 2025

simensgreen commented on Apr 24, 2025

simensgreen commented on Apr 24, 2025

simensgreen commented on Apr 24, 2025

LegNeato commented on Apr 24, 2025

LegNeato commented on May 21, 2025

simensgreen commented on May 25, 2025

nazar-pc commented on Jun 25, 2025

nazar-pc commented on Sep 15, 2025

nazar-pc commented on Sep 15, 2025

nazar-pc commented on Nov 7, 2025

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