struct Foo<'a> {
    x: i32,

    /// y points to x
    x_ref: Option<&'a i32>
}

impl<'a> Foo<'a> {
    pub fn new(x: i32) -> Self {
        Self { x, x_ref: None }
    }

    pub fn set(&'a mut self) {
        self.x_ref = Some(&self.x);
    }

    fn x(&self) -> i32 {
        self.x
    }

    fn x_ref(&self) -> Option<&i32> {
        self.x_ref
    }
}