rust/src/librustc_error_codes/error_codes/E0581.md

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

Erroneous code example:

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.