Forbid opaque types in extern blocks

src/test/ui/lint/opaque-ty-ffi-unsafe.rs

@@ -0,0 +1,16 @@
+#![feature(type_alias_impl_trait)]
+
+#![deny(improper_ctypes)]
+
+type A = impl Fn();
+
+pub fn ret_closure() -> A {
+    || {}
+}
+
+extern "C" {
+    pub fn a(_: A);
+    //~^ ERROR `extern` block uses type `A` which is not FFI-safe: opaque types have no C equivalent
+}
+
+fn main() {}

src/test/ui/lint/opaque-ty-ffi-unsafe.stderr

@@ -0,0 +1,14 @@
+error: `extern` block uses type `A` which is not FFI-safe: opaque types have no C equivalent
+  --> $DIR/opaque-ty-ffi-unsafe.rs:12:17
+   |
+LL |     pub fn a(_: A);
+   |                 ^
+   |
+note: lint level defined here
+  --> $DIR/opaque-ty-ffi-unsafe.rs:3:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+