error: `extern` block uses type `Qux`, which is not FFI-safe
  --> $DIR/lint-ctypes-73251-1.rs:23:21
   |
LL |     fn lint_me() -> <u32 as Foo>::Assoc;
   |                     ^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: opaque types have no C equivalent
note: the lint level is defined here
  --> $DIR/lint-ctypes-73251-1.rs:2:9
   |
LL | #![deny(improper_ctypes)]
   |         ^^^^^^^^^^^^^^^

error: aborting due to 1 previous error