Function expand_kernel 

pub(crate) fn expand_kernel(
    ecx: &mut ExtCtxt<'_>,
    expand_span: Span,
    _meta_item: &MetaItem,
    item: Annotatable,
) -> Vec<Annotatable>

The offload_kernel macro expands the function into two separate definitions: one on the host to handle the call, and one on the device for executing the kernel.

#[offload_kernel]
fn foo(a: &[f32], b: &[f32], c: *mut f32) {
    *c = a[0] + b[0];
}

This expands to the host-side function:

#[unsafe(no_mangle)]
#[inline(never)]
fn foo(_: &[f32], _: &[f32], _: *mut f32) {
    ::core::panicking::panic("not implemented")
}

And the device-side kernel:

#[rustc_offload_kernel]
#[unsafe(no_mangle)]
unsafe extern "gpu-kernel" fn foo(a: &[f32], b: &[f32], c: *mut f32) {
    *c = a[0] + b[0];
}