Rust - Fix QFunction FFI Inputs/Outputs

[jeremylt]

Fix new error in Rust wrapper uncovered by changes in Rust nightly

[jeremylt]

unsafe extern "C" fn trampoline(
    ctx: *mut ::std::os::raw::c_void,
    q: bind_ceed::CeedInt,
    inputs: *const *const bind_ceed::CeedScalar,
    outputs: *const *mut bind_ceed::CeedScalar,
) -> ::std::os::raw::c_int {
    let trampoline_data: Pin<&mut QFunctionTrampolineData> = std::mem::transmute(ctx);

    // Inputs
    let inputs_slice: &[*const bind_ceed::CeedScalar] =
        std::slice::from_raw_parts(inputs, MAX_QFUNCTION_FIELDS);
    let mut inputs_array: [&[crate::Scalar]; MAX_QFUNCTION_FIELDS] = [&[0.0]; MAX_QFUNCTION_FIELDS];
    // BUG STARTS HERE
    // unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
    inputs_slice
        .iter()
        .enumerate()
        .map(|(i, &x)| {
            std::slice::from_raw_parts(x, trampoline_data.input_sizes[i] * q as usize)
                as &[crate::Scalar]
        })
        .zip(inputs_array.iter_mut())
        .for_each(|(x, a)| *a = x);

    // Outputs
    let outputs_slice: &[*mut bind_ceed::CeedScalar] =
        std::slice::from_raw_parts(outputs, MAX_QFUNCTION_FIELDS);
    let mut outputs_array: [&mut [crate::Scalar]; MAX_QFUNCTION_FIELDS] =
        mut_max_fields!(&mut [0.0]);
    outputs_slice
        .iter()
        .enumerate()
        .map(|(i, &x)| {
            std::slice::from_raw_parts_mut(x, trampoline_data.output_sizes[i] * q as usize)
                as &mut [crate::Scalar]
        })
        .zip(outputs_array.iter_mut())
        .for_each(|(x, a)| *a = x);

    // User closure
    (trampoline_data.get_unchecked_mut().user_f)(inputs_array, outputs_array)
}

[jeremylt]

Found the error - fix was to limit the number of iterations for mapping the input/output C arrays to Rust

[jeremylt]

@rezgarshakeri with this fix, your branch should pass fine, so we can merge your branch after this branch merges.

[rezgarshakeri]

Great, thanks.

[rezgarshakeri]

thanks