error: `extern` block uses type `A`, which is not FFI-safe
  --> $DIR/opaque-ty-ffi-unsafe.rs:11:24
   |
LL |     pub(crate) fn a(_: A);
   |                        ^ not FFI-safe
   |
   = note: opaque types have no C equivalent
note: the lint level is defined here
  --> $DIR/opaque-ty-ffi-unsafe.rs:2:9
   |
LL | #![deny(improper_ctypes)]
   |         ^^^^^^^^^^^^^^^

error: aborting due to 1 previous error