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.