warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/opaque-ty-ffi-unsafe.rs:3:32
   |
LL | #![cfg_attr(full_tait, feature(type_alias_impl_trait))]
   |                                ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default
   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information

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; 1 warning emitted