warning: `extern` fn uses type `char`, which is not FFI-safe
  --> $DIR/naked-functions-ffi.rs:8:28
   |
LL | pub extern "C" fn naked(p: char) -> u128 {
   |                            ^^^^ not FFI-safe
   |
   = help: consider using `u32` or `libc::wchar_t` instead
   = note: the `char` type has no C equivalent
   = note: `#[warn(improper_ctypes_definitions)]` on by default

warning: 1 warning emitted