rust/tests/ui/lint/opaque-ty-ffi-unsafe.rs

#![feature(type_alias_impl_trait)]
#![deny(improper_ctypes)]

type A = impl Fn();

pub(crate) fn ret_closure() -> A {
    || {}
}

extern "C" {
    pub(crate) fn a(_: A);
    //~^ ERROR `extern` block uses type `A`, which is not FFI-safe [improper_ctypes]
}

fn main() {}