Missing unnecessary unsafe block warning

[bluss]

The following code uses a redundant unsafe block inside a closure.

This is a warning on stable but there is no warning on nightly.

To reproduce (playground):

fn main() {
    let mut v = Vec::<i32>::with_capacity(24);
    
    unsafe {
        let f = |v: &mut Vec<_>| {
            unsafe {
                v.set_len(24);
            }
        };
        
        v.set_len(0);
        f(&mut v);
    }
}

Probably due to #44700, cc @arielb1

[nikomatsakis]

triage: P-high

[arielb1]

Fixed in #45785.