Function refuse_kernel_calls 

pub fn refuse_kernel_calls(
    program: &CompiledProgram,
    interner: &Interner,
    backend: &str,
) -> Result<(), String>

The refusal for a backend that lowers compiled bytecode but cannot link the Rust kernels: Err at the first kernel call that can RUN (named when its code is a constant, as the VM compiler emits it). A kernel in a function nothing reaches is code the loader spliced in and nothing calls, so it refuses nothing — crate::vm::reach is the same answer the backends lower by.