error: `extern` block uses type `impl Fn<()>`, which is not FFI-safe
  --> $DIR/opaque-ty-ffi-unsafe.rs:14:17
   |
LL |     pub fn a(_: A);
   |                 ^ not FFI-safe
   |
note: the lint level is defined here
  --> $DIR/opaque-ty-ffi-unsafe.rs:5:9
   |
LL | #![deny(improper_ctypes)]
   |         ^^^^^^^^^^^^^^^
   = note: opaque types have no C equivalent

error: aborting due to previous error