invariant lifetimes

invariant.rs

fn foo<'left, 'right>(_: &mut &'left i32, _: &mut &'right i32)
where
    'left: 'right,
{
}

// fn foo<'both>(_: &mut &'both i32, _: &mut &'both i32) {}

fn main() {
    let a = 42;
    let mut a = &a;
    {
        let b = 43;
        let mut b = &b;
        foo(&mut a, &mut b);
    }
    {
        let b = 44;
        let mut b = &b;
        foo(&mut a, &mut b);
    }
}