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