error: type `for<'r> fn(&'r foo::S) {foo::S::f}` is private
  --> $DIR/impl_items.rs:22:23
   |
LL |         let _: () = S.f(); //~ ERROR type `for<'r> fn(&'r foo::S) {foo::S::f}` is private
   |                       ^
...
LL |     foo::m!();
   |     ---------- in this macro invocation

error: aborting due to previous error