error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
  --> $DIR/leaking-unnameables.rs:8:18
   |
LL | pub fn f<T>() -> _ {
   |                  ^
   |                  |
   |                  not allowed in type signatures
   |                  help: replace with the correct return type: `fn()`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0121`.