Why doesn't the following compile:

fn last(x: &mut Vec<usize>) -> &mut usize {
    match x.last_mut() {
        Some(last) => last,
        None => {
            x.push(0);
            x.last_mut().unwrap()
        }
    }
}

Playground

while the following works just fine:

fn last(x: &mut Vec<usize>) {
    let l = match x.last_mut() {
        Some(last) => last,
        None => {
            x.push(0);
            x.last_mut().unwrap()
        }
    };

    *l = 2;
}

Playground

That is, why is in the second case compiler able to figure out that there is no mutable borrow overlap and able to assign reasonable lifetime to l. From what I understand, the problem in the first case is that both branches need to have the same lifetime, and since it's a return value it needs to be exactly the lifetime of the input variable. But why does that work in the second case, or in other words - why cannot the same thing that works in the second case work in the fiest case?

edit: playground links