$ rustc lifetimefunc_bad.rs 
error[E0106]: missing lifetime specifier
 --> lifetimefunc_bad.rs:9:33
  |
9 | fn ref_max(x: &i32, y: &i32) -> &i32 {
  |                                 ^ expected lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `x` or `y`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0106`.