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
   |
LL - pub fn f<T>() -> _ {
LL + pub fn f<T>() -> fn() {
   |

error: aborting due to 1 previous error

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