Rust：在“ impl”函数中返回对成员变量的引用

Question

如何确定返回对成员变量的引用的函数的生存期？只要该结构还活着，该引用就有效。

这是一个可能需要这样做的示例：

struct CountUpIter<'a, T> {
    output: &'a mut T,
}

fn next<'a, T>(selff: &'a mut CountUpIter<'a, T>) -> Option<&'a T> {
    Some(&selff.output)
}

// This does not work
// first, the lifetime cannot outlive the lifetime '_ as defined on the impl so that reference does not outlive borrowed content
// but, the lifetime must be valid for the lifetime 'a as defined on the impl
impl<'a, T> Iterator for CountUpIter<'a, T> {
    type Item = &'a T;
    fn next(&mut self) -> Option<Self::Item> {
        Some(&self.output)
    }
}

// Neither does this. It fails with the same error
impl<'a, T> Iterator for &mut CountUpIter<'a, T> {
    type Item = &'a T;
    fn next(&mut self) -> Option<Self::Item> {
        Some(&self.output)
    }
}

我已经尽可能地将解决方案与this question相匹配，但是我想遍历&T而不是Vec<&T>。