Fix false positive unused_lifetime when referenced in generic bounds.

[oxalica]

It is commonly met when using lifetimes in Fn traits.

Here is an example causing false positive unused_lifetime.

fn func<'a, F: FnOnce(&'a ())>(x: &'a (), f: F) {
    f(x);
}

[oli-obk]

Removing all mentions of lifetimes makes this code still compile, so the lifetime is unnecessary. Do you have an example where removing the named lifetimes results in a compilation failure?

[oxalica]

Removing lifetimes makes F to be for<'r> FnOnce(&'r ()), it's a much stronger bound.

And the following codes will not work. Link

// fn func<'a, F: FnOnce(&'a ())>(x: &'a (), f: F) { // This works
fn func<F: FnOnce(&())>(x: &(), f: F) {
    f(x);
}

fn main() {
    let x = ();
    let p = &x;
    let mut q = &x;
    func(p, |y| q = y); // borrowed data cannot be stored outside of its closure
}

[oli-obk]

Ah that makes sense. Can you add the full test so we don't regress it in the future?

[oli-obk]

Thanks!