Change selector in match when selector is a mutable reference

I want to change enum variant based on some properties of the current enum variant in Iterator::next. I have two attempts, neither of which compile:

enum Test {
    A(Vec<usize>),
    B,
}

impl<'a> Iterator for Test {
    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {
        // attempt 1
        if let Test::A(ref a) = *self {
            if a.len() == 0 {
                *self = Test::B; // doesn't work because a is borrowed
            };
        }

        // attempt 2
        *self = match *self {
            Test::A(ref a) if a.len() == 0 => Test::B,
            _ => *self, // cannot move out of borrowed context
        };
        None
    }
}

fn main() {}

My second attempt does work if I am not working with references in the selector:

let mut a = Test::A(vec![]);
a = match a {
    Test::A(ref a) if a.len() == 0 => Test::B,
    _ => a,
};

This question is related to Is there a way to use match() in rust when modifying the selector?, but the solution proposed there is not generic: it only works if the same function is executed in both branches.

What is the Rustacean way to achieve my goal?