"Cant have a mutable operation on a const variable" when initializing variables with a expression

[jyi2ya]

reproduction, this works well if we change the 256 / 2 into 128:

use std::mem::size_of;

use cubecl::{Runtime, prelude::*};

#[cube(launch)]
fn const_mutation(output: &mut Array<u32>) {
    let thread_idx = UNIT_POS as usize;
    let num_threads = CUBE_DIM as usize;
    let mut step = 256 / 2;

    while step > 0 {
        for pos in range_stepped(thread_idx, step, num_threads) {
            output[pos] = 0;
        }
        step /= 2;
    }
}

fn main() {
    let client = cubecl::wgpu::WgpuRuntime::client(&Default::default());
    let output = client.empty(128 * size_of::<u32>());

    unsafe {
        const_mutation::launch::<cubecl::wgpu::WgpuRuntime>(
            &client,
            CubeCount::new_1d(1),
            CubeDim::new_1d(1),
            ArrayArg::from_raw_parts(output.clone(), 128),
        );
    }

    let _ = client.read(vec![output]);
}

result:

hepnode0 15:28 (master) ~/dev/gpu-zhenhaowan/repros
0 cargo run --bin const-mutation
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.23s
     Running `target/debug/const-mutation`
WARNING: radv is not a conformant Vulkan implementation, testing use only.

thread 'DSD-4-0' (1852029) panicked at /home/jyi/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cubecl-core-0.10.0/src/frontend/operation/base.rs:180:9:
Can't have a mutable operation on a const variable. Try to use `RuntimeCell`.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[nathanielsimard]

@wingertge is working on a big refactor. I think that should be fixed.

[wingertge]

This is actually a frontend bug and it's not easy to fix. The only reason it even works with a literal is that there's a specific exception for it - we really need to change how some of the traits work to fix this properly. I'll add a reproducer to the test suite and try to fix it after the refactor lands.