warning: `extern` block uses type `TransparentCustomZst`, which is not FFI-safe
  --> $DIR/repr-transparent-issue-87496.rs:8:18
   |
LL |     fn good17(p: TransparentCustomZst);
   |                  ^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: this struct contains only zero-sized fields
note: the type is defined here
  --> $DIR/repr-transparent-issue-87496.rs:6:1
   |
LL | struct TransparentCustomZst(());
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: `#[warn(improper_ctypes)]` on by default

warning: 1 warning emitted