In a `fn` type, a lifetime appears only in the return type,
and not in the arguments types.

Erroneous code example:

```compile_fail,E0581
fn main() {
    // Here, `'a` appears only in the return type:
    let x: for<'a> fn() -> &'a i32;
}
```

To fix this issue, either use the lifetime in the arguments, or use
`'static`. Example:

```
fn main() {
    // Here, `'a` appears only in the return type:
    let x: for<'a> fn(&'a i32) -> &'a i32;
    let y: fn() -> &'static i32;
}
```

Note: The examples above used to be (erroneously) accepted by the
compiler, but this was since corrected. See [issue #33685] for more
details.

[issue #33685]: https://github.com/rust-lang/rust/issues/33685