warning: the feature `track_caller` is incomplete and may cause the compiler to crash
  --> $DIR/only-for-fns.rs:1:12
   |
LL | #![feature(track_caller)]
   |            ^^^^^^^^^^^^
   |
   = note: `#[warn(incomplete_features)]` on by default

error[E0739]: attribute should be applied to function
  --> $DIR/only-for-fns.rs:3:1
   |
LL | #[track_caller]
   | ^^^^^^^^^^^^^^^
LL | struct S;
   | --------- not a function

error: aborting due to previous error