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