Function fst_replace 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn fst_replace(
    root: CLabel,
    fst_list_ptr: *mut CLabelFstPair,
    fst_list_ptr_len: size_t,
    epsilon_on_replace: bool,
    replaced_fst: *mut *const CFst,
) -> RUSTFST_FFI_RESULT

Safety

The pointers should be valid.