warning: `extern` fn uses type `()`, which is not FFI-safe
  --> $DIR/mustpass-134060.rs:11:34
   |
LL |     extern "C" fn foo_(&self, _: ()) -> i64 {
   |                                  ^^ not FFI-safe
   |
   = help: consider using a struct instead
   = note: tuples have unspecified layout
   = note: `#[warn(improper_ctypes_definitions)]` on by default

warning: 1 warning emitted

